DIFFRACTIVE STRUCTURES WITHIN POLYMERIC SUBSTRATES, THEIR MANUFACTURE AND USE

Abstract

Optical devices suitable as security devices for document authentication are described, comprising at least one two-dimensional array of laser-modified elongated tracks extending within a document substrate with a different optical refractive index compared to the unmodified substrate, and which can exhibit excellent diffraction effects. The use of such devices for document authentication and methods for their production are also described.

Description

DIFFRACTIVE STRUCTURES WITHIN POLYMERIC SUBSTRATES, THEIR MANUFACTURE AND USE FIELD OF INVENTION The present invention relates to the field of document substrates, particularly flexible polymer substrates that can be used, for example, in the production of documents and banknotes, and their modification to generate certain optical effects that can optionally form security elements for the authentication of documents. BACKGROUND OF THE INVENTION Important or high-value documents are susceptible to counterfeiting. Such documents often include specific features or devices that are difficult for forgeries to replicate. These devices are typically applied to or adhered to the document substrate. They generally need to be very thin so they don't protrude from the substrate and flexible so they can bend and flex with the substrate during normal use. Examples of such devices include holograms, thin films, and micro-optical features. There are risks associated with applying or adhering such features or devices to document substrates. With these devices, the security and integrity of the document depend on maintaining proper adhesion of the device to the document substrate. Adhesives can degrade over time or during document use, and fatigue can result in the unintentional partial or total detachment of the device, compromising the document's security value. Even if the adhesive remains intact, the security features applied to the surface may be more susceptible to wear or abrasive forces experienced during the document's lifespan. Furthermore, counterfeiters are known to intentionally detach these security features and then reattach them to counterfeit documents to make them appear authentic. For example, counterfeiters have been known to remove holographic features from genuine documents to produce new counterfeit ones, such as fake passports. In other instances, holographic features have been removed from low-denomination banknotes and then reapplied to counterfeit banknotes with a higher face value. Some security documents incorporate security features embedded or woven into the document substrate. As expected, it is more difficult to remove such integral features for counterfeiting purposes. Examples of such features are more common in paper document substrates and include features such as watermarks or metallic strips embedded in the substrate, as used, for example, in paper banknotes. IVIA / t / ZUZZ / UZ IZZO Increasingly, polymers and plastics are being used as substrates for security documents. For example, central banks often seek to use such plastic or polymer substrates for new generations of banknotes. However, polymer and plastic substrates are known to present certain limitations for the integration of security features. With the notable exception of transparent polymer windows, the security features of polymer banknotes are generally limited to those that can be adhered to the polymer surface, or those that can be printed or embossed onto the polymer surface. However, more recent efforts have included modifying the polymer substrate directly to generate specific optical effects. For example, international patent publications WO2019 / 077316 and WO2019 / 077317, both published on April 25, 2019, describe the limited laser marking of polymer substrates. Further improvements are still needed in the security features and devices integrated into document substrates. In particular, more suitable security elements and devices are needed for polymer and plastic substrates, which are less likely to detach, wear out, or be used by counterfeiters. BRIEF DESCRIPTION OF THE INVENTION It is an objective, at least for selected modalities, to provide a safety device as an integral feature of a polymer substrate material. The selected modalities provide such safety devices. Another objective of the invention, at least in selected embodiments, is to provide a method for verifying whether an article or document is a genuine or counterfeit version. The selected embodiments provide such methods. The following embodiments are illustrative only. One embodiment provides a substrate sheet comprising a material having an overall refractive index ti, wherein the substrate sheet contains at least one ordered two-dimensional array of discrete laser-modified tracks in the material generated by a beam-shaped laser with incident laser light distributed along a longitudinal laser beam axis extending through the substrate sheet, or generated by a beam-shaped laser with laser light distributed along and / or around a laser propagation path extending within the substrate sheet, each laser-modified track comprising an elongated volume of modified substrate material at least 4 times longer than its narrowest width extending at least partially through a thickness of the substrate sheet, comprising a modified shape of the substrate material,which has a refractive index that is different from the general refractive index n of the substrate sheet from which each laser-modified track originated, wherein for each ordered two-dimensional array the laser-modified tracks collectively diffract the light incident on the substrate sheet, IVI A / t / ZUZZ / UZ IZZO to form an observable shape, image, or color region. Mode 2 provides the substrate sheet of Mode 1, wherein for each laser-modified track the elongated volume of modified substrate material is at least 5 times, or at least 10 times, or at least 15 times, or at least 20 times, longer than its width, and wherein optionally the laser propagation path is linear, curved, or helical within the substrate sheet. Mode 3 provides the substrate sheet of mode 1, wherein each of the laser-modified tracks is generated by a femtolaser, such as a femtolaser with a pulse duration in the range of 0.1 fs to 100 hp for each laser pulse, with beam shaping of the femtolaser beam before or after its interaction with the substrate. Mode 4 provides the substrate sheet of mode 3, in which the femtolaser beam shaping employs a lens or sheet between a laser beam source and a substrate surface. Mode 5 provides the substrate sheet of mode 4, where the femtolaser beam shaping employs a silica sheet between the laser beam source and the substrate. Mode 6 provides the substrate sheet of Mode 1, wherein for at least some of the laser-modified tracks the elongated volume of modified substrate material is located within the substrate sheet. Mode 7 provides the substrate sheet of Mode 1, wherein for at least some of the laser-modified tracks, the elongated volume of modified substrate material is exposed on at least one surface of the substrate sheet. Mode 8 provides the substrate sheet of Mode 1, wherein for at least some of the laser-modified tracks the elongated volume of modified material in the substrate sheet includes a void in the substrate sheet, formed after the production of the substrate sheet, by melting, displacement, or decomposition of a portion of the substrate sheet material. Mode 9 provides the substrate sheet of mode 1, which has an average thickness of 10-3000 pm, preferably 50-150 pm. Mode 10 provides the substrate sheet of mode 1, wherein the substrate sheet is a polymer sheet, and each of at least one ordered two-dimensional array of discrete laser-modified tracks comprises laser-modified tracks in the polymer, generated after the production of the polymer sheet. Mode 11 provides the substrate sheet of mode 10, wherein the substrate sheet comprises BOPP, BOPET, PEN, PP, PVDF or related copolymers such as PVDF-TrFE, or Nylon-55 or 66 or other derivatives. Mode 12 provides the substrate sheet of mode 10, where the tracks Laser-modified ML / t / ZUZZ / UZ IZZO extend independently of each other within the polymer sheet from 5% to 100% of the distance between opposite surfaces of the polymer sheet, and optionally extend into one or more additional layers if present adjacent to the polymer sheet. Mode 13 provides the substrate sheet of mode 10, wherein the polymer of the polymer sheet comprises polymer chains, wherein each elongated volume of modified substrate material for at least some of the laser-modified tracks comprises polymer chains that are at least partially aligned with each other with respect to those of unmodified polymer sheet material, such that the modified material comprises aligned polymer chains that generally extend non-parallel to the surfaces of the polymer sheet, so as to cause the modified substrate material to have a different refractive index with respect to the general refractive index n for the unmodified polymer sheet material. Mode 14 provides the substrate sheet of mode 13, wherein the modified material comprises polymer chains displaced to create periodic gaps, such that the selected laser-modified tracks each comprise a gap that generally extends non-parallel to the surfaces of the polymer sheet, such that each of said tracks has a different refractive index with respect to the general refractive index n for the unmodified polymer sheet material. Mode 15 provides the substrate sheet of mode 13, comprising polymer chains that generally extend or are aligned non-parallel to the surfaces of the polymer sheet, due to melting, displacement, or decomposition of the polymer within each elongated volume of modified substrate material in the polymer sheet. Mode 16 provides the substrate sheet of mode 13, wherein at least some of the aligned polymer chains within each elongated volume of modified substrate material of each laser-modified track are generally extended or aligned perpendicular to the polymer sheet surfaces. Mode 17 provides the substrate sheet of mode 13, wherein at least some of the aligned polymer chains within each elongated volume of modified substrate material of each laser-modified track extend to one or both surfaces of the polymer sheet. Mode 18 provides the substrate sheet of mode 13, wherein at least some of the aligned polymer chains within each elongated volume of modified substrate material of each laser-modified track extend into the polymer sheet, but do not extend to the polymer sheet surfaces. Mode 19 provides the substrate sheet of mode 13, wherein the laser-modified tracks comprise amorphous regions and / or voids in the substrate that extend IVI A / t / ZUZZ / UZ IZZO optionally to one or both surfaces of the substrate sheet, wherein the amorphous regions and / or voids comprise at least one of amorphous polymer, air, vacuum, polymer decomposition and oxidation products, glassy and amorphous carbon compounds. Mode 20 provides the substrate sheet of Mode 1, wherein each elongated volume of modified substrate material of each laser-modified track is 1-5000 nm wide on average, extending partially or fully through the substrate sheet. Mode 21 provides the substrate sheet of Mode 1, wherein a two-dimensional ordered array comprises laser-modified tracks that are separated from each other by a periodicity of 0.01 to 1000 pm, preferably 0.05-10 pm, more preferably 0.1-5 pm, on average by the unmodified substrate sheet material having an overall refractive index n. Mode 22 provides the substrate sheet of Mode 1, wherein the substrate sheet comprises different sections, each comprising a plurality of said laser-modified tracks, wherein the tracks within a section have different periodicities, lengths, or orientations compared to the tracks of at least one other section of the substrate sheet, such that the optical emissions from said different sections resulting from diffraction of the incident light differ from each other when the same or equivalent incident light is simultaneously incident on the different sections. Mode 23 provides the substrate sheet of mode 22, comprising pixel-like areas of the substrate, at least some of which have different optical diffractive properties from each other, the pixel-like areas preferably 1-10,000 pm wide, more preferably 5-100 pm wide. Mode 24 provides the substrate sheet of mode 23, wherein each pixel-like area of ​​the substrate sheet comprises laser-modified tracks having the same or substantially the same periodicity within each pixel-like area, such that each pixel-like area provides a generally uniform optical diffractive output after exposure to incident light. Mode 25 provides the substrate sheet of Mode 23, wherein each pixel-like area comprises multiple subsections, each of which comprises an ordered array of said laser-modified tracks having substantially consistent periodicity within each subsection, the periodicity of the tracks extending within the subsections of any given pixel-like area being different from each other, so that the optical diffraction output of any given pixel-like area is defined by a combination of optical diffraction outputs for all subsections of that pixel-like area. Mode 26 provides the substrate sheet of mode 25, wherein each pixel-like area comprises subsections, each providing a red, green, and blue optical diffractive output IVI A / t / ZUZZ / UZ IZZO at selected angles after exposure to incident light, the relative intensity of the red, green, and blue outputs of subsections of any given pixel-like area determines the color of the combined optical diffractive output for that pixel-like area at a given angle. Mode 27 provides the substrate sheet of mode 22, wherein the laser-modified tracks have a graduation change in at least one of periodicity, spacing, length, and orientation across the substrate sheet between sections, thereby providing a graduation change in the optical diffractive output properties across the device after exposure to incident light. Mode 28 provides the substrate sheet of Mode 1, wherein at least some of the laser-modified tracks extend at least partially through the thickness of the substrate sheet in a non-perpendicular and / or non-linear path with respect to the substrate sheet surfaces. Mode 29 provides the substrate sheet of Mode 1, wherein at least some of the laser-modified tracks extend to different depths within and across a thickness of the substrate sheet from each other. Mode 30 provides the substrate sheet of Mode 1, comprising two or more ordered two-dimensional assemblies of discrete laser-modified tracks, with the assemblies at different depths from each other through a thickness of the substrate sheet. Mode 31 provides the substrate sheet of mode 30, wherein a first array of laser-modified tracks is at least partially superimposed on a second array of laser-modified tracks in the substrate sheet, when the substrate sheet is viewed in plan view from one side of it. Mode 32 provides the substrate sheet of mode 31, wherein the optical diffractive output of one of the first and second formations is further diffracted by the other of the first and second formations, when the substrate sheet is exposed to incident light. Mode 33 provides the substrate sheet of mode 31, where the optical diffraction output of the first and second formations, when viewed simultaneously, exhibits an optical interference or diffraction effect, such as the Littrow configuration or grid layers aligned on Talbot self-image planes. Mode 34 provides the substrate sheet of any of modes 31 to 33, comprising laser-modified tracks of overlay sets at different depths within the substrate, formed simultaneously by a single laser pulse with laser energy distributed along a longitudinal laser beam axis extending through the substrate sheet focused simultaneously at such different depths within the substrate sheet. IVI A / t / ZUZZ / UZ IZZO Mode 35 provides the substrate sheet of mode 1, further comprising one or more additional layers partially or totally overlaying the substrate material, the additional layer(s) being each selected interchangeably from a polymer layer, a reflective layer, a refractive layer, a diffraction filter, a transmissive filter, a protective layer, a coating, an adhesion-promoting layer, an ink, an optical interference layer, and an optical interference stack. Mode 36 provides for the use of the substrate sheet of any of modes 1 to 35 as a security feature of a security document, or as a security document. Modality 37 provides a security document comprising, as a security element, the substrate sheet in accordance with any of modalities 1 to 35. Modality 38 provides a method for manufacturing a security document or security device, wherein the method comprises the following steps: provide a substrate sheet comprising a substrate sheet material; irradiating the substrate sheet at multiple discrete positions through a flat side of the substrate sheet corresponding to a two-dimensional array, with a laser beam from a laser, such as a femtolaser, while modifying the shape of the laser beam before or after its interaction with the substrate sheet so that the incident laser light is distributed at least partially along and / or around a laser propagation path extending into the substrate sheet; melting, displacing, or decomposing at least partially or temporarily at least a portion of the substrate material in or within an elongated volume of the substrate sheet material around the longitudinal axis of the laser beam, corresponding to each of said discrete positions, thereby generating an array of laser-modified tracks within the substrate material, each comprising an elongated volume of modified substrate material at least 4 times longer than its width extending at least partially through a thickness of the substrate sheet, wherein the modified shape of the substrate material has a refractive index that is different from the general refractive index n of the unmodified substrate; where each ordered two-dimensional array of laser-modified tracks thus collectively produces diffracted light that impinges on the substrate sheet to form an observable shape, image, or color region. Mode 39 provides the method of mode 38, where the laser is a femtolaser that has a pulse duration in the range of 0.1 fs to 100 hp. Mode 40 provides the method of mode 38, wherein in the irradiation stage the laser and the substrate sheet are movable relative to each other to expose such discrete positions on the polymer sheet to the laser radiation pulses. IVI A / t / ZUZZ / UZ IZZO Mode 41 provides the method of mode 38, in which the laser beam is split into multiple beams before its interaction with the substrate, each corresponding to one of the discrete positions on the substrate sheet. Mode 42 provides the method of mode 38, wherein the substrate sheet is irradiated with multiple lasers, each with a modified laser beam shape or focus, simultaneously, to generate the laser-modified track array. Mode 43 provides the method of mode 38, wherein modifying the shape or focus of the laser beam involves passing the laser beam through a lens or silica plate before its interaction with the substrate sheet. BRIEF DESCRIPTION OF THE FIGURES Figure 1: An example of an optical image of a 2-D array of elongated 'laser-modified tracks' or modified regions embedded in BOPP, illuminated by a white light source and observed in reflection mode. Without the modified polymer structure or lattice, transparent BOPP does not produce any substantial reflection and does not scatter the wavelength spectrum as shown. Figure 2: DIC microscope optical images showing a) the bottom surface, b) the volume center, and c) the top surface of a 2-D array of laser-modified elongated tracks in BOPP. Figure 3: Scanning electron microscope images showing a) cross-section of BOPP film (~70 pm thick) embedded with laser-modified elongated tracks ~50 pm long and Lx= period of 3 pm, not extending to the top or bottom surface; b) the magnified cross-section image showing laser-modified tracks including long cylindrical nanocavities ~800 nm in diameter by ~50 pm in length; c) cross-section of BOPP film embedded with elongated tracks; and d) magnified cross-section image showing laser-modified tracks including morphology change to the BOPP film. Figure 4: Examples of large areas, 2-D arrays of laser-modified elongated tracks embedded in BOPP, optically etched, BOPP illuminated with a white LED a) at a slight tilt angle from the left side and b) normal incidence with a slight tint to show the coloring effect of arrays with different grating periods. Note that some grating periods and angles show almost no diffraction and therefore multiple colors and varying brightness can be generated by the laser-modified track array, as required. Figure 5: Scanning electron microscope images showing cross-sections of the laser-modified, embedded elongated tracks fabricated with pulse energies of a) 1004 nJ, b) 525 nJ, and c) 284 nJ. The laser was first focused through a 2 mm thick fused silica. IVI A / t / ZUZZ / UZ IZZO grating laser modified track is formed near lower BOPP samples of -70 pm thickness with a period of 2 pm. Figure 6: Scanning electron microscope images showing cross-sections of a single laser-modified embedded track fda of a 2D array fabricated with a period of 3 pm and pulse energies of a) 284 nJ and focused first through a 2 mm thick fused silica and b) 65 nJ without the fused silica. The laser-modified track length increases from -7 pm in b) to -50 pm in a) until spherical aberration by the fused silica plate. Figure 7: Set of laser-modified elongated tracks fabricated in a 2D array within BOPP with a pulse energy of 1004 nJ penetrating through the lower surface with optical microscope image a) showing the lower surface with ablation debris, SEM image b) showing the cross section on the lower surface with holes opening into the surface, and c) showing the part of the laser-modified track embedded within the BOPP film, without breaking the upper surface. Figure 8a schematically illustrates the formation of red diffractive bands on the Canadian flag by using a laser-modified track array with a given track periodicity and array periodicity in the x and y directions. Figure 8b schematically illustrates how a laser-modified via array can be optionally rotated at any angle with respect to the horizontal image plane to modify the observed diffractive light output of the array. Figure 8c schematically illustrates the formation of the white area within the Canadian flag using a series of pixel arrays containing arrays of red, green, and blue subpixels. Each subpixel array contains the track periodicity necessary to generate a red, green, or blue diffracted light output for each subpixel. The pixel output is the sum of the diffractive output of the RGB subpixels. Figure 8d schematically illustrates an alternative pixel layout consisting of hexagonal subpixel arrays that can generate the combined RGB diffractive light output for the white portion of the Canadian flag. The pixel array periodicity and spacing are identified in the illustration. Figure 9 provides a schematic illustration showing various laser-modified track arrangements on a two-dimensional periodic grid in cross-sectional view (ac) and top view (df). Figure 10 provides photos recorded from Talbot gratings that have 3 layers of a symmetric 2-D array of laser-modified short tracks generated with a femtosecond laser at a wavelength of 515 nm and pulse energies of 21.5 nJ, 36.5 nJ, 58.5 nJ, 89.0 nJ and 153.5 nJ. IVIA / t / ZUZZ / UZ IZZO Figure 11 provides optical microscope images showing the top view of a polymer sample after modeling a 3D volume lattice of short filaments, formed in three layers with a laser at pulse energies from 21.5 nJ to 89.0 nJ. The filaments were periodically aligned with symmetric transverse periods L = Lx = Ay, and the Talbot period, c, varied as indicated. Figure 12 shows reflex camera images of fluorescent lamps made by a 70 µm thick polyethylene terephthalate (PET) film observed at varying tilt angles of the sample with respect to the fixed positions of the camera and light source. The PET film was embedded with an array of symmetrical 2-D arrays of filament tracks (vertically stacked rectangular zones), formed by a femtosecond laser. Figure 13 shows top optical (a, d) and cross-sectional SEM (b, c, e, f) images of two examples of a three-dimensional lattice structure of filaments formed on three layers near the middle of a 70 pm thick BOPP substrate. The examples demonstrate Talbot configurations with hexagonal crystal symmetry in (a) and tetragonal crystal structure in (d), and lateral periodicities of a = b = a = 2 mm (top view in a) and 1.5 mm (top view in d) and longitudinally of c = 15.5 mm (cross-sectional views in b, c) and 8.7 mm (cross-sectional views in e, f). Figure 14 shows images recorded from a single layer of a symmetric 2-D array of short laser modification tracks generated in BOPP with a femtosecond laser at a wavelength of 515 nm with varying pulse energies, as indicated on the right side of the figure. The four images were recorded from the same sample, with a variable angle of incidence while irradiated with a fixed incandescent light and a camera angle in reflection (ab) and transmission (cd) modes. Figure 15 shows photos etched from a single layer of a symmetric 2-D array of long laser modification tracks in BOPP generated with a femtosecond laser at a wavelength of 515 nm and pulse energies ranging from 79.0 nJ to 141.0 nJ (from top to bottom row) as indicated on the right side of the Figure. Figure 16 graphs the diffraction efficiency expected by the FDTD simulation from periodic arrays of filaments that have been opened into gaps, providing a high refractive index contrast of Δη = 0.46. Figure 17 presents the transmission spectra generated from single filament arrays (a) and seven-layer arrays (bf) based on filaments formed in BOPP (n = ~ 1.5) and having a low refractive index contrast of Δη = 0.033020. Figure 18 illustrates the relative merits of short and long grating lengths, Littrow angle effects, and multi-layer Talbot resonances, showing graphs of combined first-order diffraction efficiency. IVI A / t / ZUZZ / UZ IZZO DEFINITIONS: Beam shaping: refers to any technique for changing the shape, focus, direction, or optical properties of a laser beam, as described herein. Laser-modified track: In the context of this application, this refers to any modification, abrogation, aberration, or generally elongated modified portion of or within a substrate, such as a polymer substrate, generally running non-parallel to the substrate surfaces, formed by any method including, but not limited to, melting, sublimation, decomposition, deformation, or displacement of the substrate. Each laser-modified track may comprise any shape of modified substrate, or an elongated region or volume of modified substrate material, wherein the modification results in a refractive index different from that of the modified substrate material before the modification.Typically, such laser-modified tracks can be formed, for example, by laser pulses on the substrate, where the shape and elongated direction of the laser-modified track generally conform to or are consistent with the direction of the laser and its interaction with the substrate material. Commonly, the laser and substrate are stationary or do not move relative to each other, or move very little relative to each other, during the duration of a laser pulse. Typically, in some modalities, the elongated nature of the laser-modified track on the substrate is generated using a laser with beam shaping so that the incident laser light is focused in a distributed manner along a longitudinal axis of the laser beam extending through the substrate sheet.In this way, the laser beam energy is not focused at a particular location or depth within the substrate. Instead, it is distributed in a controlled manner along a linear path that extends into and optionally through the substrate, generally coaxial with the laser's direction. Therefore, the laser beam can be made to modify the document substrate along a defined one-dimensional section of its beam track or path as the focused, distributed portion of the laser interacts with and passes partially or completely through the substrate. In some modalities, the laser's wavelength and pulse duration can be favorably selected to activate photothermal versus photochemical reactions at low energy exposures, or to drive nanobursts from a shaped absorption volume with higher laser pulse energy.In some modalities, the modification zone can be formed entirely by a single laser pulse or a pulse train at a high repetition rate (i.e., from 10 kHz to 100 GHz), where the latter allows substrate effects accumulated over multiple pulses to result in favorable benefits such as thermal cycling, large heat-affected zones, and material defect generation. In some modalities, laser-modified tracks may comprise regions or portions of modified polymer where the polymer chains are generally aligned with the laser direction, and may optionally form voids or amorphous regions (generally elongated voids or regions). Laser-modified tracks may comprise a volume of modified substrate material that is 1–5000 nm wide and 0.1–3000 pm long. For example, laser-modified tracks may be elongated if they comprise a volume of modified substrate that is at least four times longer (generally in the direction extending in line with the laser beam) than their individual width. Furthermore, laser-modified tracks may extend perpendicularly or non-perpendicularly (but generally not parallel) to the substrate surface, or be shaped to follow a curved or helical path. Optionally, any laser-modified track may be further modified by chemical treatments, added layers, or heat annealing as required according to any specific modality.In some modalities, the term “laser modified track” is used interchangeably with the term “filament”. Periodicity: refers to the regularly spaced spacing of laser-modified tracks on a substrate sheet, within a two-dimensional array of such tracks within the substrate sheet, or a portion thereof, as described herein. Generally, the term refers to the overall or average ordered spacing of laser-modified tracks that have been modified after the substrate sheet was produced to exhibit a change in the refractive index of the substrate sheet material, separated by unmodified areas or portions of the substrate sheet that retain their original refractive index from the original materials of, and the original fabrication of, the substrate sheet.In some embodiments, the periodicity for a small area of ​​a two-dimensional array of such laser-modified tracks may be optionally, or commonly at least substantially constant, but the periodicity for a larger area of ​​a two-dimensional array of such tracks may optionally change within or across the array, or be different between adjacent arrays, according to the specific embodiment. The common or average periodicity, in terms of the average separation of laser-modified tracks and their constituent modified substrate material, within a two-dimensional array of laser-modified tracks, may, for example, comprise tracks that are separated from each other, or from a nearest adjacent track, by 0.05–10 pm, more preferably 0.1–5 pm, on average by unmodified substrate sheet material that retains its original overall refractive index.For example, two-dimensional arrays may include periodicity intervals between array regions that can give rise to optical diffraction effects, or that may be too large or too small to produce observable optical diffraction effects within the visible light spectrum. For instance, without intending to be limited to theory, regions separated by an average of 0.1–5 pm by the unmodified substrate sheet material may be preferred for producing observable optical diffraction effects within the visible light spectrum. Selected modalities include random variation in periodicity, chirping of spatial frequencies, and overlapping gratings with different periods and / or shapes. IVI A / t / ZUZZ / UZ IZZO Polymer: refers to any polymer or polymer-like substance suitable for forming a substrate material, for example, in the form of a sheet-like configuration or roll to be formed or cut to a size suitable for use, such as in security documents. The polymer may be a substantially uniform sheet of polymeric material, or it may take the form of a laminated structure with polymeric layers or film bonded together for structural integrity, as described, for example, in International Patent Publication WO83 / 00659, published on March 3, 1983, which is incorporated herein by reference. Polymers may include, but are not limited to, polypropylene, PMMA, polycarbonate, polytetrafluoroethylene (PTFE), PET, BOPP, BOPET, PEN, PP, PVDF, and related copolymers such as PVDF-TrFE. Region (of a substrate): refers to a portion of a substrate that includes a specific or defined part of the substrate having a refractive index that differs from the rest of the substrate due to post-production modification of the substrate. Such a region may comprise, for example, a laser-modified track as described herein, or any modified substrate, polymer, voids, abrogation, or anomaly that achieves the change in refractive index for the material of the region or a portion thereof. In selected modalities, the net effect of the material modification is to redirect light propagation by optical means such as Fresnel reflection, Rayleigh or Mié scattering, or induction of localized absorption zones.In selected modalities, the collective response of such optical effects from an array of similar modification zones is then directed to spectrally filter and redirect light with controlled wavelength intervals and diffraction angles. Security document: refers to any document, whether polymer-based or not, of importance or value. In selected forms, a security document may include features or devices intended to demonstrate that the document is a genuine, legitimate, or authentic document, and not a counterfeit, illegitimate, or fake copy. For example, such security documents may include security features such as those described herein. Such security documents may include, but are not limited to, identification documents such as passports, citizenship or residency documents, driver's licenses, banknotes, checks, credit cards, bank cards, and other documents of monetary value. Security device or feature: refers to any device or feature that can be added to or incorporated into a security document for the purpose of making that security document more difficult to copy, replicate, or counterfeit, including structures or features incorporated into the substrate material or substrate sheet of the security document, or as a result of modification of the substrate material or substrate sheet. Substrate sheet / substrate material: refers to any material or combination of materials used to form the main structure or sheet of a security document. The material is formed IVIA / t / ZUZZ / UZ IZZO generally consists of a sheet or flat member and may be composed of at least one substance selected from, among others, paper, plastic, polymer, resin, fibrous material, metal, or similar materials, or combinations thereof. The substrate sheet may comprise more than one material, layered, interwoven, or bonded together. The material may be smooth or textured, fibrous, or of uniform consistency. In addition, the material may be rigid or substantially rigid, or flexible, foldable, or collapsible as required by the security document. The base material may be treated or modified in any way during the production of the final security document. For example, the material may be printed, coated, impregnated, or otherwise modified as described herein. Talbot effect: In some modalities, this refers to the preferential alignment or misalignment of grating planes at the axial periodic separation, c, which coincides or does not coincide, respectively, with the Talbot self-image distance in multibeam interference. The purpose of the alignment and misalignment is to improve or decrease, respectively, diffraction efficiency by working with or against, respectively, the interference generated solely by the single grating layer. Combining these effects in multilayered, three-dimensional volume gratings provides more degrees of freedom to control the direction and spectral content of the light flowing through the grating. DETAILED DESCRIPTION OF THE PREFERRED MODALITIES This description covers security devices or features that, at least in selected embodiments, are incorporated directly into a substrate material or substrate film of a security document, or that result from the modification of a substrate material or substrate film, rather than being applied or adhered to a substrate material or substrate film of a security document. Incorporating such security devices or features into or within the substrate film of the security document offers certain advantages, depending on the embodiment, as described herein. For example, by incorporating the security device or feature into the substrate of the security document, the problems associated with adhered or applied security devices can be substantially avoided. Embedded or incorporated security devices and features, which form an integral structural feature of the document substrate, cannot be removed from the substrate material, either intentionally or unintentionally, without compromising the substrate's integrity. Consequently, such security devices and features are generally more robust and less susceptible to damage or wear during document use. The selected methods provide safety devices and features that are formed within a polymer substrate after the polymer has been produced into a sheet material. In certain examples, a safety substrate polymer is modified using a specially adapted laser source with specific, defined properties. In this way, it is possible to generate IVI A / t / ZUZZ / UZ IZZO optically discernible security features embedded or partially embedded within polymer sheets, possessing certain advantageous optical properties that have been difficult to achieve to date. For example, using certain security methods and devices described herein, specific types of diffractive effects can be introduced directly into a security document substrate without necessarily involving or requiring the addition of other layers or additives. In some embodiments, the optical effects offer advantages over those previously known in the art. Methods for producing such security features and devices are also described, which in selected embodiments are suitable for large-scale document manufacturing, for example, for the production of banknotes. Other modalities, however, include modified material sheets, such as polymer sheets as described herein, which are applied or adhered to a substrate of a security document or banknote. Accordingly, the selected embodiments provide a substrate sheet comprising a material having an overall refractive index n, wherein the substrate sheet contains at least one ordered two-dimensional array of discrete elongated volumes of substrate, each comprising laser-modified material. For each two-dimensional array, the elongated volumes of modified substrate material, also known as 'laser-modified tracks' in the substrate material, extend at least partially through a thickness of the substrate sheet, wherein the substrate material has been modified to have a refractive index that is different from the overall refractive index n of the substrate sheet from which each laser-modified track originated.Furthermore, for each ordered two-dimensional array, the laser-modified tracks collectively diffract the light incident on the substrate sheet to form a shape or image that can be observed by reflection and / or transmission with the naked eye, or with the aid of a detection tool. Optionally, the laser-modified tracks and / or the laser propagation paths within the substrate that generate the laser-modified tracks are linear, curved, helical, or can take other configurations and orientations with different resulting optical effects. The inventors have achieved remarkable optical characteristics within document substrate materials based on such document substrate modification.In some embodiments, the assemblies may comprise longer laser modification tracks that can optionally extend substantially through or completely through a thickness of the substrate material, with Dn values ​​significant with respect to other devices known in the art. In particular, the selected modalities provide a substrate sheet comprising a material having an overall refractive index n, wherein the substrate sheet contains at least one ordered two-dimensional array of discrete laser-modified tracks in the material generated by a IVIA / t / ZUZZ / UZ IZZO beam-shaped laser with incident laser light distributed along a longitudinal laser beam axis extending through the substrate sheet, or generated by a beam-shaped laser with laser light distributed along and / or around a laser propagation path within the substrate sheet, wherein each laser-modified track comprises an elongated volume of the material preferably at least 4 times longer than its width extending at least partially through a thickness of the substrate sheet.Each laser-modified track comprises an elongated volume of a modified shape of the substrate material, having a refractive index different from the general refractive index n of the substrate sheet from which each laser-modified track originated, and wherein, for each ordered two-dimensional array, the laser-modified tracks collectively diffract light incident on the substrate sheet to form an observable shape, image, or color region, e.g., in reflection and / or transmission. In some embodiments, the laser beam is focused within the substrate in the ways described herein. These laser-modified tracks, when arranged in an array on a substrate sheet, such as a polymer sheet, provide particularly advantageous results in terms of optical effects, and in certain configurations allow for more complex and distinct optical characteristics, as explained herein. The elongation of focused laser light to form a filament-like focused laser beam can be achieved in many different ways known to the art, and such methods are not limited in this respect. Various configurations can employ any technique for beam shaping or laser refocusing, and any combination of such techniques, as required.For example, two classes of beam shaping methods can be defined around (1) external optical tools or effects that impose a shaping effect before the light reaches a focal point in the material, and / or (2) material response effects such as nonlinear optics that further distort the beam path as it propagates through the material. A combination of such effects can also be employed in selected modes so that the laser energy can be dissipated in a myriad of three-dimensional (3D) shapes that deviate from the typical Gaussian focal volume. The selected modalities also allow the advantage of a single pulsed laser exposure to modify an internal volume of substrate material where a traditional small laser focal spot is stretched along one or more preferred axes. Within this shape, the light intensity distribution is controlled to an absorption value above the threshold so that a single laser pulse can induce a preferred elongated material modification geometry, such as a filament shape, to generate laser-modified tracks in the substrate as described herein. For example, in the case of transparent substrates such as transparent polymer substrates, multi-photon absorption, avalanche ionization, above-threshold absorption, and other nonlinear optical processes can only begin to absorb the laser beam at a sufficiently high intensity. IVIA / t / ZUZZ / UZ IZZO limited near the focal volume. The resulting material modification can be easily manipulated to induce reproducibly refractive index structures,5 volume nano-lattices,6 xnano-ways,9 elongated filaments,10 and nano-channels11. Starting with external optical tools, surface and lens aberration is one example of beam distortion that can typically stretch the focal volume longitudinally along the beam's propagation direction. Aberration effects are especially pronounced with strong focusing lenses that offer a high numerical aperture to form highly convergent beams. Aberration arises when a focusing laser beam enters the first flat surface of the target material, an effect called surface aberration. This effect leads to a stretching of the focus toward the source, which increases with increasing depth of focus in the material and increasing the numerical aperture of the focusing beam. This effect can be isolated from the target sample for external control and amplification by, for example, placing a flat optic in the beam path between the lens and the target material.In this way, the plate thickness and the refractive index of the material are used to control the length of the track formation on the target material. For example, plates such as silica plates or those made of other materials can be from 0.1 mm to 10 mm thick, preferably 0.5 mm, 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, or 5 mm thick. In any spherical lens or lens combination, aberration is often set to a minimum. Therefore, aberration can be easily manipulated and amplified by misaligning optical materials, inverting optical surfaces, and other means that an optical designer typically avoids when trying to make a traditionally perfect focusing lens system. Also indicated are various optical forms containing non-spherical refractive surfaces for selected modalities, which inhibit the focusing of all light rays to a common focus. This includes the axicon, which creates a first-order Bessel-like beam by stretching the beam into a long, filament-like shape. Malfitting of compensating collar lenses, used in deep focusing,21 can be employed in selected modalities. In other modalities, a spatial light modulator (SLMS)20,22,24 provides high-resolution, two-dimensional shaping control of the beam's phase front, which, after passing through a focusing lens, allows for extensive beam shaping. With proper alignment, the lens performs a Fourier transform of the electric field distribution from the SLM surface to near the lens's focal plane, which is positioned within the target sample. Similar effects can be achieved using a liquid crystal panel that attenuates the electric field to produce a high-resolution, two-dimensional light intensity pattern. Other optional active beam-shaping tools include, but are not limited to, MEM mirrors, acousto-optical lenses, diffractive optics, holograms, phase-beam steering antennas, and flexible membrane mirrors. IVI A / t / ZUZZ / UZ IZZO In other modalities, material response effects can significantly redirect the laser beam path once it propagates within the target sample. The cumulative absorption of laser light during exposure can result in thermal gradients and physicochemical modifications that may lead to self-lensing or defocusing effects during laser interaction. In the domain of ultrashort pulse lasers, a high-intensity buildup in the focal volume can induce a wide range of nonlinear optical effects, including intensity clamping, which shifts the interaction zone backward relative to the laser propagation direction; the Kerr effect, which pulls the focal interaction volume forward; and plasma defocusing, which deflects the beam outward.Under the correct balance, the Kerr effect can induce a stable self-focusing effect that causes or forms long, high-intensity filaments that form in the material. In further examples, spatiotemporal beam shaping can be employed to influence the shape of the laser interaction volume, involving control of both pulse timing and the dimensions of the space within the focal volume. At a simple level, astigmatic beams are one option for forming filaments with non-circular cross-sections. To this end, cylindrical lenses, slit apertures, or SLMS are non-exhaustive examples of external beam shaping techniques for forming filaments with elliptical or planar shapes. Beam shaping allows for an accelerated beam to enable the option of 3D shaping of curved filament shapes. The spatiotemporal beam properties can be further manipulated to decrease autofocus effects or induce pulse-front tilt effects that create asymmetry in pulse arrival time and the resulting filament shape.In some modalities, therefore, a combination of external and internal beam shape manipulation is anticipated to exert favorable control over the morphology and structural shape of the laser modification volume. Other selected modalities may employ parallel processing, where a single laser pulse is divided and directed to multiple locations on the sample to form multiple modification points, thus accelerating manufacturing speeds. As an example, SLMs offer active control in the creation of multifocal positions that can be manipulated to new locations in real time.29,3° Other forms of parallel processing anticipate splitting the beam with diffractive optics, lens arrays, and other devices (such as those described) and projecting an array of similar beams to multiple processing points in a single exposure. For example, laser scanning can be combined in certain modalities with sample movement to allow high-speed beam scanning through a slower-moving sample. Such scanning is typically provided by galvanometer mirrors, rotating polygons, and acousto-optical scanners. Examples of suitable laser sources for use in the methods described herein ML / IZ / ZUZZ / UZZZO include, among others, high-power fiber lasers, such as any of the following commercially available models (laser wavelength is 1030 nm): Amplitude Systems Satsuma HPS: pulse duration 400 fs; power 50 W; pulse repetition rate 1 to 2 Hz; pulse energy 40 pJ; Photonic IPG YLPF-10-500: pulse duration 500 fs; power 10 W; pulse repetition rate 1 MHz; pulse energy 10 pJ; Clark MXR Impulse: pulse duration 250 fs; power 20 W; pulse repetition rate 2 MHz; pulse energy 10 pJ. Another option would be to use a cavity-based Yb laser amplifier / oscillator. Other commercially available models include, among others, the Spectrophysics Spirit and the Light Conversion Pharos. In selected modes, each of the laser-modified tracks is generated by a femtolaser, such as a femtolaser with a pulse duration in the 0 range.1 fs at 100 hp for each laser pulse, with beam shaping of the femtolaser beam before or after its interaction with the substrate. In some embodiments, for each laser-modified track on the substrate material, the elongated volume of modified substrate material is at least 5 times, or at least 10 times, preferably at least 15 times, and more preferably at least 20 times, longer than its width. Optionally, for at least some of the laser-modified tracks, the elongated volume of modified substrate material is located within the substrate sheet. In other embodiments, for at least some of the laser-modified tracks, the elongated volume of modified substrate material is exposed on at least one surface of the substrate sheet.In other modalities, for at least some of the laser-modified tracks, the elongated volume of modified material in the substrate sheet includes a void in the substrate sheet, formed after the production of the substrate sheet, by melting, displacement, or decomposition of a portion of the substrate sheet material. The substrate sheet can be of any shape, but in some embodiments it may have an average thickness of 10–3000 pm, preferably 50–150 pm. In selected embodiments, the substrate sheet is a polymer sheet, and each of at least one ordered two-dimensional array of discrete laser-modified tracks comprises laser-modified tracks in the polymer, generated after the production of the polymer sheet. For example, the substrate sheet may comprise BOPP, BOPET, PEN, PP, PVDF, or related copolymers such as PVDF-FrE. Furthermore, the independently laser-modified tracks can, in some modalities, extend within the polymer sheet from 5% to 100% of the distance between opposite surfaces of the polymer sheet, and optionally extend into one or more additional layers if present adjacent to the polymer sheet. Certain methods are particularly suitable for polymer films and document substrates, where the substrate regions comprise laser-modified tracks in the polymer, a modified post-production polymer film. The production of the security features is ML / t / ZUZZ / UZ IZZO can be adapted to achieve different degrees of substrate modification, whether in terms of area or volume of the laser-modified tracks, the separation of the laser-modified tracks within the two-dimensional laser-modified track array, or in terms of the depth, width, length, or shape of the substrate that is modified by the laser to have a different refractive index with respect to that of the original substrate material. For example, for at least some of the laser-modified tracks, the modified shape of the substrate material may be embedded within the substrate sheet. In other examples, for at least some of the laser-modified tracks, the modified shape of the substrate material may be exposed on at least one surface of the substrate sheet. A single two-dimensional array may, in certain configurations, also include laser-modified tracks that differ from each other in terms of their size, area, spacing, distance, depth, shape, length, or extent of the modified substrate, and whether the laser-modified tracks extend only within the material or fully onto one or both surfaces of the substrate material.In some embodiments, the safety devices may comprise laser-modified tracks in the substrate material (e.g., polymer) wherein the modified shape of the substrate material in at least some of the laser-modified tracks on the substrate sheet contains a void within the substrate sheet, created after the production of the substrate sheet by modifying, altering, melting, deforming, displacing, or decomposing a portion of the substrate material. Optionally, such voids may extend entirely within a width of the substrate sheet, or they may extend to one or both surfaces of the substrate sheet. The voids may comprise air, gas, amorphous polymer, or other materials, and their presence may, in certain embodiments, contribute to the optical properties of a two-dimensional array. The examples described below illustrate specific experiments in which substrate sheets comprising polymer are exposed to pulses of laser radiation from lasers such as femto-lasers to generate modified substrate sheets or substrate sheets comprising 'laser-modified tracks' as described herein. Scanning electron microscope micrographs of cross-sections through the treated substrate material indicate that the polymer is generally arranged in bundles of polymer chains that, prior to exposure to laser radiation, are relatively disorganized or randomly arranged within the substrate.However, exposure to laser radiation with a focus that extends in a linear direction causes the polymer chain beams to organize themselves in a more linear manner within an elongated volume of polymer material around the laser beam, so that the polymer chains generally adopt a more extended configuration usually in line with the orientation of the laser radiation or received laser beam (see examples for details). ML / t / ZUZZ / UZ IZZO Therefore, in selected embodiments of the substrate sheet, the polymer comprises polymer chains, wherein the modified shape of the polymer for at least some laser-modified tracks of each two-dimensional array comprises polymer chains that are at least partially ordered or aligned with respect to those of the polymer sheet outside of said laser-modified tracks. Optionally, the nanostructure of the resulting polymer sheet may include polymer chains offset to create periodic gaps in the two-dimensional array, such that the laser-modified tracks of the array (comprising modified polymer) comprise polymer chains and optionally gaps that generally extend non-parallel to the surfaces of the polymer sheet.In this way, each of the laser-modified tracks on the polymer sheet can be made to have a different refractive index with respect to the general refractive index n for the polymer sheet. In this sense, the modified polymer chains that have been influenced by the exposure of the polymer sheet to laser radiation may be caused at least partially to extend generally not parallel to the opposite surfaces of the polymer sheet, and may optionally form due to melting, displacement, or decomposition of the polymer within the laser-modified tracks of the polymer sheet as a result of the polymer sheet's exposure to laser radiation pulses. Alternatively, the modified polymer chains, which have been structurally altered by exposing the polymer sheet to laser radiation, may be caused, at least partially, to extend generally perpendicular to opposite surfaces of the polymer sheet. Optionally, they may form or align due to melting, displacement, or decomposition of the polymer within regions of the polymer sheet as a result of the sheet's exposure to laser radiation pulses. Typically, the inventors have observed that the polymer chains exposed to laser radiation appear to align with a general direction of the laser beam path within the polymer, or to align with other anomalies within the polymer itself formed in general alignment with the laser beam path within the polymer. In any of the embodiments described herein, at least some of the polymer chains mentioned above within the laser-modified tracks of the substrate sheet may optionally extend within the substrate sheet to one or both surfaces of the polymer sheet. Alternatively or additionally, at least some of the polymer chains within the laser-modified tracks of the substrate sheet may extend in an elongated manner within the polymer of the polymer sheet, but may not necessarily extend to the surfaces of the polymer sheet. In even additional modalities, the laser-modified tracks on the substrate sheet that are MA / t / ZUZZ / UZ IZZO modified to have different refractive indices with respect to the unmodified substrate sheet may comprise amorphous regions and / or voids that optionally extend to one or both surfaces of the substrate sheet. For example, in some embodiments, the various laser-modified tracks may comprise voids that in turn comprise amorphous polymer, air, vacuum, polymer decomposition or oxidation products, or glassy or amorphous carbon compounds. For example, such voids have been observed extending within the polymer substrate in general alignment with a path of the laser beam used to generate the tracks on the substrate. Some corresponding features of the polymer sheet in this respect are described in the examples with reference to scanning electron microscope (SEM) images. Each laser-modified track comprising an elongated volume of modified substrate material can therefore have one of several different types of substructure depending on the power, pulse length, shape, focal spot, beam conformation, or modification of the laser pulse or the type of laser used to incident on the substrate sheet. Thus, the type and nature of the laser radiation can be adjusted to achieve particular nanostructural features within the polymer substrate material. For example, the degree of rearrangement of the laser-modified polymer track, the size and length of the polymer anomalies or voids formed within the polymer, and the depth or extent of such features within the polymer can be adjusted according to the type of substrate material, the type of laser used to create the modified regions, and the modification of the laser beam.Furthermore, a single two-dimensional array may comprise laser-modified tracks on the substrate that have been modified in different ways by different shapes or degrees of laser energy so that the observed optical effects are inconsistent or change in different parts of a two-dimensional array. The substrate material or sheet can comprise any material suitable for modification by a laser to change its refractive index or diffraction properties in any way. For example, in terms of polymers, examples may include, but are not limited to, BOPP, PP, PET, PVDF, opaque or non-opaque fluoropolymers, non-photographic polymers, opaque or non-opaque polymer films, opaque or non-opaque inorganic and organic coatings, etc. The substrate sheets, in any of the forms described herein, may have any thickness. However, those with a general thickness suitable for the preparation and use of security documents are preferred. For example, certain forms may have an average thickness of 5–3000 µm, 10–300 µm, or 50–150 µm. Furthermore, a two-dimensional array of laser-modified tracks on the substrate may take any configuration or have any dimensions. Typically, though not necessarily, the ordered array may include modified substrate tracks arranged in rows and columns approximately equidistant from each other across the plane of the sheet. ML / t / ZUZZ / UZ IZZO substrate. Alternatively, the rows and columns may be staggered, or the laser-modified tracks may be formed in any other configuration suitable for producing a diffractive image or shape, or otherwise an image or shape observable in reflection and / or transmission, formed by the collective observation of the regions comprising the two-dimensional array. The periodicity of the laser-modified tracks comprising the two-dimensional array may also be varied or selected according to the desired optical effect. Each laser-modified track in a two-dimensional array within the substrate sheet can comprise an elongated volume of laser-modified substrate material of any dimension. However, in some configurations, the volume of laser-modified material can vary from 1 to 5,000 nm in width on average (when the substrate sheet is viewed from its widest side), extending partially or completely through the substrate sheet. Furthermore, the width of each laser-modified track can be consistent across the substrate sheet or vary according to its depth within the sheet. Again, different optical effects can be achieved by adjusting the shape of the laser-modified tracks in terms of their plan view, periodicity, orientation, or their three-dimensional shape within the substrate sheet. In any ordered two-dimensional array of laser-modified tracks, the tracks can be separated from each other by any distance, on the basis that they nevertheless achieve a collective diffraction effect when viewed together. For example, the laser-modified tracks within an array can be separated from each other by a periodicity of 0.1–5 μm on average by the unmodified substrate sheet material that retains its original post-fabrication general refractive index (e.g., sometimes referred to herein as a general refractive index n). As described herein, each laser-modified track of the substrate sheet comprises an elongated volume of modified substrate material having a modified refractive index compared to the general refractive index for the substrate sheet. Any method or system may be used to give the substrate a modified refractive index in the defined regions. However, as described, one way to induce the modification is by exposing the substrate sheet to a laser, for example, by directing a laser onto a surface of the substrate sheet or through the substrate sheet. A single laser may be used to generate all the modified refractive index regions within the substrate sheet, so that the sheet can be moved relative to the laser to achieve each of the regions in a two-dimensional array.Alternatively, a single laser beam can be split, for example, by a suitable filter, to apply laser radiation to multiple locations on a substrate sheet simultaneously. Alternatively, multiple lasers can be employed simultaneously or sequentially to modify the substrate sheet. Short pulses of laser radiation may be suitable for achieving the desired modification. MA / CZZUZZ / UZ IZZO selected substrate sheets, as can be provided, for example, by a femto-laser as described herein. In any of the described substrate sheets, the 'laser modified tracks' (e.g., polymer laser modified tracks) can be independently spaced within the polymer sheet from 1% to 100%, preferably from 5% to 90% of the distance between opposite surfaces of the polymer sheet. Furthermore, in any of the described substrate sheets, any voids present within the substrate sheet can be independently spread within the substrate sheet from 5% to 100% of the distance between opposite surfaces of the substrate sheet. In other additional forms, the use of any of the substrate sheets described herein, or any substrate sheet modified by any of the methods described herein, is provided as a security feature of a security document. In still other additional forms, a security document is provided comprising, as a security feature, any of the substrate sheets as described herein, or any substrate sheet modified by any of the methods described herein. In still other additional embodiments, a method is provided for the manufacture of any of the substrate sheets described herein, or any of the safety documents described herein, comprising the step of: Exposing a substrate sheet surface to a laser, such as a femtolaser, in a plurality of regions corresponding to a two-dimensional array of such regions, so that at least a portion of the material within each of these regions is partially and temporarily melted, displaced, decomposed, or otherwise modified, thereby altering a refractive index of the material within each of these regions. In selected embodiments, the laser beam is shaped or otherwise modified by any method or means described herein or otherwise known in the art, in order to modify the focus of the laser beam. For example, such modification of the laser beam may, in selected embodiments, employ beam shaping so that the laser light incident on the substrate is focused in a distributed manner along a longitudinal laser beam axis extending through the substrate sheet, thereby generating laser-modified tracks on the substrate. In this way, preferably each laser-modified track can be generated by a single laser pulse without the need to change the laser focus within the substrate or the position of the substrate relative to the laser. Consequently, such techniques substantially avoid any need to move the laser focus and / or the substrate relative to each other to achieve each elongated laser-modified track.Laser-modified tracks. The laser markings generated in this manner by the methods of the present invention may comprise an elongated volume of modified substrate material, with a customized shape, orientation, or configuration, generated by a single laser pulse by virtue of the linearly or shape-distributed focal energy of the single laser pulse within the substrate. This in itself presents significant advantages over certain prior art laser marking techniques, whose methods generally required the substrate to be intentionally moved about its z-axis (closer to or farther from the laser source) during laser exposure, with respect to the laser focus, in order to generate laser markings of any degree of depth. These methods, as described herein, which employ beam shaping so that the laser light incident on the substrate is focused in a distributed manner along a longitudinal axis of the laser beam extending through the substrate sheet, also reduce the risk of manufacturing tolerances. Prior art techniques, which commonly involve focused Gaussian laser beams and laser engraving or marking, can be prone to manufacturing tolerances or unintentional fluctuation or movement in the substrate sheet during laser exposure, especially during a continuous manufacturing process. This, in turn, can lead to inaccurate laser marking or incorrect depth assignment to the laser marking.The methods selected as described herein provide a more consistent and reliable means of generating laser marks with greater depth and more complex configurations, including highly elongated laser marks such as the laser-modified tracks described herein. By distributing laser energy into a desired, preselected configuration and by customizing the laser light before, during, and after interaction with the substrate, longer and more complex laser-modified tracks and track configurations can be achieved on the substrate. This, in turn, allows for greater flexibility and tunability of the optical output characteristics of the devices. As described herein, the security devices described can be formed in some ways by exposing a sheet of substrate material, such as a polymer, to pulses of laser radiation. For example, short-pulse (also known as ultrafast) lasers with a pulse duration in the range of 0.1 fs to 100 hp can, depending on the laser, be particularly useful for generating high-intensity radiation suitable for forming laser-modified regions or tracks within the polymer in a laser-modified track shape with a high aspect ratio within a polymer sheet.These laser radiation pulses are usually focused by means of a lens, but a focused beam shape of the laser can be distorted and / or stretched by (1) nonlinear interaction (Kerr effect or self-focusing) and further elongation by substrate surface aberration when the focusing beam is optionally passed through one first. A sufficiently thick optical plate (e.g., 0.1–10 mm, preferably around 1 mm) with potential for additional surface aberration on the substrate surface. In any case, the depth and / or length of the resulting laser-modified track formed within the polymer substrate can be controlled by surface aberration (plate thickness and numerical aperture of the focused beam) and pulse energy, among other factors. In this way, the depth, length, and potential shape of each laser-modified track can be adjusted to create different optical effects. Additional optical effects, as described, can be achieved by adjusting the periodicity of the laser-modified tracks and / or by adjusting the pattern and / or periodicity of the resulting laser-modified tracks and / or the grid geometry and rotation of the array. In some configurations, the laser-modified tracks thus formed on the substrate can effectively create elongated focal lines or volumes within the substrate resulting from only a portion of the laser beam being focused at any given longitudinal position. This results in aberrant focusing, concentrating light into a small spot diameter but extending it over lengths longer than the typical depth of focus or confocal beam parameter. Efficient propagation through transparent material can be achieved by tightly focusing the laser energy with a lens, and the elongated focal length is produced at a controlled position within the substrate, with energy dissipated only when it exceeds a critical energy level, driven by nonlinear absorption in the high-intensity portion of the laser beam. The selected modes can also employ a laser beam with laser light distribution at more than one depth within the substrate sheet, so that the laser-modified tracks are generated by the laser at two or more depths within the substrate. Such modes can be useful when generating multiple sets of laser-modified tracks that overlap each other at different depths in the substrate, when the sets on the substrate are viewed as a plane. Multiple 'overlapping' sets of laser-modified tracks can also lead to interference effects between the sets, as described herein. Different optical effects achieved by varying one or more of the periodicity, grid geometry, matrix rotation angle, periodicity, length, depth, rotation, and angle of the laser-modified tracks on the substrate sheet The optical properties of each two-dimensional array of laser-modified tracks, or sections or parts of a two-dimensional array, can be modeled, selected, or tuned to meet specific application requirements. For example, different parts of the substrate can be modified differently in different sections of the device to generate the modified regions. Selective laser focusing and laser beam shaping, including beam modification before interaction with the substrate material, can generate regions of modified refractive index that MA / t / ZUZZ / UZ IZZO have the following example variations, all of which are comprised of selected modalities as described herein: • Different depths for laser-modified tracks extending into the substrate • Different widths for laser-modified tracks • Different lengths for laser-modified tracks extending into the substrate and optionally to one or both substrate surfaces • Different angles for laser-modified tracks extending into the substrate with respect to the perpendicular to the substrate surfaces • Different periodicity of the laser-modified tracks within the two-dimensional array • Different lattice geometry of the laser-modified tracks within the two-dimensional array, for example, rhombic lattice, square lattice, hexagonal lattice, rectangular lattice, and parallelogram lattice • Rotation angle of the array with respect to adjacent arrays, or different rotation angles of regions within an array. • Regular (e.g., elongated or cylindrical) or irregular shape of the laser-modified tracks of the substrate material within the substrate These additional variations to the laser-modified tracks on the substrate sheet and / or between the modified regions of the substrate sheet allow for greater adaptability and finer tuning of the substrate sheet's optical properties, either for the entirety of each two-dimensional array or for specific subsets or sections of each two-dimensional array. For example, the selected modalities allow for the generation of more complex images through two-dimensional arrays of regions, as described herein, where the array regions are produced with different periodicities, spacing, depth, shape, grid geometry, array rotation angle, length, angle, or other modifications that were previously difficult or impossible to achieve.Therefore, a single two-dimensional array can comprise a consistent set of laser-modified tracks, or the regions within the two-dimensional array can vary in one or more of the variable factors listed above, both across and within the two-dimensional array. In this way, the optical properties and the optical diffraction output of a two-dimensional array can vary across and within the same array. Thus, the methods employed by the inventors to generate laser-modified tracks, as described herein, can be adapted and adjusted in terms of laser focus, beam shaping, and substrate interaction to achieve a wide variety of optical effects. Alternatively, a substrate sheet may comprise multiple two-dimensional arrays, each with selected and tuned optical properties that differ from one another. Such multiple arrays Two-dimensional ML / IZ / ZυZZ / υZΊZZZO may or may not be separated from each other on a sheet of substrate, and each may have individually tailored regions with, for example, selected periodicities for the regions within each matrix. For example, the selected modalities include substrate sheets comprising laser-modified tracks of two-dimensional ordered arrays that are separated from each other by a separation of 1 to 1000pm, preferably 1-10pm, more preferably 1-50pm, on average by the substrate sheet material having an overall refractive index n.However, the substrate sheet may alternatively comprise an ordered two-dimensional array in which different sections of the array each comprise some of said laser-modified tracks, but wherein the laser-modified tracks within one section have different relative periodicities compared to the laser-modified tracks of at least one other section of the substrate sheet, so that the optical diffraction outputs of the different sections (resulting from the diffraction of the incident light) differ from each other, even when the same or equivalent incident light simultaneously strikes the different sections of the two-dimensional array. For example, in some specific embodiments, a substrate sheet comprising a two-dimensional array of laser-modified tracks may comprise pixel-like areas of the substrate, each preferably 1–10,000 pm wide, more preferably 5–100 pm wide, wherein at least some of the pixel-like areas containing laser-modified tracks have different optical diffractive output properties compared to other pixel-like areas in the two-dimensional array. Each pixel-like area may be designed and manufactured to have customized optical properties by virtue of the nature and construction of the laser-modified tracks of the two-dimensional array of the modified substrate it contains, their periodicity, and other aspects of the regions. For example, certain modalities provide a substrate sheet as described herein, wherein each pixel-like area of ​​the substrate sheet comprises laser-modified tracks having the same or substantially the same periodicity within each pixel-like area, such that each pixel-like area provides a generally uniform optical diffractive output after exposure to incident light [e.g., see Figure 8(a)]. In other embodiments, each pixel-like area comprises multiple subpixels, each of which comprises an ordered array of such laser-modified tracks having a substantially consistent periodicity within each subpixel. However, the periodicity of the laser-modified tracks within the subpixels of any given pixel-like area may differ from one another, so that the optical diffraction output of any given pixel-like area is defined by a combination of the optical diffraction outputs for all the subpixels of that particular pixel area. In this way, the optical properties of each pixel-like area, or a group of pixel-like areas, are defined. Adjacent MA / t / ZUZZ / UZ IZZO pixels can be customized during manufacturing, and different pixel-like areas can be assigned different optical diffraction output properties by virtue of the custom laser-modified track periodicity, spacing, lattice geometry, depth, length, angle, etc., of the modified substrate material within each pixel-like area of ​​a two-dimensional array and its subpixels. This, in turn, allows the creation of additive and subtractive colors that enable the generation of images that provide a multi-color output at specific output angles when the device is exposed to incident light. For example, each pixel-like area can comprise subpixels that each provide a red, green, and blue optical diffraction output at selected angles after exposure to incident light (rather similar to the pixels of a television screen, but static in terms of their output).In this way, the relative intensities of the red, green, and blue outputs from the subsections of each pixel-like area can determine the color of the combined optical diffractive output for the pixel-like area, or a plurality of such pixel-like areas, at a given angle [e.g., see Figures 8c and 8d]. The pixel-like areas can be designed to include a stronger red output, for example, relative to blue and green, or they can allow the combination of the red, green, or blue outputs to create colors such as magenta or white that might otherwise be more difficult to generate using uniform two-dimensional arrays with uniform periodicity. In other embodiments, the substrate sheet may comprise a two-dimensional array of regions where the periodicity of the regions changes across the sheet from one section to another, thus altering the optical diffractive output properties of the device after exposure to incident light. These periodicity changes can be gradual from one region to another, or more pronounced, such as changes in periodicity from one region of the two-dimensional array to another. Again, these periodicity changes for the modified substrate regions can lead to the tailoring of diffractive optical outputs to generate complex color images adapted to a specific design, pattern, or pictorial representation. As described, the selected modalities may comprise two-dimensional arrays of laser-modified tracks on the substrate, modified to have an altered refractive index with respect to the unmodified substrate sheet, wherein at least some of the laser-modified tracks are generated by exposing the substrate sheet to laser radiation, and wherein the laser-modified tracks extend within the substrate sheet or at least partially through it. Such laser-modified tracks may extend within the substrate sheet or at least partially through it substantially perpendicular to the surface of the substrate sheet.Alternatively, laser-modified tracks can extend into or at least partially through the substrate sheet at one or more non-perpendicular angles to the substrate sheet surfaces, e.g., in circumstances where the laser beam is directed onto the surface. ΜΛ / ΙΖ / ΖυΖΖ / υΖΊΖΖΟ of the substrate sheet at an angle to the perpendicular. In additional embodiments, the substrate sheet may comprise a two-dimensional array of laser-modified tracks, wherein the modified shape of the material extends to different depths within the substrate sheet for different regions within the ordered array of said laser-modified tracks. The combined optical properties of the laser-modified tracks within any particular two-dimensional array, or part thereof, in selected embodiments, may depend on the depth of the modified substrate shape with respect to one or both substrate surfaces, or with respect to the total substrate thickness.Therefore, depth selection, for example, from the default depth focus of a laser to generate such laser-modified tracks on the substrate, can be used to adjust the diffractive optical properties of any modified region of the substrate, or any two-dimensional array of such laser-modified tracks. In still other additional configurations, laser-modified tracks on the substrate (in terms of refractive index) are elongated regions of modified substrate that extend into and generally through the substrate sheet, optionally to one or both surfaces of the substrate sheet, either perpendicular or non-perpendicular to the substrate sheet surfaces. The optical properties of the substrate sheet, and the optical diffraction properties of the laser-modified tracks, can be varied by selecting the length of the laser-modified tracks that extend into the substrate sheet. For example, a single two-dimensional array of laser-modified tracks may include differences in some areas of the array compared to other areas with respect to the length of the elongated volume of modified material of the laser-modified tracks in the substrate material.For example, the laser focusing and modification before or after the laser is directed at the substrate sheet can determine the nature, shape, and length of the modification. The laser-modified tracks thus produced can be shaped and tailored in terms of their optical diffraction properties according to the desired diffraction output and color output properties for a particular substrate sheet. In further embodiments, a substrate sheet may comprise two or more ordered two-dimensional arrays of discrete laser-modified tracks of the material, each having the same or different optical diffraction pattern, shape, or output. In such embodiments, two or more of the ordered two-dimensional arrays of discrete laser-modified tracks of the material may or may not at least partially overlap. In embodiments where the two-dimensional arrays overlap, the observable or detectable optical diffraction output for such overlapping portions may comprise a combination of outputs from each of the ordered overlapping two-dimensional arrays. In other embodiments where two or more ordered two-dimensional arrays of discrete regions of the material are present and at least partially overlap, the optical diffraction output of a two-dimensional array The ordered ML / IZ / ZυZZZ / υZΊZϖZO can be diffracted and further modified by another two-dimensional array before the device exit, when the substrate sheet is exposed to incident light. The selected modalities employ the Littrow and Talbots effect In selected examples, more than one array of laser-modified tracks may exist on the same substrate at different depths of overlap within the substrate, such that the combined observable optical output of both arrays is combined in an additive and / or subtractive manner. For example, in additional modalities, some of which employ multiple arrays of laser-modified tracks, the interaction between the outputs of the overlapping arrays may be subject to Littrow or Talbot effects that can be fine-tuned depending on the properties and dimensions of the two-dimensional arrays and the laser-modified tracks they contain, examples of which are provided herein. Interference effects provided by the Littrow configuration In a short-separation-volume, long-length (Lt) phase grating, a single low-refractive-index-contrast grating layer (see, for example, Figure 9) can provide only weak diffraction efficiency at all non-zero orders when illuminated at normal incidence (0en = 0°). Here, light diffracted from a filament length as it spreads over half a wavelength does not radiate in constructive interference. In the specialized case of the Littrow configuration

[51] , a phase-matching effect causes all diffracted light to interfere constructively, allowing the full depth of the grating volume to contribute to potentially high diffraction efficiency. For a periodic diffraction structure with refractive index contrast Δη and thickness d, the cumulative phase-shift modulation of , 2π(Δη)ά ... = ^^(D will be imposed on the phase front for the light emerging from the grating. The more scalable electromagnetic theory requires that the probing wavelength, λ, be much smaller than the period at the grating, i.e., α » λ. With weak refractive index contrast, a maximum diffraction efficiency of 4.4% is achieved for light of wavelength 532 nm when the grating thickness reaches only d = 2.65 mm for first-order diffraction, and the efficiency ranges from 0 to 4.4% with increasing thickness

[52] . Under Littrow illumination, these phase-matching limits are eliminated for a single diffraction order at a specific angle of incidence given by 2Asin(e¿) = mÁ (2) where a is the grating period, λ is the Littrow angle which is the incident angle of entry, m is the diffraction magnitude and λ is the probe wavelength. A diffraction efficiency of 97% was reported for a small grating period a = 0.6 pm at a wavelength of λ = 633 nm. ML / E / ZuZZ / uZZZ ZZO a silica air transmission grid

[51] , Talbot Effects When a periodic object is illuminated with spatially coherent light, a series of self-images of the illuminated object can be observed due to Fresnel diffraction. This observation was first discovered by H.F. Talbot

[53] , and the self-image length is called the Talbot distance (c). Chanda and Herman

[54] noted that the interference pattern generated by a period-phase mask leads to the coherent combination of diffraction orders in repeating planes with the Talbot distance, c. Conversely, when diffraction grating elements are layered at the Talbot spacing, the diffraction from each layer will combine coherently, greatly improving the otherwise weak diffraction efficiency when the grating is illuminated with normal incidence

[52] .Therefore, a second factor in the grating diffraction efficiency arises from the Talbot distance, which for the case of normal incidence sounding (6en= 0o), is given by. MA / t / ZUZZ / UZ IZZO Here, a is the period at the grating, λη= λ / ηΓ is the wavelength of light inside a material with a refractive index of nr. In the case of sounding in a multi-layered grating with angles of incidence other than normal incidence, further insights can be gained from understanding that repeated grating layers with a period of at a distance of Talbot length, c, constitute a grating that needs to satisfy the grating equation at each of the lateral (Ax) and vertical (c) periodicities. For a grating within the medium with refractive index, n, the traditional grating equation provides the relationship between the directions of the incoming and outgoing beam according to the order m and the lateral period, Ax, according to (see new Figure 1a): sin0IN= sin60UTm+ (4) The influence of an orthogonal grating along z, with period c, satisfies a similar grating equation for the same entry and exit angles, but with an independent interference order, m'. λ cos0IN= coseOUT}m, + m' ^(5). For strong diffraction efficiency for low-contrast gratings, Equations 4 and 5 must be satisfied simultaneously for the same angles and wavelength. In this case, a Talbot length, c, can be specified for a given wavelength and diffraction angle to provide high diffraction efficiency in either the reflection or transmission modes. Correspondingly, the first-order diffraction beams from each grating layer are destructively interfering with the entire half-layer at separations of 3c / 2, 5c / 2, 7c / 2, and so on, which can be forced at other wavelengths for the same viewing angle optimized for a first wavelength in Equations 4 and 5. In this way, a powerful Talbot coherent alignment effect improves diffraction efficiency without the restriction of the Littrow angles presented earlier. Furthermore, the antiresonance factors in Equation 4 and 5...5 can sharpen the wavelength responses that are enhanced. At a fixed Talbot length, this corresponds to generating high diffraction efficiency at a narrowing, restrictive viewing angle for a target wavelength or selected wavelengths, thus offering spectral filtering options for better color display control. Therefore, the selected modalities encompass each and every possible Littrow and / or Talbot effect generated by the interaction of outputs from a single or multiple arrays of laser-modified tracks on a substrate. The relative separations between the laser-modified tracks of adjacent assemblies on the substrate—in terms of the relative lengths of the tracks extending into the substrate, the placement of tracks within one assembly relative to another, and the separations of the assemblies from each other across a thickness of the substrate—lead to many optically relevant options in terms of the devices' optical output. In additional embodiments, a substrate sheet may include one or more two-dimensional arrays of laser-modified tracks as described herein, wherein the assemblies or parts thereof include regions that are rotated relative to each other about the general plane of the substrate. For example, some assemblies may be fabricated or structured to cause diffraction of incident light with asymmetric diffractive emission at a specific angle and in a specific direction away from the substrate surface, such that the angle-dependent emission is visible to a user not only at certain angles, but also only at certain positions around the substrate plane.To this end, the modified track sets on the substrate can be rotated relative to each other after fabrication so that angle- and position-dependent emission can be viewed for different regions depending on a user's line of sight, such that rotating the substrate around its general plane while maintaining a constant angle of incidence for the user's line of sight with respect to the plane of the substrate results in the observation of changing diffractive emission for different regions of the substrate and the device is rotated around its general plane, e.g., by the user [e.g., see Figure 8b]. In some embodiments, any substrate sheet as described herein may further comprise one or more additional layers that partially or fully overlap at least a portion of at least one two-dimensional array of laser-modified tracks on the substrate. For example, such additional layers may comprise one or more reflective layers, refractive layers, diffraction filters, transmissive filters, optical interference layers or stacks, ink, a protective layer, a coating, or an adhesion promoter. In any of these embodiments, the additional layer or layers may modify the incident light. MA / t / ZUZZ / UZ IZZO that impinges on the substrate sheet and / or the optical diffraction output of the substrate sheet, prior to observation or detection by a user. The following examples describe selected embodiments related to the substrate sheets described herein, their production, and use. These are provided for illustrative purposes only and are not intended to limit the object and scope of the invention or the appended claims. EXAMPLES: The following examples employ a specific substrate type (biaxially oriented polypropylene or BOPP) and a specific laser system in the form of a Yb-doped fiber femtosecond laser system (Amplitude Systemes, Satsuma) providing Gauss-shaped pulses (beam quality: M2 = 1.14) with a wavelength of 515 nm (wavelength of frequency doubled or halved) and a pulse duration of -215 fs. Such substrates and laser systems are merely illustrative, and the claimed embodiments are not limited in this respect. Other substrates and systems may be employed to generate the devices as described, as required. Preliminary investigations included shaping the laser beam to generate a single laser-modified or aberration-modified track on or within the substrate. A 0.55 NA aspherical air focusing lens (NewFocus, 5722-AH) was used to focus the laser onto the substrate. A narrow, nearly uniform laser-modified track beam with a high dimensional ratio was generated through a combination of longitudinal and nonlinear Kerr aberration effects when the laser was focused through a 2 mm thick fused silica glass plate before striking the substrate. By controlling the pulse energy in the range of 93.5 nJ to -8.0 pJ and the thickness of the fused silica glass plate from 250 pm to 2 mm, laser-modified tracks of varying lengths and widths were produced in biaxially oriented polypropylene (BOPP).Single or multiple pulses can be applied to the same interaction zone to accumulate a larger overall modification change. Multiple pulses can be applied while the sample is moved relative to the laser focus (or vice versa). Example 1: Two-dimensional array of laser-modified tracks in BOPP Figure 1 shows an example of a 2D array of laser-modified elongated tracks. Laser-modified tracks were generated using a femtosecond laser at a wavelength of 515 nm and a pulse energy of 243 nJ. The laser was focused with a 0.55 NA spherical lens and first focused through a 2 mm thick fused silica to form a laser-modified track shape at the center of the BOPP sample (i.e., 37.5 pm from the sample surface). The elongated laser focus formed laser-modified tracks -50 pm long and submicron in diameter (resolution limit) according to optical microscopy. Each laser-modified track was formed within the BOPP with a single laser pulse and arranged in a symmetrical 2D array with track separation. MA / t / ZUZZ / UZ IZZO laser by a distance of Lx= Ly= 3 pm. Under normal room lighting, the resulting 2-D array reflected and transmitted different colors (wavelengths) at different angles, as shown in Figure 1, with varying efficiency. When viewed through it, the eye observed different colors or effects at different viewing angles, which determined which wavelengths were most efficiently reflected by the embedded grid. Figure 2 shows optical microscopy images taken with an inverted differential interference contrast (DIC) microscope that enhances the contrast of small changes in refractive index at close points: i.e., it emphasizes refractive index gradients. The top and bottom surfaces of the BOPP substrate were unaffected by laser irradiation, as shown in Figure 2, and the laser-modified embedded elongated tracks formed only within the BOPP volume. Figure 3 shows scanning electron microscope (SEM) images of the laser-modified embedded tracks in BOPP, with each laser-modified track formed by a single laser pulse energy of 284 nJ. The cross-sectional SEM images shown in Figure 3 confirm that the laser-modified tracks were embedded within the BOPP film without disturbing the top or bottom surfaces, as well as the nearby radial zone surrounding the laser-modified track, which was not directly irradiated with the high-intensity laser; there was no apparently large heat-affected zone. At the irradiated sites, the elongated laser focal volume produced a long open cavity with an internal surface that may have melted, with a length and width of -50 pm and -800 nm, respectively. The polymer chains immediately surrounding the laser-modified tracks were realigned by the laser to align parallel to the direction of the laser-modified track. For BOPP, the alignment of unmodified areas placed the chains in lateral directions.This should affect the Raman signal and the optical birefringence of the structured zone of the laser-modified track. At a pulse energy of 284 nJ, a continuously open vacuum was also formed along the laser-modified track, as shown in Figure 3b. This may contain gases decomposed from the polymer by the laser, or air diffused in after the laser process. The diameter and length of the cavity in the laser-modified track are controllable. Example 2: Large-Area Two-Dimensional Arrays of Laser-Modified Tracks in BOPP and PET. A sample of BOPP was fabricated using the same technique described in Example 1. Figure 4 shows an example of 2-D arrays of laser-modified tracks, with each track fabricated using a single energy pulse of 210 nJ and varying periods of 1 pm, 1.5 pm, 2 pm, 2.5 pm, 3 pm, 3.5 pm, 4 pm, 4.5 pm, and 5 pm, as indicated. By varying the period of the 2-D array of gratings, the track is adjusted. ML / IZ / ZZ / ZZZ / ZZZZO the wavelength response of the diffraction of light that is strongly reflected and transmitted at a given angle. As shown in Figure 4a and Figure 4b, capturing the image with a camera equipped with a white LED irradiated at a slight angle from the left side (left-right tilt) and with normal incidence with a slight tint (up and down tilt), respectively, provides a vibrant array of different period-controlled reflected colors on the grating, when viewing angles that are polar and azimuth. Figure 14 shows images etched from a single layer of a symmetric 2-D array of laser-modified short tracks in BOPP, generated with a femtosecond laser at a wavelength of 515 nm and pulse energies ranging from 17.5 nJ to 142.0 nJ (from top to bottom row), as indicated on the right side of the figure. The laser exposure was focused directly onto the polymer sample without inducing aberration through a glass plate, resulting in a filament length, Lf, of less than 10 mm. The laser was focused with a 0.55 NA spherical lens to form a laser modification track shape on a single-layer 2-D array positioned at the center of the BOPP sample (i.e., 37.5 pm from the sample surface) with the transverse period (0.5 pm to 1.5 pm) incremented by 0.1 pm from left to right column as shown in the figure.The four images were recorded from the same sample, with a variable angle of incidence while irradiated with a fixed incandescent light and a camera angle in reflection mode (ab) and transmission mode (cd). The angles of the light source and the camera with respect to the normal of the sample surface were small (ayc) and large (byd). Under exposure to a low pulse energy of 17.5 nJ, the polymer appears transparent when irradiated and observed at a shallow angle in (c), where the diffraction does not satisfy the grating formula (i.e., Eq. 2 or 4). The same grating area diffracts a bright red color when aligned under a steep irradiation incidence angle in (d), satisfying the grating equations. This lower pulse energy exposure allows the formation of isolated filament structures, forming high-resolution grating arrays with a high diffraction period of at least 0.6 pm. It is expected that lower energy exposure will allow the formation of diffraction gratings with a smaller period, L, avoiding material damage and the washing away of refractive index contrast in the polymer sample. The diffuse white color observed from the grating areas in the lower left corner of the polymer sample arises from optical scattering, as the modified area has been damaged and thermally calcined by a combination of higher pulse energies and shorter periods. In the undamaged area with low pulse energy exposure, bright diffraction colors are observed that change over an increasing period (from right to left) as follows: - reflection at a small angle (a): orange (a = 1.5 pm), green (a = 1.4 pm), indigo (a = 1.3 pm), ΜΛ / ΙΖ / ΖυΖΖ / υΖΊΖΖΟ light blue (a = 1.2 pm), deep blue (a = 1.1 pm), purple (a = 1.0 pm), - reflection at a large angle (b): red (0.1 pm), orange (0.4 pm), light orange (0.3 pm), yellow (1.2 pm), green (1.1 pm), light blue (1.0 pm), deep blue (0.9 pm), purple (0.8 pm), - small angle transmission (c): deep red (α = 1.5 pm), bright red (α = 1.4 pm), orange (α = 1.3 pm), yellow (α = 1.2 pm), yellowish green (α = 1.1 pm), green (α = 1.0 pm), dark green (α = 0.9 pm), deep blue (α = 0.8 pm), purple (α = 0.7 pm), and - transmission at a large angle (d): green (a = 1.5 pm), light blue (a = 1.4 pm), purple (a = 1.3 pm), reddish purple (a = 1.2 pm), yellow (a = 1.1 pm), light green (a = 1.0 pm), dark green (a = 0.9 pm), reddish purple (a = 0.8 pm), light orange-purple (a = 0.7 pm), bright red (a = 0.6 pm). Figure 15 shows images etched from a single layer of a symmetric 2-D array of laser-modified long tracks in BOPP generated with a femtosecond laser at a wavelength of 515 nm and pulse energies ranging from 79.0 nJ to 141.0 nJ (from top to bottom row) as indicated on the right side of the figure. The laser exposure was focused through a 2 mm thick fused silica plate and into the polymer sample, inducing aberration to lengthen the laser-modified track to more than 10 pm. The laser was focused with a 0.55 NA spherical lens to form a laser modification track shape with a layer of a 2-D array of laser tracks in the center of the BOPP sample (i.e., 37.5 pm from the sample surface) with the transverse period (0.9 pm to 1.4 pm) with an increment of 0.1 pm from left to right column as shown in the figure.The four images were recorded from the same sample, with a variable angle of incidence while irradiated with a fixed incandescent light and a camera angle in reflection mode (ab) and transmission mode (cd). The angles of the light source and the camera with respect to the normal of the sample surface were small (ayc) and large (byd). The bright diffraction colors are observed changing over an increasing period (from right to left) as follows: - reflection at a small angle (a): faint purple (a = 1.4 pm), very faint red (a = 1.3 pm), faint red (a = 1.2 pm), red (a = 1.1 pm), green (a = 10 pm), blue (a = 0.9 pm), - reflection at a large angle (b): orange (a = 1.4 pm), yellowish green (a = 1.3 pm), green (a = 1.2 pm), light blue (a = 1.1 pm), blue (a = 10 pm), purple (a = 0.9 pm), - transmission at a small angle (c): faint light blue (a = 1.4 pm), faint purple (a = 1.3 pm), very faint red (a = 1.2 pm), red (a = 1.1 pm), green (a = 10 pm), blue (a = 0.9 pm), and - transmission at a large angle (d): red (a = 1.4 pm), yellow (a = 1.3 pm), light green (a = 1.2 pm), light blue (a = 1.1 pm), purple (a = 10 pm), very faint purple (a = 0.9 pm) Figure 12 shows light reflection camera images of fluorescent lamps made by a 70 pm thick polyethylene terephthalate (PET) film viewed at tilt angles ML / IZ / ZZ / ZZZ / ZZZZO sample variables with respect to fixed camera and light source positions. The PET film was embedded with a series of symmetric 2-D arrays of filament tracks (vertically stacked rectangular zones), formed by a femtosecond laser at pulse energies of 104.0 to 1005.0 nJ, and periods of 10, 1.5, and 2.0 pm. The filament tracks were elongated by aberration, prefocused through 2 mm thick silica plates. At the lower pulse energies of 150.5 nJ and 325.5 nJ, bright colors from grating diffraction can be observed at periodicities of ~1.5 pm and 2.0 pm. For a = 1.5 pm, the observed colors are a) green and cyan, c) purple and cyan, d) cyan with a hint of blue, and e) cyan. The most prominent colors are b) purple to cyan, c) cyan to yellow, d) yellow-orange to red, and e) red. At a high pulse energy of 1005.At 0 nJ, the carbonization of the PET film created a gray metallic reflection effect at all periodicities. With a moderately high pulse energy of 325.5 nJ, the small period of a = 1000 pm also resulted in a carbonization effect due to the closer separation of the filament density, creating a metallic-like reflection. Example 3: Scanning electron microscopy of two-dimensional arrays of laser-modified tracks in BOPP A sample was fabricated using the same technique described in Example 1. Figure 5 shows cross-sectional SEM images of laser-modified, embedded elongated tracks fabricated with a period of 2 pm and decreasing pulse energies of a) 1004 nJ, b) 525 nJ, and c) 284 nJ. By controlling the laser pulse energy, the lengths of the laser-modified elongated tracks can be easily adjusted to the desired length. As shown in Figure 4, laser pulse energies of 1004 nJ (Figure 4a), 525 nJ (Figure 4b), and 284 nJ (Figure 4c) produced laser-modified, elongated tracks with lengths of approximately 50 pm, -43 pm, and -36 pm, respectively. Another method for controlling the length of the laser-modified track is to vary the thickness of the fused silica plate, which affects spherical aberration when the laser beam is focused through the plate. The thicker the fused silica plate, the more spherical aberration is introduced when focusing the laser beam, which in turn provides a longer laser-modified track. Figure 6a shows cross-sectional SEM images of a 2D array of laser-modified tracks, 50 pm in length, embedded in 70 pm-thick BOPP with a pulse energy of 284 nJ focused through a 2 mm-thick fused silica plate. In contrast, Figure 6b shows a much shorter laser-modified track, 7 pm in length, when fabricated with a pulse energy of 65 nJ and without the fused silica plate. The position of the laser-modified tracks within the substrate can be controlled by changing the focus position. Depending on the application, the laser-modified tracks can be fully or partially embedded in the substrate. ML / t / ZUZZ / UZ IZZO Figure 7a shows the optical microscope image of the underside of a 2D array of partially embedded, laser-modified elongated tracks fabricated using the same technique as in Example 1, but focused 15 pm below the center of the BOPP film and with a pulse energy of 1004 nJ. The ablation debris on the underside indicates an open, laser-modified track. Figure 7b shows the cross-sectional SEM image of the lower half of the substrate where the laser-modified elongated tracks penetrated through the underside. Figure 7c shows the cross-sectional SEM image of the upper half of the substrate where the track is still embedded and does not break through the upper surface of the substrate.By controlling the length of the laser-modified track using spherical aberration and / or varying the laser pulse energy, along with controlling the laser focus position, different structural and optical requirements can be met on different substrates. Example 4 - schematic example of two-dimensional arrays of laser-modified tracks arranged in pixel-like areas to generate an image Figure 8 illustrates how a more complex multicolor image, such as the Canadian flag, can be formed from an array of pixels, each containing an array of subpixels formed by laser-modified track arrays with the array spacing required to generate a red, green, or blue diffractive light output. The orientation of the pixel arrays can also be rotated about the horizontal axis to further modify the observed color and / or intensity of the array's diffracted light output. Figure 8a schematically illustrates the formation of red diffractive bands on the Canadian flag using a laser-modified track array with a given track periodicity and periodicity matrix in the x and y directions. Figure 8b schematically illustrates how a laser-modified track array can be optionally rotated at any angle with respect to the horizontal image plane to modify the observed diffractive light output of the array. Figure 8c schematically illustrates the formation of the white area within the Canadian flag using a series of pixel arrays containing red, green, and blue subpixel arrays, with each subpixel array containing the track periodicity necessary to generate a red, green, or blue diffracted light output for each subpixel. The pixel output is the sum of the RGB subpixel diffractive outputs.Figure 8d schematically illustrates an alternative pixel layout consisting of hexagonal subpixel arrays that can generate the combined RGB diffractive light output for the white portion of the Canadian flag. The pixel array periodicity and spacing are identified in the illustration. Example 5: Simulation (FDTD) of diffraction efficiency illustrating the effects of the Talbot photonic crystal and 3D ML / t / ZUZZ / UZ IZZO In the following examples, flexible point-by-point writing of laser-modified tracks on 2D and 3D periodic refractive index modulation networks is explored using finite-difference time-domain (FDTD) simulation (numerical software) to demonstrate the spectral filtering options anticipated by the present invention. As noted in the SEM images in Figure 13, two laser-modified zones in the BOPP filament structure are expected to provide a relatively low refractive index contrast (i.e., ~0.01) for low pulse energy exposure or a high refractive index contrast (i.e., ~0.5) for high pulse energy exposure.More generally, a similar difference is anticipated in the generation of low and high refractive index contrast for other optical materials, where the control of laser pulse exposure is set respectively below or above a threshold for the opening of elongated holes. The arrangement of laser-modified tracks in different crystallographic planes or aperiodic structures is well known in the field of photonic crystals for controlling the formation of photonic stopping bands at specific propagation angles and spectral bands, or for enhancing diffraction efficiency at specific propagation angles and spectral bands. In this way, the present invention anticipates varying the structural geometry and crystallographic arrangement of laser-modified tracks in changing areas across the transparent film to control light diffraction, zone by zone, and to enhance or filter spectrally and / or angularly the fused light to produce vivid color images under specific lighting conditions. Examples of the underlying control for such spectral and angular filtering are presented for both high and low refractive index contrast cases (0.46 and 0.033) in Figures 16 and 17, respectively. Light was projected as a plane wavefront, with wavelengths ranging from 380 nm to 700 nm, and presented at an incident angle of θn = 0° to the grating plane (i.e., normal incidence in Figure 9a). The percentage efficiency of the projected power in all allowed interference modes, both transmission and reflection, was calculated as a function of wavelength. Spectral records can also present an angular distribution of light with wavelength, with the wavelength scale converted to a diffraction angle, θout, calculated by Eq. 4 for forward or reflected directions (Figure 9) and for deviations from the xy and y axes.For normal incidence, four first-order interference beams result for both transmission and reflection, with orders (mx,my) = (+1,0), (-1,0), (0,+1), and (0,-1). The sum of all fourth-order diffraction efficiencies is presented in the simulated FDTD spectra for this first-order diffraction case. The two curves in each graph represent the total transmitted or reflected light (purple or higher-value line) and the subcomponent of this light that diffracted in the zero- or first-order diffraction directions (green or lower-value line). ΜΛ / ΙΖ / ΖυΖΖ / υΖΊΖΖΟ Figure 16 plots the diffraction efficiency expected from the FDTD simulation of periodic arrays of filaments that have been opened into gaps, providing a high refractive index contrast of Δη = 0.46. Each filament was cylindrical, with a diameter of 0.8 pm and a length of 1.5 μm, and was arranged in a 2-D array with a square pattern (i.e., Δχ a,b = Ayc,d = 0 in Figure 9c) with symmetric lateral periods of a = ax = ay = 0 pm. The regional duty cycle of the open and closed zones in the grating layer was nearly 50:50, to provide the most efficient diffraction at higher orders (i.e., m = 1 or higher). Figures 16s (a), (c), and (e) plot the 0th and 1st order transmission spectra and the 1st order reflection spectra, respectively, calculated for the single-layer grating array. With such high contrast, the filament length of 1.5 mm resulted in a 3p phase contrast for 460 nm light, corresponding to the destructive interference of this green light in the center of the visible spectrum. A strong 0th order transmission rejection of ~5% (Figure 16a) and a correspondingly strong total diffraction efficiency of -80% in the combined 1st order transmission diffraction beams (Figure 16C) are observed, enabled by this high refractive index contrast when using such a thin grating zone.The single-layer grating also features a broadband filter, strongly rejecting zero-order transmission in the blue-green spectral band (Figure 16a) and redirecting this light into four beams for first-order transmission. The single-layer grating structure provides only modest reflection, peaking at -2% in the reflected first-order beams (Figure 16e). Reflection resonances arise from a thin-film effect due to the 1.5 pm-thick grating layer, resulting in a wavelength-free spectral range of -460 nm. Including the multiple orders, total internal reflection across all orders reaches a maximum of only 5% at 432 nm. Therefore, most of the incident light in the visible spectrum is directed to the first-order beams (Figure 16a). Figures 16(b), (d), and (f) consider the additional influence of multi-layered 3D gratings where the same 2-D grating array design above was replicated and optimally spaced to probe green light (520 nm wavelength) on Talbot planes separated by c = 5.59 pm (Eq. 5). Layer-to-layer laser-modified tracks were axially aligned (i.e., Δχι,2 = Ayi,? = 0 for layers 1 and 2, etc.), forming a tetragonal crystal arrangement of filaments. With the addition of 2 more layers, which have a total of 3 layers, the sharpening of the spectral features is observed, broadening and flattening the destructive interference resonance in the 0th order transmission (Figure 16(b)) through a wide blue-green band (425 to 530 nm) while passing the red band of wavelengths (0.630 ± 50 nm) that fall out of resonance with the Talbot condition in Eq. 5.The first-order diffraction spectrum (Figure 16(d)) is no longer a well-matched complement to the 0th-order diffraction as in the single-layer box (Figure 16(ayc)), since more light is diffracted (-10% in 3 layers versus -3% in a single layer). ML / IZ / ZυZZZ / υZΊZZZΟ now interferes consistently in the direction of reflection due to Talbot's enhancement effect. Figure 16(f) demonstrates the strong improvement in grating reflection due to Talbot and other resonance effects, resulting in multi-wavelength peaks appearing across the visible spectrum that achieve an efficiency exceeding 20% ​​for combined first-order beams. This is 10 times stronger than the single-layer case, demonstrating the strong potential for managing the visual color of transparent films through spectral and angular filtering, as shown in Figure 16 for the case of highly contrasting 2D and 3D grating patterns.Figure 17 presents the transmission spectra generated from single-filament (a) and seven-layer (bf) arrays based on filaments formed in BOPP (n = -1.5) and having a low refractive index contrast of Δη = 0.033020. Each grating layer is constructed from cylindrical filament shapes 0.518 pm in diameter and Lf = 1.12485 pm in length, modeled on a two-dimensional symmetric grating with a periodic basis dea = ax = ay = 0.65 pm and having no lateral displacement between the columns and rows of laser-modified tracks (i.e., αχ a,b = ac,d = 0 in Figure 9c). In a single grating layer, a small phase contrast of only φ = ~0.14π (Eq. 1) has been accumulated to obtain light by passing the grating at a central wavelength of 520 nm.The result is a weak diffraction efficiency that extends across the entire visible spectrum, as shown in Figure 17a for the first-order diffraction case in the forward direction. This diffraction varies monotonically from ~3% to <1% efficiency for blue to red wavelengths. Most of the light remains undeflected, exceeding >97% transmittance in the zero-order beam. Increasing the grating length in a single layer does not significantly improve diffraction efficiency, except when weakly contrasting laser-modified tracks have been segmented and aligned in Talbot planes. In this latter case, with seven grating layers having filaments positioned in tetragonal symmetry and axially aligned row by row (i.e., Δχι,2 = Ayi,2 = 0 in Figure 9c), a moderately strong stopping band (47% attenuation) is found to form in a narrow spectrum (~40 nm bandwidth) centered near the 520 nm design resonance (Eq. 3), as shown in Figure 17(b). Most of the visible spectrum is transmitted undeflected, except for the narrow band of greenish light that is -47% redirected in four beams appearing in the first transmitted order, as depicted in Figure 17c.This light of central wavelength diffracts at an angle of 0out,m = i = -20°, or -31° externally in air. Given the -40 nm bandwidth of this stopping band, the 7-layer grating will only pass a narrow band of light, from -480 to 535 nm (3 dB bandwidth), at diffraction angles ranging from -19 to -21 degrees. This 3D grating leaves the viewing area significantly darkened at all other viewing angles, except for zero-order transmission, where the complementary colored light will appear, making the design of spectral and angular filters possible. ML / t / ZUZZ / UZ IZZO The spectral and angular filtering as presented in the first-order transmission for the 7-layer tetragonal structure in Figure 17(c) can be further adjusted by shifting or tilting the filament grating, for example, by selecting non-zero values ​​for any of the row (Δχ ab) or column (Aye,d) transverse shifts or the row (Δχι^) or column (Ayi.z) axial shifts (see Figure 9c). A half-period lateral sieving of filaments along the x-direction, applied between adjacent layers (Δχ a,b = ax / 2 = 325 nm), leads to a weakening of the central top band, resulting in a drop in first-order transmission from 45% in Figure 17(c) (tetragonal) to 30% in Figure 17(d) (tilted tetragonal). Other crystal structures can be considered in the design of laser-modified track patterns. The insertion of additional filaments into the present tetragonal design (Figure 17C) leads to body-centered cubic (bbc) or face-centered cubic (fcc) arrangements, except with axially broken symmetry due to the Talbot periodicity exceeding the lateral periodicity (i.e., c > x). The result of such changes in crystal symmetry is a broadening of the stopband to 100 nm, as shown in the first-order transmission for stretched bcc symmetry in Figure 17(e). A narrowing of the stopband to -12% is observed for first-order transmission, along with a -30 nm blue shift in the peak resonance wavelength, as shown in Figure 17(f) for the stretched fcc symmetry case. Example 6: Examples of Talbot effects Figure 9 schematically illustrates various laser-modified track arrangements on two-dimensional periodic grids in cross-sectional (ac) and top (df) views. In Figure 9a), a single layer of identical filament tracks of length Lf is uniformly spaced over period Lx. Lf can be controlled by the laser pulse energy and focusing conditions. An incident laser beam enters the grating layer at an angle of 6in, leading to diffraction of the transmitted and reflected beams at an outward angle, Qoutward,m, which varies with the diffraction orders, shown here for m = 0 and ±1 for both reflection and transmission. Multiple layers of a similar grating array are depicted in (b) and (c), where the center-to-center spacing of the layers defines a Talbot length, c, for the normal incidence case (9in = 0).A misalignment of the filament positions in rows a and b, Δχ a, b, is shown for the zero and non-zero cases in (b) and (c), respectively. The top-view images of the filament end, dae, refer to the respective cross-sectional views in ac, where Lxy and Ly define the periodic separation of the filaments along the x and y axes, respectively. In (c) and (d), the rows and columns are aligned on a grid, while in (d), the rows and columns are offset by Ayc,dy and Axa.b, respectively. Here, Ayc,d defines the y-axis offset between the filament positions in layer c and layer d. The Ayc,dy and Axa,b offsets provide glow-like grating efficiency effects to strengthen diffraction orders. specific ΜΛ / ΙΖ / ΖυΖΖ / υΖΊΖΖΟ at target light wavelengths. Figure 10 shows images recorded from Talbot gratings consisting of three layers of a symmetric 2D array of short laser-modified tracks generated with a femtosecond laser at a wavelength of 515 nm and pulse energies of 21.5 nJ, 36.5 nJ, 58.5 nJ, 89.0 nJ, and 153.5 nJ. The laser exposure was focused on the polymer sample without inducing aberration using a glass plate, to keep the filament length smaller than the Talbot spacing (c) in all cases. The three images were recorded from the same sample, with varying angles of incidence while irradiated with a fixed white LED light and a reflection detected at a fixed camera angle. The incident angle of irradiation relative to the sample surface normal increases from Figure 8a to Figure 8b to Figure 8c. The laser was focused with a spherical lens of 0.55 NA to form a laser-modified track shape with a layer of a 2-D array of laser-modified tracks at the center of the BOPP sample (i.e., 37.5 μm from the sample surface), and a layer of the same 2-D array above and a layer below by a side length (c) of 3.9 pm, 8.7 pm and 15.5 pm with the transverse period (Lx, Ly) of LO pm, 1.5 pm and 2.0 pm, respectively. The advantages of spectral filtering and the enhancement due to the Talbot effect are observed by comparing gratings modeled with the same pulse energy of 21.5 nJ, but optimized for two different Talbot configurations. In the first Talbot configuration of c = 3.9 pm, and a transverse period of 10 pm (top row of grating images in Figure 10a, b, c), a narrow spectrum is filtered and strongly reflected as expected, with the smaller incident angle enhancing the short-wavelength light (blue in Figure 10a) while the longer wavelength is tuned to a larger incident angle (blue-green in Figure 10B to green in Figure 10C). In the second configuration with a larger periodic separation of c = 8.7 pm and ya = 1.5 pm (second top row of grating images in Figure 10a, b, c), the red wavelength light at the edge of the visible spectrum is enhanced at the smaller incident angle in Figure 10a.As the incident angle increases, the optical wavelength for constructive interference shifts into the infrared and shows a barely visible red in Figure 8b, while a further increase in angle begins to reveal short-wavelength blue hues as the next order of diffraction with lower diffraction efficiency comes into resonance in Figure 8c. Figure 11 provides optical microscope images showing the top view of a polymer sample exposed with a laser at pulse energies from 21.5 nJ to 89.0 nJ, forming a 3D volume lattice of short, three-layered filaments with symmetrical transverse periods, L = Ax = Ay and the Talbot period, c, given respectively: a) at 10 pm and c = 3.9 pm, b) at 1.5 pm and c = 8.7 pm, and c) at 2.0 pm and c = 15.5 pm. In the case of (a), the limits of optical resolution for imaging the filaments in a non-perfectly transparent medium have been reached. With a small period and an energy of ML / IZ / ZυZZZ / υZΊZZZO high pulse (i.e., a = 1.0 pm with pulse energies from 36.5 nJ to 89.0 nJ, ya = 1.5 pm with pulse energies from 58.5 nJ to 89.0 nJ), carbonization and coalescence of the laser modification zones lead to scattering of the incident light, providing a poor diffraction grating. Figure 13 shows top optical (a, d) and cross-sectional SEM (b, c, e, f) images of two examples of a three-dimensional lattice structure of laser-modified tracks formed on three layers near the middle of a 70 pm thick BOPP substrate. The examples demonstrate Talbot configurations with hexagonal crystal symmetry in (a) and tetragonal crystal structure in (d), and lateral periodicities of a = b = A (top views in a and d) and longitudinal periodicities of caa (cross-sectional views in b, c, and f). The whitish horizontal lines in (b) and (e) mark the center of the film, being 35 pm from the top and bottom surfaces.The horizontal dotted lines in (a) and (d) mark the film cleavage position and the relative intersection positions of the laser-modified tracks, counting down the periodic observation of the cross-sectional side views of the vertical laser-modified track, shown respectively in (b) and (c) and in (e) and (f). In the first example, formed with a lower pulse energy of 21.5 nJ, a structural period of a = 2 pm is observed in (a), and a Talbot length of c = 15.5 pm in the three laser-modified track layers, as shown in (b). This cross-sectional SEM image (b) has captured one out of every three laser-modified tracks in the three 2-D laser-modified track grating layers. The magnified two-filament, single-layer SEM image in (c) reveals laser realignment of the polymer chain from the predominant lateral direction in an unexposed volume to only vertical alignment in the laser-exposed volume, with a length of -5.5 pm and a diameter of ~0.7 pm. The low energy exposure was below the threshold for opening a cylindrical void, at least not with a diameter above the resolution limit of ~50 nm here.In this exposure, only a modest contrast in the refractive index can be expected at the level of several percent of the laser modification. In the second example, formed with smaller periods of a = 1.5 pm laterally and c = 8.7 pm vertically, and a higher pulse energy of 58.5 nJ, a stronger morphological change is observed in the top optical image view of Figure 13(d). The cleavage plane aligned more closely with the lateral laser-modified track pattern, resulting in two of the three track layers being visible in the cross-section SEM view in (e). The filament tracks aligned at the expected periodicities of a = 1.5 pm and c = 8.7 pm. At higher magnification of a single layer in (f), the cross-section SEM image confirms a vertical realignment of the polymer chain, which forms as the boundary around a cylindrical void shape with a vertical length of -8.0 pm and a diameter of ~0.8 pm. The open structure formed by this stronger laser exposure will result in a much higher refractive index contrast of ~0.5 pm in such volume grids with three-dimensional patterns. Example 7: Example of grating diffraction efficiency with short to long grating length in a single layer, and with Littrow and Talbot effects in multiple layers Two-dimensional laser-modified track assemblies were laser-fabricated on BOPP films using various laser pulse energies, grating periods (L and c), laser-modified track lengths, and numerical grating layers. Their diffraction efficiency was evaluated in both reflection and transmission modes. The relative merits of short and long grating lengths, Littrow angle effects, and multi-layer Talbot resonances are presented in Figure 18, which shows plots of the combined first-order diffraction efficiency. The gratings were illuminated with collimated 520 nm wavelength light applied at normal incidence and Littrow angle for the cases of a single-layer 2D grating consisting of (a) short laser-modified tracks and (b) long laser-modified tracks in transmission mode, and (c) short filaments in reflection mode.An additional case of (d) laser-modified short-track three-layer gratings was optimized for the 520 nm probe light by separating the grating layers at the Talbot period of c = 5.59 pm, and was measured in transmission mode. The first-order diffraction efficiency is plotted as a function of the grating period. For short laser-modified tracks (length less than 10 pm), the laser exposure was focused directly onto the polymer (BOPP) sample without inducing aberration through a glass plate, while long laser-modified tracks were fabricated by passing the focused laser through a 1 mm fused silica plate before entering the sample, thus lengthening the filament modification track to more than 10 pm. For low refractive index contrast gratings fabricated with a low pulse energy of 23.5 nJ in (a), the first-order diffraction efficiency is low for the short laser-modified track length at normal incidence. The diffraction efficiency increases modestly to -5.5% with an increase in pulse energy to 31 nJ and 40 nJ due to stronger material modification resulting in greater refractive index contrast.With a longer laser-modified track length, higher pulse energies were required to compensate for energy propagation over the longer track length, leading to low first-order diffraction efficiency for normal incidence, as shown in (b). The results point to low contrast in the refractive index. Overall, higher diffraction efficiencies were observed in transmission when probing at Littrow angles for both (a) short and (b) long laser-modified tracks. An approximately 3-fold improvement was observed for (a) short laser-modified tracks when separated at a period of α = 1.0 pm. Much larger improvement factors of up to 10x were observed for (b) long laser-modified track gratings with periods in the range of 0.9 to 1.4 pm. The highest efficiency of 35% was observed at a period of α = 1.2 pm and a pulse energy of 92.5 nJ. IVIA / t / ZUZZ / UZ IZZOIn reflection from the same short-wavelength laser-modified track grating in (a), only very weak first-order diffraction efficiencies (less than 0.5%) were observed at normal incidence, as shown in Figure 18(c). In contrast, the Littrow angle provided a strong -10x improvement, with first-order efficiencies as low as -5.7%. Overall, the relatively low refractive index contrast and the single-layer 2-D grating configuration resulted in poor diffraction efficiency. Using the Talbot effect to overcome this deficit, a strong Talbot enhancement effect is observed in Figure 18(d) from the three-layer structure. For normal incidence, moderately strong diffraction efficiencies of -17.2% (at 36.5 nJ) and -31.4% (at 58.5 nJ) were observed at periods of 1.5 pm and 2 pm, respectively.0 pm, respectively, demonstrating the high potential diffraction efficiency available from a low refractive index contrast grating. The invention anticipates achieving much higher diffraction efficiencies (Example 5) by improving the grating structural design to optimize phase matching (i.e., Eqs. 2 and 4) at different wavelengths and by controlling the filament length, refractive index contrast, and Talbot length. It is understood that the safety devices and features, the methods for their production, and the related technology employed in the illustrative embodiments may be modified in a variety of ways that will be readily apparent to those skilled in the art from the teachings described herein. All such modifications and variations of the illustrative embodiments shall be deemed to be within the scope and spirit of the present invention as defined, or partially defined, by the appended claims herein. ΜΛ / ΙΖ / ΖυΖΖ / υΖΊΖΖΟ References: 1. Booth, MJ, Neil, MAA & Wilson, T. Aberration correction for confocal imaging in refractive-index-mismatched media. Journal of Microscopy 192, 90-98 (1998). 2. Rayner, D., Naumov, A. & Corkum, P. Ultrashort pulse non-linear optical absorption in transparent media. Opiles express 13, 3208-3217 (2005). 3. Li, J., Ertorer, E. & Hermán, P. R. Ultrafast láser burst-train filamentation for non-contact scribing of optical glasses. Optics Express 27, 25078 (2019). 4. Hermán, P. R., Marjoribanks, R. & Oettl, A. Burst-ultrafast láser machining method. 5. Gattass, R. R. & Mazur, E. Femtosecond láser micromachining in transparent materials. Nature photonics 2, 219-225 (2008). 6. Shimotsuma, Y., Kazansky, P. G., Qiu, J. & Hirao, K. 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S., Sekita, H., Sumiyoshi, T. & Kamata, M. Display glass cutting by femtosecond láser induced single shot periodic void array. Applied Physics A: Materials Science and Processing 93, 189-192 (2008). 19. Chung, S. H. & Mazur, E. Surgical applications of femtosecond lasers. Journal of Biophotonics 2, 557-572 (2009). 20. Booth, M. J. Adaptive optical microscopy: The ongoing quest for a perfect image. Light: Science and Applications 3, el65-el65 (2014). 21. Hnatovsky, C. et al. High-resolution study of photoinduced modification in fused silica produced by a tightly focused femtosecond láser beam in the presence of aberrations. Journal of Applied Physics 98, 013517 (2005). 22. Chen, Y. C. et al. Láser writing of coherent color centers in diamond. Nature Photonics 11, 7780 (2017). 23. Huang, L., Salter, P. S., Payne, F. & Booth, M. J. Aberration correction for direct láser written waveguides in a transverse geometry. Optics Express 24, 10565-10574 (2016). 24. Cumming, B. P. et al. Adaptive optics enhanced direct láser writing of high refractive índex gyroid photonic crystals in chalcogenide glass. Optics Express 22, 689-698 (2014). 25. Osellame, R. et al. Femtosecond writing of active optical waveguides with astigmatically shaped beams. Journal ofthe Optical Society of America B 20, 1559—1567 (2003). 26. Ams, M., Marshall, G., Spence, D. & Withford, M. Slit beam shaping method for femtosecond láser direct-write fabrication of symmetric waveguides in bulk glasses. Optics Express 13, 5676-5681 (2005). 27. Salter, P. S. el al. Adaptive slit beam shaping for direct láser written waveguides. Optics Lellers 37, 470-472 (2012). 28. Salter, P. S. & Booth, M. J. Focussing over the edge: adaptive subsurface láser fabrication up to the sample face. Optics Express 20, 19978-19989 (2012). 29. Ren, H., Lin, H., Li, X. & Gu, M. Three-dimensional parallel recording with a Debye diffractionlimited and aberration-free volumetric multifocal array. 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IEEE Transactions on Semiconductor Manufacturing 20, 259-265 (2007). 35. Tamaki, T., Watanabe, W., Nishii, J. & Itoh, K. Welding of transparent materials using femtosecond láser pulses. Japanese Journal of Applied Physics, Parí 2: Letters 44, L687-L689 (2005). 36. Duocastella, M. & Arnold, C. B. Bessel and annular beams for materials processing. Láser and Photonics Reviews 6, 607-621 (2012). 37. He, F. et al. Tailoring femtosecond 1.5-pm Bessel beams for manufacturing high-aspect-ratio through-silicon vías. Scientific Reports 7, 1-9 (2017). 38. Yao, Z. et al. Non-diffraction-length, tunable, Bessel-like beams generation by spatially shaping a femtosecond láser beam for high-aspect-ratio micro-hole drilling. Optics Express 26, 21960-21968 (2018). 39. Mishchik, K. et al. Improved láser glass cutting by spatio-temporal control of energy deposition using bursts of femtosecond pulses. Optics Express 25, 33271—33282 (2017). 40. Liu, X. et al. Front-surface fabrication of modérate aspect ratio micro-channels in fused silica by single picosecond Gaussian-Bessel láser pulse. Applied Physics A: Materials Science and Processing 124, 206 (2018). 41. Ashcom, J. B., Gattass, R. R., Schaffer, C. B. & Mazur, E. Numerical aperture dependence of damage and supercontinuum generation from femtosecond láser pulses in bulk fused silica. Journal ofthe Optical Sociely of America B 23, 2317-2322 (2006). 42. Lapointe, J. & Kashyap, R. A simple technique to overeóme self-focusing, filamentation, supercontinuum generation, aberrations, depth dependence and waveguide interface roughness using fs láser Processing. Scientific Reports Ί, 1-13 (2017). 43. Kammel, R. et al. Enhancing precisión in fs-laser material processing by simultaneous spatial and temporal focusing. Light: Science and Applications 3, el69-el69 (2014). 44. Vitek, D. N. et al. Spatio-temporally focused femtosecond láser pulses for nonreciprocal writing in optically transparent materials. Optics express 18, 24673-24678 (2010). 45. Salter, P. S. & Booth, M. J. Dynamic control of directional asymmetry observed in ultrafast láser direct writing. Applied Physics Letters 101, 141109 (2012). 46. Patel, A., Svirko, Y., Durfee, C. & Kazansky, P. G. Direct Writing with Tilted-Front Femtosecond Pulses. Scientific Reports 7, 1-14 (2017). 47. Yariv, A. Quantum Electronics, in 486-487 (Wiley, 1988). MA / t / ZUZZ / UZ IZZO 48. Ertorer, E., Haque, M., Li, J. & Hermán, P. R. Femtosecond láser filaments for rapid and flexible writing of fiber Bragg grating. Opt. Express 26, 9323-9331 (2018). 49. Müller, M. et al. 1 kW 1 mJ eight-channel ultrafast fiber láser. Optics Letters 41, 3439-3442 (2016). 50. Milam, D. Review and assessment of measured valúes of the nonlinear refractive-índex coefficient of fused silica. Applied Optics 37, 546-550 (1998). 51. T. Clausnitzer, T. 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Claims

1. A substrate sheet comprising a material having an overall refractive index n, wherein the substrate sheet contains at least one ordered two-dimensional array of discrete laser-modified tracks in the material generated by a beam-shaped laser with laser light distributed along and / or around a laser propagation path extending within the substrate sheet, each laser-modified track comprising an elongated volume of modified substrate material at least 4 times longer than its narrowest width extending at least partially through a thickness of the substrate sheet, comprising a modified shape of the substrate material having a refractive index different from the overall refractive index n of the substrate sheet from which each laser-modified track originated,where for each ordered two-dimensional array the laser-modified tracks collectively diffract the light incident on the substrate sheet to form an observable shape, image, or color region.

2. The substrate sheet according to claim 1, wherein for each laser-modified track the elongated volume of modified substrate material is at least 5 times, preferably at least 8 times, preferably at least 10 times, preferably at least 15 times, more preferably at least 20 times, longer than its narrowest width, and optionally the laser propagation path within the substrate sheet is linear, curved, or helical.

3. The substrate sheet according to claim 1, wherein each of the laser-modified tracks is generated by a femtolaser, such as a femtolaser having a pulse duration in the range of 0.1 fs to 100 hp for each laser pulse, with beam shaping of the femtolaser beam before or after its interaction with the substrate.

4. The substrate sheet according to claim 3, wherein the femtolaser beam shaping employs a lens or sheet between a laser beam source and a substrate surface.

5. The substrate sheet according to claim 4, wherein the femtolaser beam shaping employs a silica sheet between the laser beam source and the substrate.

6. The substrate sheet according to claim 1, wherein for at least some of the laser-modified tracks, the elongated volume of modified substrate material is located within the substrate sheet.

7. The substrate sheet according to claim 1, wherein for at least some of the laser-modified tracks the elongated volume of modified substrate material is exposed on at least one surface of the substrate sheet.

8. The substrate sheet according to claim 1, wherein for at least some of the laser-modified tracks the elongated volume of modified material in the substrate sheet includes a void in the substrate sheet, formed after the production of the substrate sheet by melting, displacement, or decomposition of a portion of the substrate sheet material.

9. The substrate sheet according to claim 1, wherein it has an average thickness of 10-3000 pm, preferably 50-150 pm.

10. The substrate sheet according to claim 1, wherein the substrate sheet is a polymer sheet, and each of at least one ordered two-dimensional array of discrete laser-modified tracks comprises laser-modified tracks in the polymer, generated after the production of the polymer sheet.

11. The substrate sheet according to claim 10, wherein the substrate sheet comprises BOPP, BOPET, PEN, PP, PVDF or related copolymers such as PVDF-TrFE, or Nylon 55 or 66 or other derivatives.

12. The substrate sheet according to claim 10, wherein the laser-modified tracks extend independently of each other within the polymer sheet from 5% to 100% of the distance between opposite surfaces of the polymer sheet, and optionally extend into one or more additional layers if present adjacent to the polymer sheet.

13. The substrate sheet according to claim 10, wherein the polymer of the polymer sheet comprises polymer chains, wherein each elongated volume of modified substrate material for at least some of the laser-modified tracks comprises polymer chains that are at least partially aligned with each other with respect to those of unmodified polymer sheet material, such that the modified material comprises aligned polymer chains that generally extend non-parallel to the surfaces of the polymer sheet, so as to cause the modified substrate material to have a different refractive index with respect to the general refractive index n for the unmodified polymer sheet material.

14. The substrate sheet according to claim 13, wherein the modified material comprises polymer chains displaced to create periodic gaps, such that the selected laser-modified tracks each comprise a gap that generally extends non-parallel to the surfaces of the polymer sheet, such that each of said tracks has a different refractive index with respect to the general refractive index n for the unmodified polymer sheet material.

15. The substrate sheet according to claim 13, wherein it comprises polymer chains that generally extend or are aligned non-parallel to the surfaces of the polymer sheet, due to melting, displacement, or decomposition of the polymer within each elongated volume of modified substrate material in the polymer sheet.

16. The substrate sheet according to claim 13, wherein at least some of the MA / t / ZUZZ / UZ IZZO aligned polymer chains within each elongated volume of modified substrate material of each laser-modified track generally extend or align perpendicular to the surfaces of the polymer sheet.

17. The substrate sheet according to claim 13, wherein at least some of the aligned polymer chains within each elongated volume of modified substrate material of each laser-modified track extend to one or both surfaces of the polymer sheet.

18. The substrate sheet according to claim 13, wherein at least some of the aligned polymer chains within each elongated volume of modified substrate material of each laser-modified track extend into the polymer of the polymer sheet, but do not extend to the surfaces of the polymer sheet.

19. The substrate sheet according to claim 13, wherein the laser-modified tracks comprise amorphous regions and / or voids in the substrate that optionally extend to one or both surfaces of the substrate sheet, wherein the amorphous regions and / or voids comprise at least one of amorphous polymer, air, vacuum, polymer decomposition and oxidation products, glassy and amorphous carbon compounds.

20. The substrate sheet according to claim 1, wherein each elongated volume of modified substrate material of each laser-modified track is 1-5000 nm wide on average, extending partially or fully through the substrate sheet.

21. The substrate sheet according to claim 1, wherein a two-dimensional ordered array comprises laser-modified tracks that are separated from each other by a periodicity of 0.01 to 1000 pm, preferably 0.05-10 pm, more preferably 0.1-5 pm, on average by the unmodified substrate sheet material having an overall refractive index n.

22. The substrate sheet according to claim 1, wherein the substrate sheet comprises different sections, each comprising a plurality of said laser-modified tracks, wherein the tracks within a section have different periodicities, lengths, or orientations compared to the tracks of at least one other section of the substrate sheet, such that the optical emissions from said different sections resulting from diffraction of the incident light differ from each other when the same or equivalent incident light is simultaneously incident on the different sections.

23. The substrate sheet according to claim 22, comprising pixel-like areas of the substrate, at least some of which have different optical diffractive properties from each other, the pixel-like areas preferably being 1-10,000 pm wide, more preferably 5-100 pm wide.

24. The substrate sheet according to claim 23, wherein each pixel-like area of ​​the substrate sheet comprises laser-modified tracks having the same or substantially the same periodicity within each pixel-like area, such that each pixel-like area provides a generally uniform optical diffractive output after exposure to incident light.

25. The substrate sheet according to claim 23, wherein each pixel-like area comprises multiple subsections, each of which comprises an ordered array of said laser-modified tracks having substantially consistent periodicity within each subsection, the periodicity of the tracks extending within the subsections of any given pixel-like area being different from one another, such that the optical diffraction output of any given pixel-like area is defined by a combination of optical diffraction outputs for all subsections of that pixel-like area.

26. The substrate sheet according to claim 25, wherein each pixel-like area comprises subsections each providing a red, green, and blue optical diffractive output at selected angles after exposure to incident light, the relative intensity of the red, green, and blue outputs of the subsections of any given pixel-like area determining the color of the combined optical diffractive output for that pixel-like area at a given angle.

27. The substrate sheet according to claim 22, wherein the laser-modified tracks have a graduation change in at least one of periodicity, spacing, length, and orientation across the substrate sheet between sections, thereby providing a graduation change in the optical diffractive output properties across the device after exposure to incident light.

28. The substrate sheet according to claim 1, wherein at least some of the laser-modified tracks extend at least partially through the thickness of the substrate sheet in a non-perpendicular and / or non-linear path with respect to the surfaces of the substrate sheet.

29. The substrate sheet according to claim 1, wherein at least some of the laser-modified tracks extend to different depths within and through a thickness of the substrate sheet from each other.

30. The substrate sheet according to claim 1, comprising two or more ordered two-dimensional arrays of discrete laser-modified tracks, the assemblies being at different depths from each other through a thickness of the substrate sheet.

31. The substrate sheet according to claim 30, wherein a first array of laser-modified tracks is at least partially superimposed on a second array of laser-modified tracks in the substrate sheet, when the substrate sheet is viewed in plan view from one side thereof.

32. The substrate sheet according to claim 31, wherein the optical diffraction output of one of the first and second arrays is further diffracted by the other of the first and second arrays when the substrate sheet is exposed to incident light. ML / t / ZUZZ / UZ IZZO 33. The substrate sheet according to claim 31, wherein the optical diffraction output of the first and second arrays, when observed simultaneously, exhibits an optical interference or diffraction effect, such as the Littrow configuration or Talbot self-image grid layers aligned.

34. The substrate sheet according to any of claims 31 to 33, comprising laser-modified tracks of overlay sets at different depths within the substrate, formed simultaneously by a single laser pulse with laser energy distributed along a longitudinal laser beam axis extending through the substrate sheet focused simultaneously at said different depths within the substrate sheet.

35. The substrate sheet according to claim 1, wherein it further comprises one or more additional layers partially or totally overlaying the substrate material, the additional layer(s) being selected individually from a polymer layer, a reflective layer, a refractive layer, a diffraction filter, a transmissive filter, a protective layer, a coating, an adhesion-promoting layer, an ink, an optical interference layer, and an optical interference stack.

36. The use of the substrate sheet according to any of claims 1 to 35 as a security feature of a security document or as a security document.

37. A security document comprising, as a security feature, the substrate sheet according to any of claims 1 to 35.

38. A method for manufacturing a security document or security device, wherein the method comprises the steps of: providing a substrate sheet comprising a substrate sheet material; irradiating the substrate sheet at multiple discrete positions through a flat side of the substrate sheet corresponding to a two-dimensional array, with a laser beam from a laser, such as a femtolaser, while modifying the shape of the laser beam before or after its interaction with the substrate sheet so that the incident laser light is distributed at least partially along and / or around a laser propagation path extending within the substrate sheet;melting, displacing, or decomposing at least partially or temporarily at least a portion of the substrate material in or within an elongated volume of the substrate sheet material around the longitudinal axis of the laser beam, corresponding to each of said discrete positions, thereby generating an array of laser-modified tracks within the substrate material, each comprising an elongated volume of modified substrate material at least 4 times longer than its width extending at least partially through a thickness of the substrate sheet, wherein the modified shape of the substrate material has a refractive index that is different from the general refractive index n of the unmodified substrate; wherein each ordered two-dimensional array of laser-modified tracks thereby collectively produces diffracted light incident on the substrate sheet to form an observable shape, image, or color region.

39. The method according to claim 38, wherein the laser is a femtolaser having a pulse duration in the range of 0.1 fs to 100 hp.

40. The method according to claim 38, wherein in the irradiation step the laser and the substrate sheet are movable relative to each other to expose said discrete positions on the polymer sheet to the laser radiation pulses.

41. The method according to claim 38, wherein the laser beam is divided into multiple beams prior to its interaction with the substrate, each corresponding to one of the discrete positions on the substrate sheet.

42. The method according to claim 38, wherein the substrate sheet is irradiated with multiple lasers, each with a modified laser beam shape or focus, simultaneously, to generate the laser-modified track array.

43. The method according to claim 38, wherein the modification of the shape or focus of the laser beam comprises passing the laser beam through a lens or silica plate before its interaction with the substrate sheet.

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