Manipulation of optical interference from reflective microstructures via evanescent field absorption

Abstract

Provided is a substrate that exhibits an interference pattern upon reflection of incident electromagnetic radiation, articles incorporating said substrate, and methods of making said substrate.

Description

MANIPULATION OF OPTICAL INTERFERENCE FROM REFLECTIVEMICROSTRUCTURES VIA EVANESCENT FIELD ABSORPTIONCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to U.S. Provisional Application No. 63 / 434,823, filed December 22, 2022, which is incorporated by reference herein in its entirety.GOVERNMENT SUPPORT CLAUSE

[0002] This invention was made with government support under Grant No. IIP2016420 awarded by the National Science Foundation and under Grant No. FA9550-21-1-0150 awarded by the U.S. Air Force Office of Scientific Research. The Government has certain rights in the invention.BACKGROUND

[0003] Coloration significantly influences how people perceive and interact with the world. Structural colors created by interference of light are of special interest because they do not fade (unlike dyes and pigments) and exhibit iridescence, meaning the colors shift position or hue with illumination or viewing angle. Beyond aesthetic applications of color in selfexpression, such as for cosmetics and apparel, structural color and tailored spectral reflectance of coatings are useful for technological, safety / security, and military applications with broad societal impact. Examples include color-shifting holograms on security labels and anti-counterfeiting measures, camouflaging coatings, retroreflective signs and road markings, and laser-guided navigation. However, current strategies to enable structural coloration have not yet been widely adopted in industrial and commercial application due to a number of challenges. Structural colors can be expensive to process and may have limited optical customizability (e.g. hue / shade / tint, angular positions and separation of colors, interference pattern) due to restrictions in geometry imposed by nano-manufacturing challenges.

[0004] Accordingly, there is a need for improved structural coloration and articles including the same. These needs and others are at least partially satisfied by the present disclosure.SUMMARY

[0005] Disclosed herein is modified structural color produced by surfaces containing reflective microstructures through the enhanced absorption of electromagnetic radiation bychemical dyes and other molecules. This can be realized by adding dyes and / or other optically active species (e.g., absorptive molecules) between two layers forming an interface responsible for TIR. In some aspects, the optically active species can comprise an organic, optically active species (e.g. a dye, such a dye that absorbs light in the visible region, the UV region, and / or the IR region of the electromagnetic spectrum). In other aspects, the optically active species can comprise an inorganic, optically active species (e.g., a plasmonic nanoparticle or nanostructure). A combination of organic and inorganic optically active species can also be used.

[0006] Due to the forward propagation of evanescent waves generated when light experiences a TIR event, optically active species (e.g., dyes and molecules) located near the interface may interact with the reflecting light, for example, partially absorbing its intensity at specific wavelength. Due to the multiple TIR events occurring within microstructures, an enhancement in the absorption of light can be observed to occur by the dye molecules greater than if placed on a similar unstructured surface. Surfaces and coatings with distinctive and tunable optical reflectance properties are relevant to variety of applications including security features, transportation markings, optical sensors for chemicals and biomolecules, decorative embellishments, and military camouflage. Manufactured films utilizing such a surface can exhibit visually distinguished optically variable appearances differentiated by their unique color-shifts (angular separation of colors, ordering of colors, shades and hues). The final product could be applied to a target surface (such as a banknote or passport documents) through lamination or using an adhesive layer. The surfaces can also be integrally formed within an article if desired.

[0007] In an aspect, provided is a substrate that exhibits an interference pattern upon reflection of incident electromagnetic radiation, the substrate including: a first material having a first refractive index; a second material having a second refractive index disposed on the first material; and an absorptive layer including an organic, optically active species disposed between and abutting the first material and the second material. The substrate can include a plurality of microstructures, each of which can include the first material, the second material abutting the first material, an interface between the first material and the second material, and the absorptive layer disposed at the interface. The interface can be configured such that at least a portion of electromagnetic radiation incident a surface of the substrate at least one illumination angle interacts with the interface such that the substrate exhibits the interference pattern upon reflection of the incident electromagnetic radiation.

[0008] In another aspect, provided is a substrate that exhibits an interference pattern upon reflection of incident electromagnetic radiation, the substrate including: a first material having a first refractive index; a second material having a second refractive index disposed on the first material; and an absorptive layer including an inorganic, optically active species disposed between and abutting the first material and the second material. The substrate can include a plurality of microstructures, each of which can include the first material, the second material abutting the first material, an interface between the first material and the second material, and the absorptive layer disposed at the interface. The interface can be configured such that at least a portion of electromagnetic radiation incident a surface of the substrate at least one illumination angle interacts with the interface such that the substrate exhibits the interference pattern upon reflection of the incident electromagnetic radiation.

[0009] In another aspect, provided is an article including any of the disclosed substrates.

[0010] In another aspect, provided is a method of manufacturing any of the disclosed substrates, including: forming the plurality of microstructures with the first material; coating the plurality of microstructures with the absorptive layer; and coating the absorptive layer with the second material.

[0011] In another aspect, provided is a substrate, comprising: a first component and a second component disposed on the first component; and a curved, microscale interface between the first component and the second component, the interface configured such that at least a portion of electromagnetic radiation incident to a surface of the interface undergoes total internal reflection between the first component and the second component; an absorptive layer comprising an optically active species disposed between and abutting the first component and the second component; wherein the first component has a first refractive index greater than a second refractive index of the second component; and wherein the electromagnetic radiation undergoes a change in amplitude during total internal reflection.

[0012] In another aspect, provided is a substrate, comprising: a plurality of domed structures formed on a surface, the surface comprising a first material; a second component adjacent the plurality of domed structures and comprising a second material; an absorptive layer comprising an optically active species disposed between and abutting the first material and the second material; wherein the first material has a refractive index greater than a refractive index of the second material, such that incident electromagnetic radiation undergoes total internal reflection at a curved surface of each domed structure at a microscale interface between the first material and the second material.

[0013] Other systems, methods, features and / or advantages will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features and / or advantages be included within this description and be protected by the accompanying claims.BRIEF DESCRIPTION OF DRAWINGS

[0014] FIGURES 1A-1D depict exemplary substrates in accordance with the present disclosure.

[0015] FIGURE 2A depicts an exemplary method of fabricating a substrate in accordance with the present disclosure.

[0016] FIGURE 3A shows a schematic of the process to replicate epoxy microstructures from a master sample fabricated through greyscale lithography. Silicone molds are cured off of the greyscale master sample to end with convex hemicylinders. OG-142 epoxy is UV- cured from the convex hemicylinder silicone mold to yield concave hemicylinders. Black pigmented silicone is cured onto the back of the concave hemicylinders to yield bright, iridescent colors. Reproduced with permission from ref 1. 2023 John Wiley and Sons. FIG. 3B shows an optical micrograph of the cross section of the iridescent, hemicylinder microstructures.

[0017] FIGURE 4 depicts approaches for fabricating microstructured substrates exhibiting optically variable spectral properties including evanescent absorbing layers. Layers of absorbing components may be applied to surfaces containing either concave or convex microstructures after microreplication processing. Additional layers of solid material (e.g. polymers, oxides, glass) may be then applied on top of the microreplicated substrate coated with absorbing components to prepare a microstructured interface with the appropriate combination of refractive indices to generate optical paths for reflected light that includes multiple TIR events.

[0018] FIGURES 5A-5C depict tuning reflected colors by altering absorptive layer density. FIG. 5A shows schematics of prepared microstructures with no dye (I), 0.5% w / v dye evaporated (II), and 1.4% w / v dye evaporated (III). The associated images of the far field projections are shown below taken at 9=45°, 4>=0° . FIG. 5B shows a normalized reflectance spectra of microstructure samples I-III of the indicated position on the far field projections in (a). Normalized absorbance spectrum of Sudan Yellow shown in the dotted line. FIG. 5C shows a CIE space plot of each reflectance spectra shown in FIG. 5B.

[0019] FIGURES 6A-6C depict tunable effects on visible reflected color using different optically absorbing layers. FIG. 6A shows images of far field projections of each visible absorbing dye thermally evaporated onto the microstructures taken at 9=45°, 4>=0°. Associated molecular structures of each dye shown to the left. FIG. 6B shows a normalized reflectance spectra of non-dyed and dyed microstructures of the indicated position on the far field projections in FIG. 6A. Normalized absorbance spectrum of the molecular dye shown in the dotted line. FIG. 6C shows a CIE space plot of each reflectance spectra shown in FIG. 6B. The arrow indicates the color shift from non-dyed to dyed microstructures.

[0020] FIGURES 7A-7B depict patterning of infrared dyes for covert optically variable reflectance effects. FIG. 7A shows schematics of checkerboard patterned microstructure with airbrushed IR-813 toluenesulfonate illuminated under visible room light and under an IR flashlight emitting at 850 nm. Images of the patterned microstructures are shown below. The dye is only visible under IR lighting conditions. FIG. 7B shows anormalized absorbance spectrum of IR-813 toluenesulfonate.

[0021] FIGURES 8A-8B depict alterative approaches for patterning the deposition of absorptive components onto microstructured surfaces. FIG. 8A shows asymmetry in the distribution of optically absorbing components across each microstructure interface may be achieved by for example: shadowing during the deposition process. FIG. 8B shows the patterning of one or multiple different chemically absorbing components onto surfaces using one or more of the described methods such as digital or analog printing, masked evaporation or spray coating.

[0022] FIGURE 9 depicts a schematic of thermal evaporation setup. The dye evaporates from the donor glass substrate on a hot plate to the epoxy microstructures separated by cover slips for masking.

[0023] FIGURES 10A-10C depict evanescent wave absorption by dye at the TIR interface is more effective at color modulation than dye at the top interface. FIG. 10A depicts experimental and computational far-field caustic from the microstructures in FIG. 14B. A flat screen was used here (instead of a hemispherical screen) to enable direct comparison with ray tracing simulations which use a flat detector. We only show here one half of the far field caustic; the other half is a mirror image of what is shown. Bounce trajectories from reflections on the two sides of the microstructure are mapped below the caustic and range from 3 to 6+ bounces (where 6+ encompasses m>6 bounces). Solid lines show the bounce trajectory ranges which result when the left half of the hemicylinder is illuminated and dotted lines show the bounce ranges which result when the right half is illuminated; the overallcoloration results from the additive overlap of interference colors from these various trajectories. Refer to reference 16. FIG. 10B top row shows: schematics of (i) microstructures with no dye, (ii) Sudan yellow dye evaporated onto the TIR interface, (iii) Sudan yellow dye evaporated onto the top flat surface. FIG. 10B bottom row shows: the associated optical photographs of the far field projections, taken at 9in=0°. The exit angles (0out) of the caustics are given. FIG. IOC shows the reflectance spectra of microstructure samples from (FIG. 10B, i-iii) at 9out=22°. The absorbance spectrum of Sudan yellow dye is shown in the dotted line.

[0024] FIGURES 11A-11C depict a schematic of the reflectance measurement setup at 9in=45° from the top view (FIG. 11 A) and side view (FIG. 11B). The light source was placed directly above the macro lens attached to the spectrograph. FIG. 11C shows a portion of the far-field caustic was collected by the spectrograph for spectral analysis.

[0025] FIGURE 12A shows an example of spectra collected from the imaging spectrograph of a masked microstructure sample which was half dyed with Sudan yellow (rows 875-1025) and half non-dyed (rows 1025-1225). FIGURE 12B shows a zoomed in portion of spectra collected in FIG. 12A from rows 900-1150 showing the transition from Sudan yellow dyed microstructures to non-dyed microstructures spectra. FIGURE 12C shows a non-dyed microstructure reflectance spectra from FIG. 12B showing the slight changes in intensity at various positions of the sample collected.

[0026] FIGURE 13A shows an averaged reflectance spectra of non-dyed (black) and Sudan yellow dyed (orange) with background subtracted evenly across all wavelengths. FIGURE 13B shows an averaged white reference reflectance spectra with background subtracted evenly across all wavelengths. Two spectra are included to account for the slight difference in intensity at two different positions of the microstructure array. FIGURE 13C shows the reflectance spectra from FIG. 13A divided by the white reference reflectance spectra from FIG. 13B to remove the white light source from the spectrum. FIGURE 13D shows reflectance spectra were normalized to 1 by using the maximum of the non-dyed spectra and the minimum of the dyed spectra.

[0027] FIGURES 14A-14C depict tuning structural color through dye absorption of evanescent waves. FIG. 14A shows a general schematic of interference occurring from TIR at a concave microscale interface. Here, light rays reflecting two, three, and four times undergo interference to generate structural color. The specific ray trajectories shown in the schematic are not necessarily the same as in the microstructures used in experiments and are drawn here for illustrative purposes only. Radius of curvature (r), contact angle (0CA), andrefractive indices (n) are defined. Each TIR event generates an evanescent wave with penetration depth dP. Dye present at the interface interacts with the evanescent wave through wavelength-specific absorption. FIG. 14B shows scanning electron micrographs of the hemicylinder microstructure array used in this work. The lower magnification image shows the microstructure array near the center of the sample (20° tilt) and the higher magnification shows that cylindrical edge of the microstructures (30° tilt). FIG. 14C shows macroscale photographic images of the iridescent microstructure array where half of the sample is coated with an evaporated layer of Sudan yellow or quinoline yellow dye. The sample remained in the same position relative to the incident light while the viewing angle (camera angle) was moved away from the normal (directly above microstructures) by the angle given.

[0028] FIGURES 15A-15C depict evanescent wave absorption by dye tunes the reflected color produced by TIR interference. FIG. 15A shows a schematic of the far field iridescence produced by the microstructures and definition of the illumination and exit angles. A translucent material, such as a half of a ping pong ball, is used as a screen onto which reflected colors are projected and easily visualized. FIGS. 15B-15C show angularly resolved reflectance spectra of the non-dyed microstructure (FIG. 15B) and Sudan yellow dyed-coated structure (thermally evaporated from a 1.4% w / v coated donor substrate) (FIG. 15C). The caustic with labeled exit angles (bout) are given. Both spectra are normalized such that the intensities are directly comparable. The absorbance spectrum of Sudan yellow dye is shown at far right.

[0029] FIGURE 16 depicts a schematic of microstructures with a yellow pigment added to the silicone (top) and the associated optical photographs of the far field projections, taken at 9in=0° (bottom).

[0030] FIGURES 17A-17B depict an optical micrograph (FIG. 17A) and optical profilometry heatmap (FIG. 17B) of a flat epoxy sample that was masked during dye deposition such that the upper left corner has no dye and the lower right corner having thermally evaporated Sudan yellow dye. 5 nm of iridium was sputter coated onto the sample to increase reflectivity as needed to obtain the profilometry data. FIGURE 17C shows a plot of the height along the slice outlined in FIG. 17A.

[0031] FIGURES 18A-18C depict patterning of near-infrared absorbing dyes for generation of “invisible” iridescent signatures. FIG. 18A shows a schematic of the microstructure array used. This sample was composed of hemicylinder structures arranged in rings, where each ring had a different orientation of the hemicylinder axis, as seen in the zoomed-in transmission optical micrograph. There are also small arrays of hemispheres in the squareinterstitial areas (not drawn). A near-infrared absorbing dye, IR-813 toluenesulfonate (0.06% w / v in methanol) was deposited onto the center pattern of the array. FIG. 18B shows a normalized absorbance spectrum of IR-813 toluenesulfonate. FIG. 18C shows photographs of the microstructure array under broad spectrum room lighting (left column) and nearinfrared (850 nm, right column). Color-shifting and motion effects were seen from the sample under the white light illumination. The near- infrared absorbing dye, which was deposited in the center only, was not observable under visible light. However, when near-infrared illumination (850 nm) was used, and imaged with a cell phone camera, the centermost region becomes largely invisible due to evanescent wave absorption by the dye. The uncoated regions of the sample still exhibited the motion effects upon changing illumination angle.

[0032] FIGURE 19 depicts a control experiment with a flat piece of epoxy coated with the near-IR absorbing dye patterned in a square in the center of the image. As expected, there is no reflection from the sample, under either white light or near-IR light, because there are no structures present to support TIR. Thus the entire sample appears black, both in the regions with dye and the regions without dye.

[0033] FIGURES 20A-20B depict optical sensing based on evanescent field detection of interfacially adsorbed molecular species. Embodiments of the invention may be created using a substrate containing convex microstructures formed in a high index material (e.g. a polymer, glass, or oxide) backed with a lower index liquid (e.g. water). The composition of the liquid may be adjusted, and the substrate interfacial adsorption properties prepared such that changes to the presence of solutes in the liquid result in observable differences to the spectral reflectance from microstructures. The liquid may contain a known or unknown concentration of chemical solutes (e.g. proteins, enzymes, DNA, RNA) that may be reversibly or irreversibly interfacially active and possess with unique optically absorption properties such that their presence and concentration be correlated with the spectral reflectance signature detected by microscope or by eye. The targeted adsorption of molecular species to the reflecting interface may be primed by the attachment of receptor ligands with binding properties tailored to interact with specific molecules.

[0034] The components in the drawings are not necessarily to scale relative to each other. Like reference, numerals designate corresponding parts throughout the several views.DETAILED DESCRIPTION

[0035] It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate aspects, can also be provided in combination with a single aspect. Conversely, various features of the disclosure, which are, for brevity, described in the contextof a single aspect, can also be provided separately or in any suitable subcombination. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure.DEFINITIONS

[0036] In this specification and in the claims that follow, reference will be made to a number of terms, which shall be defined to have the following meanings:

[0037] Throughout the description and claims of this specification, the word “comprise” and other forms of the word, such as “comprising” and “comprises,” means including but not limited to, and are not intended to exclude, for example, other additives, segments, integers, or steps. Furthermore, it is to be understood that the terms comprise, comprising, and comprises as they relate to various aspects, elements, and features of the disclosed invention also include the more limited aspects of “consisting essentially of’ and “consisting of.”

[0038] As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a “polymer” includes aspects having two or more such polymers unless the context clearly indicates otherwise.

[0039] Ranges can be expressed herein as from “about” one particular value and / or to “about” another particular value. When such a range is expressed, another aspect includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It should be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0040] As used herein, the terms “optional” or “optionally” mean that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

[0041] For the terms “for example” and “such as,” and grammatical equivalences thereof, the phrase “and without limitation” is understood to follow unless explicitly stated otherwise.

[0042] The terms “iridescent” and “iridescence” as used herein are each given its ordinary meaning in the art and generally refer to color that changes as a function of light incidence and / or viewing angle.

[0043] As used herein, a “fluid” is given its ordinary meaning, i.e., a liquid or a gas. A fluid cannot maintain a defined shape and will flow during an observable time frame to fill thecontainer in which it is put. Thus, the fluid may have any suitable viscosity that permits flow. If two or more fluids are present, each fluid may be independently selected among essentially any fluids (liquids, gases, and the like) by those of ordinary skill in the art.EXAMPLE SUBSTRATES AND ARTICLES

[0044] FIGS. 1A-1D depict example substrates that exhibit an interference pattern upon reflection of incident electromagnetic radiation in accordance with the present disclosure. FIG. 1A shows a cross section of an exemplary substrate 100a which includes a first material 102 and a second material 104 disposed on the first material 102, thereby forming an interface 106a. The first material 102 has a first refractive index, and the second material 104 has a second refractive index.

[0045] In some aspects, the first material can comprise a polymer (e.g., polyethylene, polydimethylsiloxane). In certain aspects, the polymer is a block copolymer. In certain aspects, the polymer is a liquid crystal polymer (e.g., a thermotropic liquid crystal polymer, a reflective liquid crystal). In certain aspects, the polymer is a biopolymer (e.g., gelatin, alginate). Non-limiting examples of suitable polymers include poly dimethyl siloxane, polycarbonate, acrylics (e.g., polymethyl methacrylate), polyesters, polyethylene, polyethylene terephthalate, polyethylene glycol, polyolefins, polypropylene, and polystyrene. In certain aspects, multiple polymers layers are used to create a reflective surface, such as a distributed Bragg reflector. Other polymers are also possible and those of ordinary skill in the art would be capable of selecting such polymers based upon the teachings of this specification. In some aspects, the polymer includes a thermoset, such as an epoxy, a polyurethane, Bakelite, a polyimide, or any combination thereof.

[0046] In certain aspects, the first material can comprise a thermoplastic, such as a polyester, a polyolefin, acrylic, acrylonitrile butadiene styrene (ABS), a polyamide, or any combination thereof. In some aspects, the first material can comprise a glass. In some embodiments, the first material can comprise a metal. In some embodiments, the first material can comprise a semiconductor.

[0047] The term material, as used herein, generally refers to a portion of a substrate comprising a group of substantially similar molecules, a group of substantially similar compounds, and / or a phase of matter (e.g., a non-aqueous phase, an aqueous phase) comprising such molecules and / or compounds. Those skilled in the art would understand that the term material is not intended to refer to a single molecule or atom. In some embodiments, a material is a liquid phase (e.g., a gas phase, an aqueous phase, non-aqueous phase) comprising a group of substantially similar compounds and / or molecules. In someembodiments, a material is a solid phase (e.g., a polymer, glass). In some embodiments, a material is a gel. For example, in some cases, each material may occupy at least about 1 vol %, at least about 2 vol %, at least about 5 vol %, at least about 10 vol %, at least about 20 vol %, at least about 50 vol %, at least about 70 vol %, at least about 90 vol %, at least about 95 vol %, or at least about 99 vol % of the total volume of the two or more materials.

[0048] In some aspects, the first material is present in each substrate in an amount greater than or equal to 10 vol %, greater than or equal to 15 vol %, greater than or equal to 20 vol %, greater than or equal to 25 vol %, greater than or equal to 30 vol %, greater than or equal to 35 vol %, greater than or equal to 40 vol %, greater than or equal to 45 vol %, greater than or equal to 50 vol %, greater than or equal to 55 vol %, greater than or equal to 60 vol %, greater than or equal to 65 vol %, greater than or equal to 70 vol %, greater than or equal to 75 vol %, greater than or equal to 80 vol %, or greater than or equal to 85 vol % on average versus the total volume of all materials within each substrate.

[0049] In certain aspects, the first material is present in each substrate in an amount less than or equal to 90 vol %, less than or equal to 85 vol %, less than or equal to 80 vol %, less than or equal to 75 vol %, less than or equal to 70 vol %, less than or equal to 65 vol %, less than or equal to 60 vol %, less than or equal to 55 vol %, less than or equal to 50 vol %, less than or equal to 45 vol %, less than or equal to 40 vol %, less than or equal to 35 vol %, less than or equal to 30 vol %, less than or equal to 25 vol %, less than or equal to 20 vol %, or less than or equal to 15 vol % on average versus the total volume of all materials within each substrate.

[0050] Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 10 vol % and less than or equal to 90 vol %, greater than or equal to 35 vol % and less than or equal to 65 vol %, greater than or equal to 45 vol % and less than or equal to 55 vol %). Other ranges are also possible.

[0051] In some aspects, the second material comprises a polymer (e.g., polyethylene, polydimethylsiloxane). In certain aspects, the polymer is a block copolymer. In certain embodiments, the polymer is a liquid crystal polymer (e.g., a thermotropic liquid crystal polymer, a reflective liquid crystal). In certain aspects, the polymer is a biopolymer (e.g., gelatin, alginate). Non-limiting examples of suitable polymers include polydimethylsiloxane, polycarbonate, acrylics (e.g., polymethyl methacrylate), polyesters, polyethylene, polyethylene terephthalate, polyethylene glycol, polyolefins, polypropylene, and polystyrene. In certain aspects, multiple polymers layers are used to create a reflective surface, such as a distributed Bragg reflector. Other polymers are also possible and those of ordinary skill in theart would be capable of selecting such polymers based upon the teachings of this specification.

[0052] In some aspects, the second material can comprise a thermoplastic, such as a polyester, a polyolefin, acrylic, acrylonitrile butadiene styrene (ABS), a polyamide, or any combination thereof. In some aspects, the second material can comprise a glass. In some aspects, the second material can comprise a metal. In some aspects, the second material can comprise a semiconductor. In some aspects, the second material includes a curable resin. In some aspects, the second material includes a UV curable resin. In some aspects, the second material includes a thermosetting resin. In some aspects, the second material includes an inorganic material, such as MgF2, SiCh, TiCh, or AI2O3. In some aspects, the second material includes a fluid.

[0053] In certain aspects, the second material is present in each substrate in an amount greater than or equal to 10 vol %, greater than or equal to 15 vol %, greater than or equal to 20 vol %, greater than or equal to 25 vol %, greater than or equal to 30 vol %, greater than or equal to 35 vol %, greater than or equal to 40 vol %, greater than or equal to 45 vol %, greater than or equal to 50 vol %, greater than or equal to 55 vol %, greater than or equal to 60 vol %, greater than or equal to 65 vol %, greater than or equal to 70 vol %, greater than or equal to 75 vol %, greater than or equal to 80 vol %, or greater than or equal to 85 vol % on average versus the total volume of all materials within the substrate.

[0054] In some aspects, the second material is present in each substrate in an amount less than or equal to 90 vol %, less than or equal to 85 vol %, less than or equal to 80 vol %, less than or equal to 75 vol %, less than or equal to 70 vol %, less than or equal to 65 vol %, less than or equal to 60 vol %, less than or equal to 55 vol %, less than or equal to 50 vol %, less than or equal to 45 vol %, less than or equal to 40 vol %, less than or equal to 35 vol %, less than or equal to 30 vol %, less than or equal to 25 vol %, less than or equal to 20 vol %, or less than or equal to 15 vol % on average versus the total volume of all materials within each substrate.

[0055] Combinations of the above-referenced ranges are also possible (e.g., greater than or equal to 10 vol % and less than or equal to 90 vol %, greater than or equal to 35 vol % and less than or equal to 65 vol %, greater than or equal to 45 vol % and less than or equal to 55 vol %). Other ranges are also possible.

[0056] In some aspects, the difference between the first refractive index and the second refractive index (measured at 20°C. at the wavelength of electromagnetic radiation undergoing total internal reflection) is at least 0.01, such as from 0.01 to 2, or from 0.05 to1.5, or from 0.1 to 1.25, or from 0.15 to 1, or from 0.2 to 0.75, or from 0.25 to 0.5, or from 0.3 to 0.45, or from 0.35 to 0.4, or from 0.01 to 0.5, or from 0.02 to 0.5, or from 0.03 to 0.5 or from 0.03 to 0.5, or from 0.04 to 0.5, or from 0.05 to 0.5, or from 0.1 to 0.25, or from 0.5 to 2, or from 0.55 to 1.75, or from 0.6 to 1.5, or from 0.65 to 1.25, or from 0.6 to 1.

[0057] The interface 106a is configured such that at least a portion of electromagnetic radiation incident a surface of the substrate 100a at least one illumination angle interacts with the interface 106a such that the substrate 100a exhibits the interference pattern upon reflection of the incident electromagnetic radiation. In some cases, the interface 106a can be arcuate and concave relative to the incident electromagnetic radiation.

[0058] An absorptive layer 108 is disposed at the interface 106a, i.e., between and abutting the first material 102 and the second material 104. The absorptive layer can comprise any suitable optically active species (e.g., a species which absorbs and / or emits incident electromagnetic radiation). In some aspects, the absorptive layer includes an organic, optically active species. In some aspects, the absorptive layer includes an inorganic optically active species.

[0059] In some embodiments, the optically active species can comprise a dye, a pigment, or a combination thereof. Examples of dyes and pigments include, but are not limited to, Direct Yellow 86, Acid Red 249, Direct Blue 199, Direct Black 168, Direct Yellow 132, Reactive Black 31, Direct Yellow 157, Reactive Yellow 37, Acid Yellow 23, Reactive Red 180, Acid Red 52, Acid blue 9, Direct Blue 86k Reactive Red 4, Reactive Red 56, Acid Red 92, Reactive Red 31, the Pro-Jet series of dyes available from Avecia Ltd., including Pro-Jet Yellow I, Pro-Jet Magenta I, Pro-Jet Cyan I, Pro-Jet Black I, and Pro-Jet Yellow 1-G; Aminyl Brilliant Red F-B (Sumitomo Chemical Co.); the Duasyn line of "salt-free" dyes available from Hoechst, such as Duasyn Direct Black HEF-SF, Duasyn Black RL-SF, Duasyn Direct Yellow 6G-SF VP216, Duasyn Brilliant Yellow GL-SF VP220, Duasyn Acid Yellow XX-SF VP413, Duasyn Brilliant Red F3B-SF VP218, Duasyn Rhodamine B-SF VP353, Duasyn Direct Turquoise Blue FRL-SF VP368, and Duasyn Acid Blue AE-SF VP344; mixtures thereof; and the like; Tricon Acid Red 52, Tricon Direct Red 227, and Tricon Acid Yellow 17 (Tricon Colors Incorporated), Bernacid Red 2BMN, Pontamine Brilliant Bond Blue A, BASF X-34, Pontamine, Food Black 2, Catodirect Turquoise FBL Supra Cone. (Carolina Color and Chemical), Special Fast Turquoise 8GL Liquid (Mobay Chemical), Intrabond Liquid Turquoise GLL (Crompton and Knowles), Cibracron Brilliant Red 38-A (Aldrich Chemical), Drimarene Brilliant Red X-2B (Pylam, Inc.), Levafix Brilliant Red E-4B (Mobay Chemical), Levafix Brilliant Red E-6B A (Mobay Chemical), Pylam Certified D&C Red #28 (Pylam), Direct Brill Pink B Ground Crude (Crompton & Knowles), Cartasol Yellow GTF Presscake(Sandoz, Inc.), Tartrazine Extra Cone. (FD&C Yellow #5, Acid Yellow 23, Sandoz, Inc.), Catodirect Yellow RL (Direct Yellow 86, Carolina Color and Chemical), Cartasol Yellow GTF Liquid Special 110 (Sandoz, Inc.), D&C Yellow #10 (Yellow 3, Tricon), Yellow Shade 16948 (Tricon), Basacid Black X34 (BASF), Carta Black 2GT (Sandoz, Inc.), Neozapon Red 492 (BASF), Orasol Red G (Ciba-Geigy), Direct Brilliant Pink B (Crompton- Knolls), Aizen Spilon Red C-BH (Hodagaya Chemical Company), Kayanol Red 3BL (Nippon Kayaku Company), Levanol Brilliant Red 3BW (Mobay Chemical Company), Levaderm Lemon Yellow (Mobay Chemical Company), Aizen Spilon Yellow C-GNH (Hodagaya Chemical Company), Spirit Fast Yellow 3G, Sirius Supra Yellow GD 167, Cartasol Brilliant Yellow 4GF (Sandoz), Pergasol Yellow CGP (Ciba-Geigy), Orasol Black RL (Ciba-Geigy), Orasol Black RLP (Ciba- Geigy), Savinyl Black RLS (Sandoz), Dermacarbon 2GT (Sandoz), Pyrazol Black BG (ICI Americas), Morfast Black Cone A (Morton-Thiokol), Diazol Black RN Quad (ICI Americas), Orasol Blue GN (Ciba-Geigy), Savinyl Blue GLS (Sandoz, Inc.), Luxol Blue MBSN (Morton-Thiokol), Sevron Blue 5GMF (ICI Americas), and Basacid Blue 750 (BASF); Levafix Brilliant Yellow E-GA, Levafix Yellow E2RA, Levafix Black EB, Levafix Black E-2G, Levafix Black P-36 A, Levafix Black PN-L, Levafix Brilliant Red E6BA, and Levafix Brilliant Blue EFFA, all available from Bayer; Procion Turquoise PA, Procion Turquoise HA, Procion Turquoise Ho5G, Procion Turquoise H-7G, Procion Red MX-5B, Procion Red H8B , Procion Red MX 8B GNS, Procion Red G, Procion Yellow MX- 8G, Procion Black H-EXL, Procion Black P-N, Procion Blue MX-R, Procion Blue MX- 4GD, Procion Blue MX-G, and Procion Blue MX-2GN, all available from ICI Americas; Cibacron Red F-B, Cibacron Black BG, Lanasol Black B, Lanasol Red 5B, Lanasol Red B, and Lanasol Yellow 46, all available from Ciba-Geigy; Baslien Black P-BR, Baslien Yellow EG, Baslien Brilliant Yellow P-3 GN, Baslien Yellow M-6GD, Baslien Brilliant Red P-3B, Baslien Scarlet E-2G, Baslien Red E- B, Baslien Red E-7B, Baslien Red M-5B, Baslien Blue E-R, Baslien Brilliant Blue P-3R, Baslien Black P-BR, Baslien Turquoise Blue P-GR, Baslien Turquoise M- 2G, Baslien Turquoise E-G, and Baslien Green E-6B, all available from BASF; Sumifix Turquoise Blue G, Sumifix Turquoise Blue H-GF, Sumifix Black B, Sumifix Black H-BG, Sumifix Yellow 2GC, Sumifix Supra Scarlet 2GF, and Sumifix Brilliant Red 5BF, all available from Sumitomo Chemical Company; Intracron Yellow C-8G, Intracron Red C-8B, Intracron Turquoise Blue GE, Intracron Turquoise HA, and Intracron Black RL, all available from Crompton and Knowles, Dyes and Chemicals Division; Pro-Jet 485; Magenta 377; carbon black, titanium oxide, cobalt blue (COO-AI2O3), chrome yellow (PbCrCL), and iron oxide. Suitable organic pigments include, for example, azo pigmentsincluding diazo pigments and monoazo pigments, polycyclic pigments(e.g., phthalocyanine pigments such as phthalocyanine blues and phthalocyanine greens, perylene pigments, perynone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, pyranthrone pigments, and quinophthalone pigments), insoluble dye chelates (e.g., basic dye type chelates and acidic dye type chelate), nitropigments, nitroso pigments, phthalocyanine blues including copper phthalocyanine blue and derivatives thereof (Pigment Blue 15), quinacridones including Pigment Orange 48, Pigment Orange 49, Pigment Red 122, Pigment Red 192, Pigment Red 202, Pigment Red 206, Pigment Red 207, Pigment Red 209, Pigment Violet 19 and Pigment Violet 42, anthraquinones including Pigment Red 43, Pigment Red 194 (Perinone Red), Pigment Red 216 (Brominated Pyranthrone Red) and Pigment Red 226 (Pyranthrone Red), perylenes including Pigment Red 123 (Vermillion), Pigment Red 149 (Scarlet), Pigment Red 179 (Maroon), Pigment Red 190 (Red), Pigment Violet 19, Pigment Red 189 (Yellow Shade Red) and Pigment Red 224, thioindigoids including Pigment Red 86, Pigment Red 87, Pigment Red 88, Pigment Red 181 , Pigment Red 198, Pigment Violet 36, and Pigment Violet 38, heterocyclic yellows including Pigment Yellow 1 , Pigment Yellow 3, Pigment Yellow 12, Pigment Yellow 13, Pigment Yellow 14, Pigment Yellow 17, Pigment Yellow 65, Pigment Yellow 73, Pigment Yellow 74, Pigment Yellow 151 , Pigment Yellow 117, Pigment Yellow 128, Pigment Yellow 155, Pigment Yellow 83, Pigment Yellow 213, Pigment Yellow 138, CABO-JET 250C, CABO- JET 260M, and CABO-JET 270Y, PALIOGEN Orange, PALIOGEN Orange 3040, PALIOGEN Blue L 6470, PALIOGEN Violet 5100, PALIOGEN Violet 5890, PALIOGEN Yellow 1520, PALIOGEN Yellow 1560, PALIOGEN Red 3871 K, PALIOGEN Red 3340, HELIOGEN Blue L 6901 F, HELIOGEN Blue NBD 7010, HELIOGEN Blue K 7090, HELIOGEN Blue L 7101 F, HELIOGEN Blue L6900, L7020, HELIOGEN Blue D6840, HELIOGEN Blue D7080, HELIOGEN Green L8730, HELIOGEN Green K 8683, HELIOGEN Green L 9140, CHROMOPHTAL Yellow 3G, CHROMOPHTAL Yellow GR, CHROMOPHTAL Yellow 8G, IGRAZIN Yellow 5GT, IGRALITE Rubine 4BL, IGRALITE Blue BCA, MONASTRAL Magenta, MONASTRAL Scarlet, MONASTRAL Violet R, MONASTRAL Red B, MONASTRAL Violet Maroon B, DALAMAR Yellow YT-858-D, HEUCOPHTHAL Blue G XBT-583D, Permanent Yellow GR, Permanent Yellow G, Permanent Yellow DHG, Permanent Yellow NCG-71 , Permanent Yellow GG, Hansa Yellow RA, Hansa Brilliant Yellow 5GX-02, Hansa Yellow-X, NOVOPERM Yellow HR, NOVOPERM Yellow FGL, Hansa Brilliant Yellow 1OGX, Permanent Yellow G3R-01,HOSTAPERM Yellow H4G, HOSTAPERM Yellow H3G, HOSTAPERM Orange GR, HOSTAPERM Scarlet GO, HOSTAPERM Pink E, Permanent Rubine F6B, the HOSTAFINE series, QUINDO Magenta, INDOFAST Brilliant Scarlet, QUINDO Red R6700, QUINDO Red R6713, INDOFAST Violet, L74-1357 Yellow, L75-1331 Yellow, and L75-2577 Yellow, Normandy Magenta RD-2400, Permanent Violet VT2645, Argyle Green XP-111-S, Brilliant Green Toner GR 0991 , Sudan Blue OS, PV Fast Blue B2GO1 , Sudan III, Sudan II, Sudan IV, Sudan Orange G, Sudan Orange 220, Ortho Orange OR 2673, Lithol Fast Yellow 0991 K, Paliotol Yellow 1840, Lumogen Yellow D0790, Suco-Gelb L1250, Suco- Yellow D1355, Fanal Pink D4830, Cinquasia Magenta, Lithol Scarlet D3700, Toluidine Red, Scarlet for Thermoplast NSD PS PA, E. D. Toluidine Red, Lithol Rubine Toner, Lithol Scarlet 4440, Bon Red C, Royal Brilliant Red RD-8192, Oracet Pink RF, and Lithol Fast Scarlet L4300. Examples of other suitable colored pigments are described in the Colour Index, 3rd edition (The Society of Dyers and Colourists, 1982).

[0060] In some aspects, the organic, optically active species includes a visible dye (a dye that absorbs electromagnetic radiation in the visible region of the electromagnetic spectrum). In some aspects, the organic, optically active species includes Sudan yellow, quinoline yellow, solvent blue 59, phthalocyanine green, or any combination thereof. In some aspects, the organic, optically active species includes a non-visible dye (a dye that absorbs electromagnetic radiation in a region of the electromagnetic spectrum outside of the visible region).

[0061] In some aspects, the non-visible dye includes an infrared dye (a dye that exhibits a maximum absorption or a significant absorption in the infrared or near-infrared region of the electromagnetic spectrum), such as IR-813 toluenesulfonate, polymethyl indoliums, metal complex IR dyes, indocyanine green, polymethine dyes, croconium dyes, cyanine dyes, merocyanine dyes, squarylium dyes, chalcogenopyryloarylidene dyes, metal thiolate complex dyes, bis(chalcogenopyrylo)polymethine dyes, oxyindolizine dyes, bis(aminoaryl)polymethine dyes, indolizine dyes, pyrylium dyes, quinoid dyes, quinone dyes, phthalocyanine dyes, naphthalocyanine dyes, azo dyes, (metalized) azomethine dyes, DyLight-680, DyLight-750, VivoTag-750, DyLight-800, IRDye-800, VivoTag-680, Cy5.5, or an indocyanine green (ICG) and any derivative of the foregoing, cyanine dyes, acradine orange or yellow, ALEXAFLUORs and any derivative thereof, 7-actinomycin D, 8- anilinonaphthalene-1 -sulfonic acid, ATTO dye and any derivative thereof, auraminerhodamine stain and any derivative thereof, bensantrhone, bimane, 9-10- bis(phenylethynyl)anthracene, 5,12 - bis(phenylethynyl)naththacene, bisbenzimide, brainbow, calcein, carbodyfluorescein and any derivative thereof, l-chloro-9,10-bis(phenylethynyl)anthracene and any derivative thereof, DAP I, DiOC6, DyLight Fluors and any derivative thereof, epicocconone, ethidium bromide, FlAsH-EDT2, Fluo dye and any derivative thereof, FluoProbe and any derivative thereof, Fluorescein and any derivative thereof, Fura and any derivative thereof, GelGreen and any derivative thereof, GelRed and any derivative thereof, fluorescent proteins and any derivative thereof, m isoform proteins and any derivative thereof such as for example mCherry, hetamethine dye and any derivative thereof, hoeschst stain, iminocoumarin, indian yellow, indo-1 and any derivative thereof, laurdan, lucifer yellow and any derivative thereof, luciferin and any derivative thereof, luciferase and any derivative thereof, mercocyanine and any derivative thereof, nile dyes and any derivative thereof, perylene, phloxine, phyco dye and any derivative thereof, propium iodide, pyranine, rhodamine and any derivative thereof, ribogreen, RoGFP, rubrene, stilbene and any derivative thereof, sulforhodamine and any derivative thereof, SYBR and any derivative thereof, synapto-pHluorin, tetraphenyl butadiene, tetrasodium tris, Texas Red, Titan Yellow, TSQ, umbelliferone, violanthrone, yellow fluorescent protein and YOYO-1. Other Suitable fluorescent dyes include, but are not limited to, fluorescein and fluorescein dyes (e.g., fluorescein isothiocyanine or FITC, naphthofluorescein, 4', 5'-dichloro-2',7' - dimethoxyfluorescein, 6-carboxyfluorescein or FAM, etc.), carbocyanine, merocyanine, styryl dyes, oxonol dyes, phycoerythrin, erythrosin, eosin, rhodamine dyes (e.g., carboxytetramethyl-rhodamine or TAMRA, carboxyrhodamine 6G, carboxy-X-rhodamine (ROX), lissamine rhodamine B, rhodamine 6G, rhodamine Green, rhodamine Red, tetramethylrhodamine (TMR), etc. ), coumarin and coumarin dyes (e.g., methoxy coumarin, dialkylaminocoumarin, hydroxycoumarin, aminomethylcoumarin (AMCA), etc.), Oregon Green Dyes (e.g., Oregon Green 488, Oregon Green 500, Oregon Green 514, etc ), Texas Red, Texas Red-X, SPECTREIM RED, SPECTREIM GREEN, cyanine dyes (e.g, CY-3, Cy-5, CY-3.5, CY-5.5, etc ), ALEXA FLUOR dyes (e g, ALEXA FLUOR 350, ALEXA FLUOR 488, ALEXA FLUOR 532, ALEXA FLUOR 546, ALEXA FLUOR 568, ALEXA FLUOR 594, ALEXA FLUOR 633, ALEXA FLUOR 660, ALEXA FLUOR 680, etc ), BODIPY dyes (e g., BODIPY FL, BODIPY R6G, BODIPY TMR, BODIPY TR, BODIPY 530 / 550, BODIPY 558 / 568, BODIPY 564 / 570, BODIPY 576 / 589, BODIPY 581 / 591, BODIPY 630 / 650, BODIPY 650 / 665, etc. ), IRDyes (e.g, IRD40, IRD 700, IRD 800, etc ), or any combination thereof.

[0062] In some aspects, the non-visible dye includes an ultraviolet dye (a dye that exhibits a maximum absorption or a significant absorption in the UV region of the electromagnetic spectrum), such as ALEXA FLLiOR 350 and AMCA dyes (e.g., AMCA-X Dyes),derivatives of 7-aminocoumarin dyes, dialkylaminocoumarin reactive versions of ALEXA FLLIOR 350 dyes, ALEXA FLUOR 430 (and reactive UV dyes that absorb between 400 nm and 450 nm have appreciable fluorescence beyond 500 nm in aqueous solution), Marina Blue and Pacific Blue dyes (based on the 6,8- difluoro-7-hydroxy coumarin fluorophore), exhibit bright blue fluorescence emission near 460 nm, hydroxy coumarin and alkoxy coumarin derivatives, Zenon ALEXA FLUOR 350, Zenon ALEXA FLUOR 430 and Zenon Pacific Blue, succinimidyl ester of the Pacific Orange dye, Cascade Blue acetyl azide and other pyrene derivatives, ALEXA FLUOR 405 and its derivatives, pyrene succinimidyl esters, Cascade Yellow dye, PyMPO and pyridyl oxazole derivatives, aminonaphthalene-based dyes and dansyl chlorides, dapoxyl dyes (e.g., Dapoxyl sulfonyl chloride, amine-reactive Dapoxyl succinimidyl ester, carboxylic acid-reactive Dapoxyl (2- aminoethyl)sulfonamide), bimane dyes (e.g., bimane mercaptoacetic acid) and its derivatives, NBD dyes and its derivatives, QsY 35 dyes and its derivatives, fluorescein, or any combination thereof.

[0063] In some aspects, the absorptive layer includes an inorganic, optically active species. In some aspects, the inorganic, optically active species can comprise an inorganic pigment. In some aspects, the inorganic, optically active species includes plasmonic nanoparticles or nanostructures, such as Au, Ag, Cr, or Al nanoparticles or nanostructures. In some aspects, the inorganic, optically active species includes a metallic thin film, such as Al, Cr, Au, Ag, or any combination thereof.

[0064] In some aspects, the absorptive layer further includes a recognition element, such as a protein (e.g., an antibody), enzymes, DNA, RNA, or any combination thereof. These recognition elements can be selected so as to have affinity for and selectively bind to an analyte of interest. Binding of the analyte of interest at the recognition element can subsequently induce a change in the optical properties of the absorptive layer (and consequently a change in the visual appearance of the substrate / article). In this way, substrates and articles with sensory behavior for an analyte of interest can be generated.

[0065] In some aspects, the absorptive layer has a thickness of less than 250 nm, such as from 1 nm to 250 nm, or from 5 nm to 225 nm, or from 10 nm to 200 nm, or from 15 nm to 175 nm, or from 20 nm to 150 nm, or from 25 nm to 125 nm, or from 30 nm to 100 nm, or from 35 nm to 75 nm, or from 40 nm to 70 nm, or from 45 nm to 65 nm, or from 50 nm to 60 nm, or from 1 nm to 100 nm, or from 5 nm to 95 nm, or from 10 nm to 90 nm, or from 15 nm to 85 nm, or from 20 nm to 80 nm, or from 25 nm to 75 nm, or from 30 nm to 70 nm, or from 35 nm to 65 nm, or from 40 nm to 60 nm, or from 45 nm to 55 nm, or from 100 nm to 250 nm, or from 110 nm to 240 nm, or from 120 nm to 230 nm, or from 130 nm to 220 nm, orfrom 140 nm to 210 nm, or from 150 nm to 200 nm, or from 160 nm to 190 nm, or from 170 nm to 180 nm.

[0066] The substrate 100a further includes a plurality of microstructures 110a, each of which includes the first material 102, the second material 104 abutting the first material 102, the interface 106a between the first material 102 and the second material 104, and the absorptive layer 108 disposed at the interface 106a. In some aspects, the plurality of microstructures are disposed in a regular 2-dimensional array. In some embodiments, at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% of the plurality of TIR microstructures present within the substrate are arranged in a regular two-dimensional array. In some embodiments, less than or equal to 100%, less than or equal to 90%, less than or equal to 80%, less than or equal to70%, less than or equal to 60%, less than or equal to 50%, less than or equal to 40%, less than or equal to 30%, or less than or equal to 20% of the plurality of TIR microstructures present within the substrate are arranged in a regular two-dimensional array. Combinations of the above-referenced ranges are also possible (e.g., at least 10% and less than or equal to 100%). Other ranges are also possible.

[0067] In some aspects, the plurality of microstructures are disposed in a regular 3- dimensional array. In certain embodiments, at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% of the plurality of TIR microstructures present within the substrate are arranged in a regular three-dimensional array. In some embodiments, less than or equal to 100%, less than or equal to 90%, less than or equal to 80%, less than or equal to 70%, less than or equal to 60%, less than or equal to 50%, less than or equal to 40%, less than or equal to 30%, or less than or equal to 20% of the plurality of TIR microstructures present within the substrate are arranged in a regular three- dimensional array. Combinations of the above-referenced ranges are also possible (e.g., at least 10% and less than or equal to 100%). Other ranges are also possible.

[0068] The TIR microstructures described herein may have any suitable size. In some embodiments, the TIR microstructures have a largest cross-sectional dimension (e.g., diameter) of less than or equal to 250 microns, less than or equal to 200 microns, less than or equal to 100 microns, less than or equal to 50 microns, less than or equal to 25 microns, less than or equal to 10 microns, less than or equal to 5 microns, or less than or equal to 2 microns. In certain embodiments, the TIR microstructures may have a largest cross-sectional dimension of greater than or equal to 1 micron, greater than or equal to 2 microns, greater than or equal to 5 microns, greater than or equal to 10 microns, greater than or equal to 25 microns, greater than or equal to 50 microns, greater than or equal to 100 microns, or greaterthan or equal to 200 microns. Combinations of the above-referenced ranges (e.g., greater than or equal to 1 micron and less than or equal to 250 microns) are possible. Other ranges are also possible.

[0069] In some aspects, electromagnetic radiation reflected by the plurality of TIR microstructures exhibits variable intensity based on the illumination angle. In some aspects, electromagnetic radiation reflected by the plurality of TIR microstructures exhibits variable intensity based on an angle of observation relative to the surface. In some aspects, electromagnetic radiation reflected by the plurality of TIR microstructures exhibits structural color.

[0070] The substrate 100a can further comprise a third material 112 on top of the substrate 100a. In some aspects, the third material comprises a polymer (e.g., polyethylene, polydimethylsiloxane). In certain aspects, the polymer is a block copolymer. In certain embodiments, the polymer is a liquid crystal polymer (e.g., a thermotropic liquid crystal polymer, a reflective liquid crystal). In certain aspects, the polymer is a biopolymer (e.g., gelatin, alginate). Non-limiting examples of suitable polymers include polydimethylsiloxane, polycarbonate, acrylics (e.g., polymethyl methacrylate), polyesters, polyethylene, polyethylene terephthalate, polyethylene glycol, polyolefins, polypropylene, and polystyrene. In certain aspects, multiple polymers layers are used to create a reflective surface, such as a distributed Bragg reflector. Other polymers are also possible and those of ordinary skill in the art would be capable of selecting such polymers based upon the teachings of this specification.

[0071] In some aspects, the third material can comprise a thermoplastic, such as a polyester, a polyolefin, acrylic, acrylonitrile butadiene styrene (ABS), a polyamide, or any combination thereof. In some aspects, the third material can comprise a glass. In some aspects, the third material can comprise a metal. In some aspects, the third material can comprise a semiconductor. In some aspects, the third material includes a curable resin. In some aspects, the third material includes a UV curable resin. In some aspects, the third material includes a thermosetting resin. In some third, the second material includes an inorganic material, such as MgF2, SiCh, TiCh, or AI2O3. In some aspects, the third material includes a fluid. In some aspects, the substrate does not include a third material.

[0072] FIG. IB shows a cross section of an exemplary substrate 100b with an interface 106b and a plurality of microstructures 110b, which differs from the exemplary substrate 100a in that the interface 106b is convex relative to the incident electromagnetic radiation. FIG. 1C shows a cross section of an exemplary substrate 100c with an interface 106c and a plurality ofmicrostructures 110c, which differs from the exemplary substrate 100a in that the interface 106c includes a plurality of sides 116, thereby forming a truncated arcuate interface. FIG. ID shows a cross section of an exemplary substrate lOOd with an interface 106d and a plurality of microstructures 1 lOd, which differs from the exemplary substrate 100a in that the interface 106d includes a plurality of sides 116’, thereby forming a truncated arcuate interface, and is convex relative to the incident electromagnetic radiation. Interfaces 106c and 106d are depicted as having three sides for each microstructure 110c, 1 lOd. In some aspects, each microstructure can have four sides, or five sides, or six sides, or seven sides, or eight sides, or nine sides, or ten sides, or more than ten sides.

[0073] In another aspect, provided is an article including any of the disclosed substrates. In some aspects, the article is a bank note, passport document, driver’s license, government ID, or other article with a security film. In some aspects, the article is a sensor. In some aspects, the article is a display.METHODS

[0074] FIG. 2A depicts an exemplary method 200 of manufacturing a substrate in accordance with the present disclosure. First, the plurality of microstructures are formed 202 with the first material. In some aspects, forming the plurality of microstructures with the first material includes injection molding, cast molding, embossing, or reel-to-reel processing. Next, the plurality of microstructures are coated 204 with the absorptive layer. In some aspects, coating the plurality of microstructures with the absorptive layer includes deposition, spray coating, spin coating, blade coating, solvent or thermal evaporation, dip coating, sputtering, layer-by-layer assembly, or digital or analog printing processes. In some aspects, coating the plurality of microstructures with the absorptive layer includes embossing, thermoforming, or UV casting. Finally, the absorptive layer is coated 206 with the second material.EXAMPLES

[0075] The invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of non- critical parameters which can be changed or modified to yield essentially the same results.Materials and Methods

[0076] Materials and supplies: OG-142 UV-cure epoxy resin (Epoxy Technology), Moldstar3 IT silicone (Smooth On), black silicone pigment (Smooth On), glass slides, ping pong balls,l / 32”-thick translucent polystyrene sheet for far field projection screen (McMaster Carr, item #8734K31), 6 inch <100> test grade silicon wafers (University Wafer), AZ 50-XT photoresist (Integrated Micro Materials), AZ-400K developer, yellow silicone pigment (Smooth On), Sudan yellow (TCI, >98.0%), quinoline yellow (Sigma Aldrich, 95%), solvent blue 59, phthalocyanine green (Epolin), toluene (EMD, 90-100%), hexanes (MP), chloroform (Macron Fine Chemicals, >99.8%), methanol (Supelco, >99.9%), IR-813 toluenesulfonate (TCI, >98.0%).

[0077] Fabrication of microstructures with greyscale lithography: Microstructured surfaces were fabricated using Penn State’s shared user facilities following method detailed in a previous publication28and summarized in brief here. In general, microwell arrays were created in photoresist using greyscale lithography and the microwells were then molded into UV-cured epoxy “domes" via an intermediate silicone molding step.

[0078] 1) Spin coating: A six-inch diameter silicon wafer cut at a <100> orientation and single side polished was used as a substrate. The substrate was cleaned with acetone and isopropyl alcohol and subsequently pretreated with bis(trimethylsilyl)amine (HMDS). The wafer was baked at 110°C for 1 minute. A 35 pm layer of AZ 50-XT photoresist was deposited onto the silicon wafer using static deposition and subsequent spin coating at 2200 RPM for 30 seconds. The silicon wafer was then baked at 90°C for 2 minutes and 110°C for 3 minutes.

[0079] 2) Exposure: A Heidelberg DWL 66+ equipped with a 10 mm write head (500 nm pixel size) was used to expose photoresist coated wafers in greyscale-mode (256 grey shades). Design files for exposure were prepared in Adobe Illustrator and exported as 8-bit .bmp files. Exposure of the resist was conducted at an intensity of 100 mW at a constant write head focus of -10%.

[0080] 3) Development: A 3 : 1 ratio of DI water and AZ-400K developer was used as a bath to develop the exposed silicon wafers for 25 minutes with constant stirring using a magnetic stir bar (100 RPM). The exposed wafer was quenched with DI water followed by air drying. The wafer was baked at 90 °C for 15 minutes before replication into silicone molds described below.

[0081] Preparation of microstructure replicas: Preparation of microstructured polymer samples was conducted as described elsewhere in detail and outlined in FIG. 328. UV-cured microstructure arrays were replicated from greyscale masters as described here. Moldstar 31- T two-part silicone resin (Smooth On) was mixed in equal parts and poured onto the greyscale master wafer and put under vacuum to remove all bubbles (~5 minutes). Vacuumwas released and the silicone resin was allowed to cure for at least 2 hours at room temperature. The cured elastomer was peeled off to give the first generation copy of convex hemicylinders. The first-generation copy was plasma treated (Harrick Plasma PDC-001 plasma cleaner, air used as input gas, 2-minute treatment) and then Moldstar 31-T silicone pre-mixed resin was poured on top, degassed in vacuum, and allowed to cure for at least 2 hours at room temperature. The second-generation silicone mold was pulled off to yield concave hemicylinders. OG-142 epoxy resin (Epotek) was poured onto the second-generation silicone mold, covered on top with a glass slide, and placed into a UV chamber to cure under a 368 nm lamp for 10 minutes (Boekel Scientific UV Crosslinker; SKU: 234100). The UV cured OG-142 on glass was peeled away from the second-generation silicone mold to yield convex hemicylinders. At this stage, dye may be deposited onto the hemicylinder microstructures if desired. Lastly, a layer of black pigmented Moldstar 31-T pre-mixed resin (<1 vol% black silicone pigment added) was poured onto the cured OG-142 and left to cure for at least 1 hour. Iridescent structural color could be observed reflecting from the epoxysilicone interface.

[0082] One or more additional layers of solid material (e.g. polymers, oxides, glass) may be applied on top of the microreplicated substrate coated with absorbing components to prepare a microstructure interface with the appropriate combination of refractive indices to generate optical paths for reflected light that includes multiple TIR events (FIG. 4).

[0083] Addition of dyes onto microstructures: The application of the dyes or chemical layers to microstructured surface may be carried out using various deposition techniques, a spray coating, spin coating, blade coating, solvent or thermal evaporation, dip coating, sputtering, layer-by-layer assembly, digital or analog printing processes. Substrates such as polymeric films utilizing the invention may be produced using industrially established coatings processes, such as roll-to-roll or plate-to-plate microreplication techniques including embossing, thermoforming, and UV casting. Layers of absorbing components may be applied to surfaces containing either concave or convex microstructures depending on the polarity of the mold used during microreplication processing.

[0084] Optically absorbing components localized to the interface for realization of the described invention include: visible chemical dyes (e.g Sudan yellow, quinoline yellow, solvent blue 59, phthalocyanine green) (FIG. 5 and FIG. 6), non-visible dyes (FIG. 7), plasmonic nanoparticles or nanostructures, metallic thin films (e.g. Al, Cr, Au, Ag), and biomolecules (e.g. proteins, enzymes, DNA, RNA).

[0085] Optically absorbing components may be distributed uniformly across the interface or include a varying degrees of asymmetry on the scale of the individual microstructures. The introduction of nonuniformity in the distribution of optically absorbing components may be achieved by for example: shadowing during the deposition process, the self-assembly of components to parts of the structure interface such as three-phase contact line, or via the application of external electric or magnetic fields during deposition (FIG. 8A).

[0086] Variations on the above-described approaches may be realized by spatially patterning the deposition of one or multiple different chemically absorbing components onto surfaces using one or more of the described methods such as digital or analog printing, masked evaporation or spray coating (FIG. 8B)

[0087] Most dyes were thermally evaporated from a donor substrate onto the microstructures via the following method. Dyes were weighed and dissolved as follows: Sudan yellow (0.5% and 1.4% w / v) in toluene, quinoline yellow (0.4% w / v) in chloroform, and solvent blue 59 (0.7% w / v) in chloroform. Sudan yellow, quinoline yellow and solvent blue 59 dye solutions (400 pL) were spin coated onto a clean glass slide substrate (2.5 X 2.5 cm) with dynamic deposition at 500 rpm with 50 rpm acceleration for 45 seconds. A hot plate was set at the appropriate temperature, depending on the dye used: Sudan yellow (120 °C), solvent blue 59 (160 °C), quinoline yellow (160 °C). The dye coated glass slide was placed on the hot plate and two cover slips (130-160 pm thick) were placed on both ends of the glass substrate to prevent direct contact with the microstructures and to mask a portion of the microstructures. The epoxy microstructures on glass was placed face down onto the cover slips (one cover slip was placed in the center of the microstructure array for masking) (FIG. 9). A petri dish was placed over the setup to localize the evaporation and was left for 5 minutes. Black pigmented silicone (1 : 1 mixture of parts A and B) was poured onto the dye coated microstructures and left to cure for at least 1 hour.

[0088] Phthalocyanine green could not be thermally evaporated onto the microstructures. Therefore, phthalocyanine green in hexane (1% w / v, 400 pL) was spin coated directly onto the epoxy microstructures at 500 rpm with 50 rpm acceleration for 45 seconds. Care was given to choosing the solvent and spin coating conditions, as some solvents will swell and deform the polymer microstructures if left on the surface too long. Black pigmented silicone was poured onto the microstructures and left to cure.

[0089] Measurement of the far-field caustic: A Thor Labs fiber optic cold white LED positioned at a specific incident angle (0in =0° or 45°) was used to illuminate a microstructure array (FIG. 10A). The reflected colors were projected onto a translucent hemispherical dome(half a ping pong ball) or a translucent polystyrene sheet. The cylinder length was oriented perpendicular to the incident light source. Images were captured with a Canon EOS Rebel T6 DSLR camera.

[0090] Optical microscopy: For imaging the cross section of a microstructure array, an already prepared silicone backed epoxy microstructure sample as discussed previously was used and the silicone backing was peeled away from the epoxy microstructures. The silicone mold was cut perpendicular to the cylinder length with a razor in a thin slice to create a crosssection. The cross section of the silicone molds and patterned epoxy microstructure arrays were imaged with an inverted optical microscope (Nikon, Eclipse Ti-U) in brightfield transmission mode at x20 and x4 magnification, respectively. Images were obtained using a PCO Panda camera.

[0091] Reflectance spectra of far field caustics at 0in=O°: Reflectance spectra were collected by directly imaging the far-field caustic of a microstructure array which was focused by a macro lens attached to a Kymera 193i spectrograph with an attached Zyla 4.2 sCMOS camera28. The microstructure sample was illuminated with a Thor Labs cold white LED at 9in=0° . Spectra were collected from 380-776 nm at 0.5 nm increments using a 100 pm slit width, an exposure time of 2.00 seconds and 10 cumulative scans. A custom normalization procedure described previously was used to view the interference signatures of the microstructure sample28.

[0092] Reflectance spectra of far-field caustics at 9in=45°: Reflectance spectra were collected by directing a single position of the far-field caustic of a microstructure array (9out=20°, 9out=25°) into a macro lens attached to an imaging Kymera 193i spectrograph with an attached Zyla 4.2 sCMOS camera (FIGS. 11A-11B). An Amscope 50 W white LED gooseneck lamp was placed above the lens of the spectrograph to illuminate the sample within the same vertical plane. The illuminated sample was half dyed and half non-dyed which allows for a single scan to capture both portions of the sample. While the image plane of the microstructure sample was in focus, a portion of the far-field caustic (parallel to slit of spectrograph) was projected into the spectrograph and a measurement was taken with a 200 pm slit width and varying exposures (0.001-0.0028 sec) of the camera to maximize intensity (FIG. 11C) Since the entire microstructure sample was imaged at once with the imaging spectrometer, the reflectance spectrum could be measured at different positions along the spectrometer slit. The non-dyed portion of the microstructure sample was positioned at a higher point on the slit while the dyed portion was positioned below (FIG. 12A). The reflectance spectra across the sample could be viewed in a 3D mode to see all spectracollected along the slit (FIGS. 12B-12C). When measuring multiple samples for comparison (FIG. 6), the non-dyed spectrum for each sample was compared as a control to confirm that the same 0out on the far field projection was being measured.

[0093] Three separate reflectance spectra were taken for each sample: raw reflection spectrum from the microstructure sample, a white reference, and background. For the reflection from the microstructure sample, four positions near the center of the sample were averaged for both the non-dyed and dyed sides. The spectra near the center of the sample within the slit were found to have the most consistent and highest intensity of light to be directly compared for analysis. The white reference is taken with the same illumination conditions as the sample measurement but with a white piece of paper to act as a Lambertian scatterer. The background is taken of the dark current of the spectrograph. Background was removed via baseline subtraction and evenly applied with both the raw signal of the microstructure sample (FIG. 13A) and the white reference (FIG. 13B). The raw signal of the sample was divided by the white reference to remove the white light source from the reflectance spectra (FIG. 13C). Lastly, the spectra was normalized to 1 (FIG. 13D). When comparing non-dyed and dyed reflectance spectra, both were normalized to the non-dyed spectra to maintain the relative intensities with respect to each other.

[0094] Converting spectra to CIE space: Experimental spectra taken every 0.5 nm from 400- 750 nm were converted to a 100 row data set by averaging every 4 nm (since the light source did not have much intensity from 380-400 nm and 750-776 nm, these wavelengths were assigned 0 intensity). A Matlab script titled, ‘calculate CIEspace.m’ converted experimental hyperspectral data to CIE 1931 XYZ color space and outputs x and y values to plot onto the chromaticity diagram. The x and y values are plotted onto the chromaticity diagram using Matlab script titled ‘plot CIE.m’.

[0095] Absorbance spectra of molecular dyes: Transmission spectra of thermally evaporated dyes on epoxy coated glass slides were measured using the imaging Kymera 193i spectrograph and Zyla 4.2 sCMOS camera. Sudan yellow, quinoline yellow, and solvent blue 59 were thermally evaporated onto a flat layer of UV-cured OG-142 on a glass substrate. The dyed samples were level with the macro lens attached to the imaging spectrograph and white light from an Amscope white LED-50 W gooseneck lamp was transmitted through the sample into the lens. The measurement was taken with a 200 pm slit width and at varying exposures of the camera to maximize intensity. A spectrum of the light source was measured by using a plain glass slide with the same setup as previously stated. The white light sourcewas removed from the raw data spectrum as well as converting the transmission spectrum to an absorbance (A) spectrum using the following equation.The resulting spectrum was normalized to 1 for each dye. Since phthalocyanine green could not be thermally evaporated, the absorbance spectrum was measured using the liquid solution (0.1% w / v in hexane) as well as IR-813 toluenesulfonate (0.06% w / v in methanol) on an Evolution 201 UV-Vis spectrophotometer (Thermo Scientific).

[0096] Zemax modeling: A setup similar to previously described was used to simulate the far-field caustic of the illuminated non-dyed hemicylinder sample used in each experiment28. The greyscale fabricated microstructures used in this work were not perfectly hemispherical, so the part designer tool in Zemax was used to build a shape (CAD Part: Zemax Part Designer) that more closely resembled the microstructures. To build the hemicylinder part, a line was drawn at y=0 for the width (15 pm), two lines were drawn on both sides to increase the depth of the structure (2 pm), and an arc connected the two lines (r= 15 pm). The part was extruded, rotated and moved to be under the light source and was scaled by +3%. A ray trace was done using 2x107 analysis rays for a single wavelength (380-776 nm at 4 nm increments) and the interference pattern was collected with a 500x500 pixel detector screen in the far- field. The interference intensity pattern collected from Zemax was converted to color space using two Matlab scripts (‘Make_Multidimensional_Array.m’ and‘Colorize Multidimensional Array.m’) described previously to observe the far-field caustic28.

[0097] Assembly and characterization of near-IR dyed patterned microstructures: Patterned microstructures were fabricated using greyscale lithography and replicated into UV-cured OG142 on a glass slide. The microstructure array was masked by a stencil, exposing only the center pattern, and an airbrush was used to deposit IR-813 toluenesulfonate (0.06% w / v) in methanol over the stencil masked area. Black pigmented silicone was poured onto the airbrushed microstructure array and left to cure for at least an hour. The near-IR dyed microstructure sample was illuminated with both room lighting (-400-700 nm) and a near-IR LED flashlight (850 nm). Under room lighting, the sample was illuminated at 0° and the sample stage was moved to view the iridescence. Under near-IR lighting, the sample was illuminated at various angles and the sample stage was held constant at 0°. Images were captured by an iPhone placed directly above the sample (0°).Results

[0098] Given that an evanescent wave is generated each time light undergoes TIR, a study was conducted to explore to whether evanescent wave absorption could be an efficient route to tuning the iridescent coloration produced from multibounce TIR interference. For instance, it was hypothesized that evanescent waves generated by TIR events could be absorbed by a chemical species, such as a molecular dye localized at the interface, and thus alter the reflected colors produced (FIG. 14A). To test this concept, an array of hemi cylinder microstructures were first fabricated through greyscale lithography which were further replicated into a UV-cured epoxy (refractive index, m=1.58) affixed to a glass substrate (FIG. 14B). Separately, a common organic dye, Sudan yellow (Xmax= 488 nm) in toluene (0.5% w / v), was spin coated onto a second glass substrate. The Sudan yellow was then thermally evaporated off the donor glass substrate and onto the microstructures’ surface by heating on a hot plate (120°C for 5 minutes) (FIG. 3). Half of the structures were masked during the dye evaporation to create a sample that was half-coated with the dye to facilitate direct comparison. Black pigmented silicone (n2=1.41) was then coated onto the back of the microstructures to reduce the refractive index contrast at the TIR interface, which enhances the color vibrancy; the black pigment reduces stray light scattered from behind the structures, and because the pigment particles themselves are on the 100 pm scale and are far from the TIR interface, they cannot interact with the evanescent waves16. For the refractive indices used (m=1.58, n2=1.41) and the range of incidence angles (0) anticipated for the structure shown in FIG. 14B with illumination from the normal, dPwas estimated to be on the scale of 100 to 500 nm for visible wavelengths (k=400 to 700 nm). Macroscale photographic images of the microstructures, both with and without the evaporated dye, were taken from several viewing angles to qualitatively capture the iridescence (FIG. 14C). The non-dyed portion shows a significant change in hue with viewing angle, while the Sudan yellow coated portion of the microstructure sample exhibit a more subtle change in color saturation. A second dye, quinoline yellow, was also evaporated onto the structures and examined for qualitative comparison (FIG. 14C). The two different dyes had visually different effects on the observed iridescence, as described in detail later. Overall, initial qualitative observations of the changes to the samples’ iridescence upon addition of the dyes suggested that evanescent wave absorption by dyes at the TIR interface was indeed capable of altering the reflected colors.

[0099] To easily visualize the far-field iridescence of the samples, a translucent hemispherical dome (half a ping pong ball) was used as a screen onto which light reflecting off of the microstructure surface was projected. In this manner, all reflected colors could bevisually observed and positionally mapped as a function of exit angle (0out) for a constant illumination angle (0in) (FIG. 15A). When hemicylinders are illuminated by a collimated light source, the far-field caustic is reflected along a single axis perpendicular to the orientation of the cylinder length16. To collect angularly resolved reflectance spectra of the entire far-field, an imaging spectrometer was used to visualize the Fourier plane16(FIGS. 15B-15C). Angle-resolved reflection spectra for 0in=O° were collected for the non-dyed microstructures (FIG. 15B) as well as Sudan yellow dyed microstructures, prepared by thermal evaporation from a 1.4% w / v spin coated donor substrate (FIG. 15C). Due to the dye’s absorption profile across blue-green wavelengths (FIG. 15C), the sample’s reflectance intensity from 425-530 nm is completely diminished at all exit angles, but the intensity from 530-680 nm remains the same, as compared to the non-dyed sample. The fact that the reflection intensity remained unaltered for wavelengths not absorbed by the dye indicated that the presence of the dye did not affect the TIR condition. If the presence of dye altered TIR, for instance by decreasing the effective refractive index contrast at the interface, an overall decrease in reflection intensity would have been observed across all wavelengths and all exit angles, but this was not the case. As an additional control, a yellow silicone pigment (pigment particle diameter ~ 100 pm) was dispersed within the silicone backing of the microstructures; the far-field caustic of the pigmented sample showed a yellow glow due to significant scattering within the silicone, but reflected blue coloration was still observed in the caustic, suggesting that the large, microscale pigment particles were ineffective at evanescent wave absorption because they fall outside the predicted penetration depth (FIG. 16). This result is expected, since the evanescent wave absorptive species has to be within nanometer-scale distances of the interface to absorb visible light under the sample conditions. Through optical profilometry, it was observed that the coating of dye on flat epoxy had a nanoscale thickness, although the thickness was non-uniform (FIG. 17). As such, the study concluded that the dye is acting to modulate the reflected light via evanescent wave absorption.

[0100] The study next aimed to determine the efficiency of the placement of dye at the TIR interface, as opposed to the top interface. For a dye layer placed on top of the sample, there would only be two opportunities for absorption: when the light enters and exits the microstructure. However, for the microstructure, light bounces numerous times by TIR (denoted by m), enabling more absorption events. Using Zemax OpticStudio for ray tracing, combined with previously reported custom color analysis16, it was found that this specific microstructure geometry supported optical paths ranging from m=3 to 51 bounces for 0in=O°, with most light having m<6 bounces. The number of bounces m is correlated with exit anglein FIG. 10 A. For example, the green reflected color at approximately 9out=32° to 40° results from interference between the m=4 and 5 bounce trajectories. To compare the efficiency of the two configurations (dye at the top and dye at the TIR interface), identical dye-donor substrates were used, and dye was thermally evaporated onto each interface (FIG. 10B). When dye was placed on the flat top surface, it was found visually that the iridescence more so resembled that of the non-dyed sample, with purple and blue hues still persisting at 9out=22° (FIG. 10B, iii). Hence, the change in coloration was more subtle when dye was placed at the top surface as compared to placing dye at the TIR interface. To quantify the difference, the reflectance spectra was collected from each sample at 9out=22° and compared the reflection intensity change at = 463 nm (FIG. 10C). For the top-coated sample, the reflection intensity decreased by only 18.6% compared to the control without dye, whereas the sample with the dye at the TIR interface had a 94.4% intensity decrease (FIG. 10C). It is noted also that the microstructures have approximately 2.5x higher surface area than the flat surface, so since these surfaces were coated from identical dye-donor substrates, there is actually less dye per area on the TIR interface. The enhancement in absorption efficiency achieved when placing dye at the TIR interface compared to the top surface may be attributed to the fact that the reflected light undergoes multiple reflections within the microstructure, allowing for more absorption events. The results indicate that the sum optical effect is not simply a superposition of the contribution from a dye film and interference colors, but rather, the multi-bounce mechanism is critical to the efficiency of the color tunability.

[0101] The study next sought to examine how the reflected colors could be modulated by tuning the amount of dye coating the TIR interface. It was expected that increasing the amount of dye per surface area would increase both the total surface coverage of dye (since it was observed by optical profilometry that coverage is not completely uniform (FIG. 17)) as well as the coating thickness, leading to a higher degree of evanescent wave absorption. Sudan yellow was thermally evaporated onto the microstructures in the same manner as discussed previously, but donor substrates were prepared by spin coating the dye at different concentrations (0.5% w / v and 1.4% w / v). The microstructures were illuminated at 9in=45° to observe changes in a wider range of visible wavelengths and colors. Qualitatively, at 9out=20- 45° the far-field iridescence of a sample prepared with the 0.5% w / v Sudan yellow showed a color change from purple-blue (with no dye) to primarily green (with dye) (FIG. 5A, i, ii). When using the higher concentration (1.4% w / v), a more pronounced color change was observed at 9out=20-45°, yielding more orange-green (FIG. 5A, i, iii). Reflectance spectra were collected at 9out=20° for each far-field caustic (indicated by the arrows in FIG. 5A).Within the wavelength range strongly absorbed by the Sudan yellow (X ~425 nm to 530 nm) both dyed microstructure samples showed a decrease in reflection intensity, although a higher percent decrease at X=488 nm was observed for the sample with more Sudan yellow (97% decrease compared to 73%) (FIG. 5B). The reflectance spectra were also converted to CIE 1931 color space to observe the shift in chromaticity. As expected, the CIE space plot shows a much larger shift in color space for the sample prepared with the higher dye concentration (FIG. 5C). Therefore, the amount of dye evaporated can be used to tune the reflected light intensity for the wavelengths where the dye absorbs.

[0102] Although the preliminary work was conducted with Sudan yellow dye, it was expected that the same evanescent wave absorption concept could be extended to any number of molecular dyes that could be deposited at the TIR interface. Several molecular dyes were chosen to be tested in addition to the Sudan yellow, with absorption spanning the visible spectrum: quinoline yellow (Amax= 447 nm), solvent blue 59 (kmax = 649 nm), and phthalocyanine green (kmax = 642 nm). Although solvent blue 59 and phthalocyanine green have similar kmax, they have quite different absorbance spectra, where the phthalocyanine green absorbance spans a much narrower wavelength range (FIG. 6). Each dye was thermally evaporated onto the microstructures as described previously using comparable concentrations for the donor substrates, except for phthalocyanine green which was difficult to evaporate and thus was spin coated from hexane directly onto the microstructures. Images of the far-field iridescence (9in=45°) were taken of each dye-coated microstructure array and compared to the non-dyed control sample (FIG. 6A). Qualitatively, Sudan yellow and quinoline yellow absorbed blue and purple hues positioned from 9Out=25° to 45° and 9out=-10° to -30°. Solvent blue 59, with its broad absorption spectrum spanning the green and red wavelengths, caused the caustic to appear largely blue. Phthalocyanine green, which has a relatively narrow absorbance spectrum in the red wavelengths, did not influence the caustic substantially, except for a slight change in the orange band from 9Out=0° to -15°.

[0103] Reflectance spectra were collected at 9out=25° (indicated by the arrows in FIG. 6A) for all samples. A maximum in reflection intensity was reached at = 463 nm for the nondyed microstructures (FIG. 6B). Since the maximum reflection intensity was blue, the largest change in color and intensity at this ©out would be expected to result from addition of dyes that absorb blue wavelengths. As previously noted, Sudan yellow absorbed nearly all reflected light (98.1% at = 463 nm) due to the strong, broad absorption profile from = 425 to 525 nm, and the maximum reflection intensity shifted to = 549 nm, making green the dominant color in the caustic at 9out=25°. Quinoline yellow also absorbed a significant amount of light(72.4% intensity drop at = 463 nm) and shifted the maximum reflection intensity to a longer wavelength of = 481 nm, exhibiting a bluish green. On the other hand, solvent blue 59 and phthalocyanine green do not absorb blue wavelengths and therefore, the reflectance spectrum changed minimally. Both dyes absorb strongly at longer wavelengths (525-675 nm) which would be expected to have a more substantial impact at other 90Ut angles where orange and red are more prominently reflected. Each reflectance spectrum was converted to CIE chromaticity coordinates to represent the changes resultant from the addition of each dye for this specific exit angle, 0out=25° (FIG. 6C). As expected, both Sudan yellow and quinoline yellow show large shifts in chromaticity from blue to yellow-green. Importantly, as noted previously, the reflection intensity was not modified for spectral regions where the dye did not absorb, indicating that the presence of the dye itself did not disrupt TIR for any of these samples.

[0104] Hemicylinder microstructures which generate structural color via TIR interference can be patterned across a surface in varying orientations to yield color-traveling effects and the appearance of “flashing” colors at changing viewing angles16. Such overt visual effects have utility for security and anti-counterfeiting applications2. By combining the iridescence with a near-infrared absorbing dye, which would be invisible to the human eye but detectable with a cell phone camera, such structural color features may also have relevance for covert security applications. To test this concept, an array of hemicylinders organized in circular rings was fabricated, where each ring had a different orientation of the cylinder axis; the areas in between the circles were patterned with hemispheres (FIG. 18A). The near-infrared absorbing dye, IR-813 toluenesulfonate (0.06% w / v in methanol, FIG. 18B) was then airbrushed onto the center of the sample (in the area marked in FIG. 18A), and the sample was backed with black pigmented silicone. Under ambient room lighting ( ~ 400-700 nm), the sample was iridescent with rings of colors that appeared to shift position, and there were no observable difference between the areas that were coated and uncoated with dye (FIG. 18C). When illuminated with a near-infrared LED flashlight (X = 850 nm) and imaged with a cell phone camera, the impact of the dye became readily apparent (FIG. 18C); the center area became largely invisible due to evanescent wave absorption of the near-infrared light by the dye, while the uncoated regions still exhibited the shifting ring patterns upon changes in illumination angle. A control experiment in which a flat sample with no microstructures was patterned with the dye yielded no reflection and appeared black under both white and near-IR light, further indicating that the visual effects result from the combination of the TIR interference and dye absorption (FIG. 19).Discussion

[0105] Color is generated by numerous mechanisms, such as by dyes and pigments which selectively absorb and reflect certain visible light wavelengths, as well as by nano- or microstructured materials which create structural color via optical interference1. Structural color generated by mechanisms such as Bragg’s diffraction and thin film interference usually involve periodic nano- structured materials with feature sizes on the order of the wavelength of visible light1. Unlike materials colored by dyes, structurally colored materials are often iridescent (i.e. the color changes as a function of illumination and viewing angles) and thus have been extensively studied both fundamentally and for application in areas including anticounterfeiting, displays, decorative coatings, and sensors2 l (). Combining optical effects from interference as well as absorption is widely used as a strategy to modulate the reflective color properties of materials in ways that neither mechanism can achieve in isolation1 1 l 5. For example, metal films have been incorporated into nanostructures (cavity and hole arrays) to act as spectral filters through strong absorption within the visible spectrum to enhance color saturation and brightness13,15. Here, the effects of evanescent wave absorption by dyes on the reflected iridescent structural color produced are explored via total internal reflection interference.

[0106] Recently, a design principle was reported for generating iridescent structural color from interference which occurs between light rays undergoing different multibounce trajectories of total internal reflection (TIR) in a microstructure16 l 8. TIR is an optical effect in which light that impinges at an interface with a high-to-low refractive index contrast is completely reflected back into the high index medium, rather than refracting through the interface. TIR occurs only when the incident light is at or above a certain incidence angle, called the critical angle. Iridescent coloration from TIR interference can be tuned by changing parameters such as the microstructure’s geometry (e.g. radius of curvature, contact angle, symmetry) and refractive index contrast16 19, with demonstrated use in applications such as sensors19, decorative films16,18printing20, and structural color pigments21. A unique phenomenon induced by a TIR event is the creation of an evanescent wave, which is an oscillating field generated on the surface of the second, lower refractive index medium22. The amplitude of the evanescent wave decreases as a function of distance from the interface. The penetration depth (dP) of the evanescent wave can be calculated based on the wavelength of incident light (X), the incidence angle at the interface (9) and the refractive index of the two materials at the interface (m, m).23If the lower index medium is lossless, then all incident light is reflected, hence the term “total” internal reflection. However, absorptive chemical species localized within the penetration depth can interact with the evanescent wave and thereby modify the properties of the reflected light. The evanescent wave is thus highly sensitive to the chemical environment near the TIR interface, and this phenomenon is routinely exploited for sensing and spectroscopy applications23 26. For example, optical fiber sensors utilize the evanescent wave generated by each TIR event propagated along the fiber core to interact (absorption, fluorescence, Raman scattering, etc.) with analyte and alter the output signal26,27. Structural coloration from TIR interference is typically generated by light rays undergoing TIR with near lossless reflection events; however, inspired by evanescent wave sensors, it was hypothesized that evanescent wave absorption could also be utilized in combination with TIR interference to modify the reflected light properties and create tunable iridescent color effects. By combining the absorptive properties of molecular dyes and the optical interference events generated from TIR trajectories, it was wondered whether certain hues could be selectively removed from the reflected iridescent color palette, potentially with high efficiency due to the multi-bounce absorbance mechanism.

[0107] In this work, it is explored how the iridescent color produced by TIR interference in microstructures can be tuned through evanescent wave absorption by molecular dyes localized at the TIR optical interface. It was found that localizing the dye at the microstructures’ TIR interface is more efficient in modulating the reflected color than placing a dye coating at the topmost surface, as the multibounce TIR trajectories allow for numerous evanescent wave absorption events. The iridescent colors were modulated by changing both the amount of dye at the TIR interface as well as the specific dye used. Using Fourier plane spectroscopy, angle-resolved reflection spectra of both uncoated and dye-coated surfaces were analyzed and compared. By introducing a near-infrared absorbing dye, which is invisible to the human eye, combined overt-covert optical motion effects with potential uses in anti-counterfeiting were demonstrated. In summary, it was found that the TIR events at the microscale interface are key to the proposed coloration mechanisms; structural color is created by optical interference occurring between light rays propagating by different multibounce trajectories of TIR, while upon each TIR event, an evanescent wave is generated that is absorbed by the dye. These two mechanisms, occurring simultaneously, lead to the overall coloration. By combining the structural coloration from TIR interference with evanescentwave absorption, greater tunability of the reflective colors of materials was enabled with potential impact in areas including sensors, security, displays, and decorative coatings.

[0108] Alternative substrates may be created using convex microstructures formed in a high index material (e.g. a polymer, glass, or oxide) backed with a lower index liquid (e.g. water). The composition of the liquid may be adjusted and the substrate interfacial adsorption properties prepared such that changes to the presence of solutes in the liquid result in observable differences to the spectral reflectance from microstructures. The liquid may contain a known or unknown concentration of chemical solutes (e.g. proteins, enzymes, DNA, RNA) that may be reversibly or irreversibly interfacially active and possess with unique optically absorption properties such that their presence and concentration be correlated with the spectral reflectance signature detected by microscope or by eye. The targeted adsorption of molecular species to the reflecting interface may be primed by the attachment of receptor ligands with binding properties tailored to interact with specific molecules (FIG. 20).

[0109] Devising strategies to tune the reflective optical properties and iridescent coloration of structurally colored materials is broadly important both for fundamental research and for applications in areas such as sensors, displays, coatings, and security. Interference generated from multi -bounce trajectories of TIR within microstructures is emerging as a strategy for creating tunable structural colors; here, presented is an approach for the customization of iridescent coloration using both optical interference events generated by TIR at a concave interface and evanescent wave absorption by dye localized at the interface. Reflectance spectra taken at specific exit angles, as well as angle-resolved spectra obtained from imaging of the Fourier plane, allowed for quantitative analysis of the impact of the evanescent wave absorption by the dyes. It was found that the multibounce trajectories of TIR enable numerous evanescent wave absorption events at the TIR interface, providing a more efficient way to tune the iridescent colors as compared to placing an absorptive dye at the top, flat interface. Furthermore, when using this approach, the dye is encased between polymer layers, thus providing protection from tampering or removal which could be advantageous for security films and coatings applications. Changing the dye surface coverage at the TIR interface, along with the specific dye used, facilitated tuning of the iridescent color and hue as determined by comparison of the relative reflection intensities and CIE color space mapping. Lastly, the effect of near-infrared absorbing dyes on the color-traveling properties of patterned microstructure arrays was investigated, and combined covert-overt iridescence spanning the visible and near-infrared wavelengths was demonstrated. The use of evanescentwave absorption by dyes, as showcased in this report, thus can be an effective strategy to manipulate coloration generated by multibounce TIR interference with possible uses in sensors, displays, and security.

[0110] The following patents, applications and publications as listed below and throughout this document are hereby incorporated by reference in their entirety herein.References(1) Kinoshita, S.; Yoshioka, S.; Miyazaki, J. Physics of Structural Colors. Reports Prog. Phys. 2008, 71 (7), 76401. https: / / doi.Org / 10.1088 / 0034-4885 / 71 / 7 / 076401.(2) Hong, W.; Yuan, Z.; Chen, X. Structural Color Materials for Optical Anticounterfeiting. Small 2020, 16 (16), 1907626. https: / / doi.Org / https: / / doi.org / 10.1002 / smll.201907626.(3) Zhao, Y.; Zhao, Y.; Hu, S.; Lv, J.; Ying, Y.; Gervinskas, G.; Si, G. Artificial Structural Color Pixels: A Review. Materials (Basel). 2017, 10 (8), 944. https: / / doi.org / 10.3390 / mal0080944.(4) Chen, F.; Huang, Y.; Li, R.; Zhang, S.; Wang, B.; Zhang, W.; Wu, X.; Jiang, Q.; Wang, F.; Zhang, R. Bio-Inspired Structural Colors and Their Applications. Chem. Commun. 2021, 57 (99), 13448-13464. https: / / doi.org / 10.1039 / DlCC04386B.(5) Qin, M.; Sun, M.; Hua, M.; He, X. Bioinspired Structural Color Sensors Based on Responsive Soft Materials. Curr. Opin. Solid State Mater. Sci. 2019, 23 (1), 13-27. https: / / doi.Org / https: / / doi.org / 10.1016 / j.cossms.2018.10.001.(6) Sun, J.; Bhushan, B.; Tong, J. Structural Coloration in Nature. RSC Adv. 2013, 3 (35), 14862-14889. https: / / doi.org / 10.1039 / C3RA41096J.(7) Li, R.; Zhang, S.; Zhang, R. Recent Progress in Artificial Structural Colors and Their Applications in Fibers and Textiles. Chemistry-Methods 2023, e202200081. https: / / doi.Org / https: / / doi.org / 10.1002 / cmtd.202200081.(8) Hou, X.; Li, F.; Song, Y.; Li, M. Recent Progress in Responsive Structural Color. J. Phys. Chem. Lett. 2022, 13 (13), 2885-2900. https: / / doi.org / 10.1021 / acs.jpclett. lc04219.(9) Cai, L.; Wang, Y.; Xu, D.; Chen, H.; Zhao, Y. Bio-Inspired Pigment Particles with Dual -Variation Modes of Structural Colors and Fluorescence. Chem. Eng. J. 2023, 461, 142000. https: / / doi.Org / https: / / doi.org / 10.1016 / j.cej.2023.142000.(10) Parker, R. M.; Guidetti, G.; Williams, C. A.; Zhao, T.; Narkevicius, A.; Vignolini, S.; Frka-Petesic, B. The Self-Assembly of Cellulose Nanocrystals: Hierarchical Design of Visual Appearance. Adv. Mater. 2018, 30 (19), 1704477. https: / / doi.Org / https: / / doi.org / 10.1002 / adma.201704477.(11) Yang, Z.; Ji, C.; Cui, Q.; Guo, L. J. High-Purity Hybrid Structural Colors by Enhancing Optical Absorption of Organic Dyes in Resonant Cavity. Adv. Opt. Mater. 2020, 8(12), 2000317. https: / / doi.Org / https: / / doi.org / 10.1002 / adom.202000317.(12) Zhang, W.; Anaya, M.; Lozano, G.; Calvo, M. E.; Johnston, M. B.; Miguez, H.; Snaith, H. J. Highly Efficient Perovskite Solar Cells with Tunable Structural Color. Nano Lett. 2015, 15 (3), 1698-1702. https: / / doi.org / 10.1021 / nl504349z.(13) Park, C.-S.; Lee, S.-S. Vivid Coloration and Broadband Perfect Absorption Based on Asymmetric Fabry-Perot Nanocavities Incorporating Platinum. ACS Appl. Nano Mater. 2021, 4 (4), 4216-4225. https: / / doi.org / 10.1021 / acsanm.lc00699.(14) Gao, H.; Liang, Y.; Li, R.; Huang, H.; Peng, W. Eye-Friendly Reflective Structural Colors with Shortwave Infrared Shielding. Adv. Opt. Mater. 2022, 10 (2), 2101342. http s : / / doi . org / http s : / / doi . org / 10.1002 / adom .202101342.(15) Cheng, F.; Gao, J.; Luk, T. S.; Yang, X. Structural Color Printing Based on Plasmonic Metasurfaces ofPerfect Light Absorption. Sci. Rep. 2015, 5 (1), 11045. https: / / doi.org / 10.1038 / srepl l045.(16) Stumiolo, N.; Hirsch, K.; Meredith, C. H.; Beshires, B.; Khanna, S.; Rayes, M.; Gallegos, M.; McGee, S.; Kaehr, B.; Zarzar, L. D. Iridescence from Total Internal Reflection at 3D Microscale Interfaces: Mechanistic Insights and Spectral Analysis. Adv. Mater. 2023, 35 (19), 2210665.(17) Goodling, A. E.; Nagelberg, S.; Kaehr, B.; Meredith, C. H.; Cheon, S. I.; Saunders, A. P.; Kolle, M.; Zarzar, L. D. Colouration by Total Internal Reflection and Interference at Microscale Concave Interfaces. Nature 2019, 566 (7745), 523-527. https: / / doi.org / 10.1038 / s41586-019-0946-4.(18) Goodling, A. E.; Nagelberg, S.; Kolle, M.; Zarzar, L. D. Tunable and Responsive Structural Color from Polymeric Microstructured Surfaces Enabled by Interference of TotallyInternally Reflected Light. ACS Mater. Lett. 2020, 2 (7), 754-763. https: / / doi.org / 10.1021 / acsmaterialslett.0c00143.(19) Saunders, A.; Zarzar, L. Structural Coloration from Total Internal Reflection at Microscale Concave Surfaces and Use for Sensing in Complex Droplets. Proc.SPIE 2020, 11292, 32. https: / / doi.org / 10.1117 / 12.2545151.(20) Li, K.; Li, T.; Zhang, T.; Li, H.; Li, A.; Li, Z.; Lai, X.; Hou, X.; Wang, Y.; Shi, L.; Li, M.; Song, Y. Facile Full-Color Printing with a Single Transparent Ink. Sci. Adv. 2022, 7 (39), eabhl992. https: / / doi.org / 10.1126 / sciadv.abhl992.(21) Yang, Y.; Kim, J. Bin; Nam, S. K.; Zhang, M.; Xu, J.; Zhu, J.; Kim, S.-H. Nanostructure-Free Crescent-Shaped Microparticles as Full-Color Reflective Pigments. Nat. Commun. 2023, 14 (1), 793. https: / / doi.org / 10.1038 / s41467-023-36482-4.(22) Milosevic, M. On the Nature of the Evanescent Wave. Appl. Spectrosc. 2013, 67 (2), 126-131. https: / / doi.org / 10.1366 / 12-06707.(23) Xiong, Y.; Wang, Q.; Duan, M.; Tan, J.; Fang, S.; Wu, J. Real-Time Monitoring of Azo Dye Interfacial Adsorption at Silica-Water Interface by Total Internal Reflection- Induced Surface Evanescent Wave. Langmuir 2018, 34 (26), 7612-7623. https: / / doi.org / 10.1021 / acs.langmuir.8b00722.(24) Song, Y.; Ling, L.; Westerhoff, P.; Shang, C. Evanescent Waves Modulate Energy Efficiency of Photocatalysis within TiO2 Coated Optical Fibers Illuminated Using LEDs. Nat. Commun. 2021, 12 (1), 4101. https: / / doi.org / 10.1038 / s41467-021-24370-8.(25) Azargoshasb, T.; Navid, H. A.; Parvizi, R.; Heidari, H. Evanescent Wave Optical Trapping and Sensing on Polymer Optical Fibers for Ultra-Trace Detection of Glucose. ACS Omega 2020, 5 (35), 22046-22056. https: / / doi.org / 10.1021 / acsomega.0c01908.(26) Messica, A.; Greenstein, A.; Katzir, A. Theory of Fiber-Optic, Evanescent-Wave Spectroscopy and Sensors. Appl. Opt. 1996, 35 (13), 2274-2284. https: / / doi.org / 10.1364 / AO.35.002274.(27) Sharma, A. K.; Gupta, J.; Sharma, I. Fiber Optic Evanescent Wave Absorption-Based Sensors: A Detailed Review of Advancements in the Last Decade (2007-18). Optik (Stuttg). 2019, 183, 1008-1025. https: / / doi.Org / https: / / doi.org / 10.1016 / j.ijleo.2019.02.104.(28) Stumiolo, N.; Hirsch, K.; Meredith, C. H.; Beshires, B.; Khanna, S.; Rayes, M.;Gallegos, M.; McGee, S.; Kaehr, B.; Zarzar, L. D. Iridescence from Total Internal Reflection at 3D Microscale Interfaces: Mechanistic Insights and Spectral Analysis. Adv. Mater. 2023, 35 (19), 2210665.

Claims

What is claimed is:

1. A substrate that exhibits an interference pattern upon reflection of incident electromagnetic radiation, the substrate comprising: a first material having a first refractive index; a second material having a second refractive index disposed on the first material; and an absorptive layer comprising an organic, optically active species disposed between and abutting the first material and the second material; wherein the substrate comprises a plurality of microstructures, each of which comprises the first material, the second material abutting the first material, an interface between the first material and the second material, and the absorptive layer disposed at the interface; wherein the interface is configured such that at least a portion of electromagnetic radiation incident a surface of the substrate at least one illumination angle interacts with the interface such that the substrate exhibits the interference pattern upon reflection of the incident electromagnetic radiation.

2. The substrate of claim 1, wherein the interface comprises an arcuate interface.

3. The substrate of claim 2, wherein the arcuate interface is concave relative to the incident electromagnetic radiation.

4. The substrate of claim 2, wherein the arcuate interface is convex relative to the incident electromagnetic radiation.

5. The substrate of claim 1, wherein the interface comprises a plurality of sides.

6. The substrate of claim 5, wherein the interface comprises a truncated arcuate interface.

7. The substrate of any one of claims 1-6, wherein electromagnetic radiation reflected by the plurality of TIR microstructures exhibits variable intensity based on the illumination angle.

8. The substrate of any of claims 1-7, wherein electromagnetic radiation reflected by the plurality of TIR microstructures exhibits variable intensity based on an angle of observation relative to the surface.

9. The substrate of any of claims 1-8, wherein electromagnetic radiation reflected by the plurality of TIR microstructures exhibits structural color.

10. The substrate of any of claims 1-9, wherein the first material comprises a polymer.

11. The substrate of claim 10, wherein the polymer comprises a thermoplastic, such as a polyester, a polyolefin, acrylic, acrylonitrile butadiene styrene (ABS), a polyamide, or any combination thereof.

12. The substrate of claim 10, wherein the polymer comprises a thermoset, such as an epoxy, a polyurethane, Bakelite, a polyimide, or any combination thereof.

13. The substrate of any of claims 1-12, wherein the second material comprises a curable resin.

14. The substrate of claim 13, wherein the second material comprises a UV curable resin.

15. The substrate of claim 13, wherein the second material comprises a thermosetting resin.

16. The substrate of any of claims 1-12, wherein the second material comprises an inorganic material, such as MgF2, SiCh, TiCh, or AI2O3.

17. The substrate of any one of claims 1-12, wherein the second material comprises a fluid.

18. The substrate of any of claims 1-17, wherein the difference between the first refractive index and the second refractive index is at least 0.01, such as from 0.05 to 1.5.

19. The substrate of any of claims 1-18, wherein the plurality of microstructures are disposed in a regular 2-dimensional array.

20. The substrate of any of claims 1-19, wherein the plurality of microstructures are disposed in a regular 3-dimensional array.

21. The substrate of any one of claims 1-20, wherein the absorptive layer has a thickness of less than 250 nm, such as from 5 nm to 225 nm.

22. The substrate of any one of claims 1-21, wherein the organic, optically active species comprises Sudan yellow, quinoline yellow, solvent blue 59, phthalocyanine green, or any combination thereof.

23. The substrate of any one of claims 1-22, wherein the organic, optically active species comprises a non-visible dye.

24. The substrate of claim 23, wherein the non-visible dye comprises an infrared dye, such as IR-813 toluenesulfonate.

25. The substrate of any one of claims 23-24, wherein the non-visible dye comprises an ultraviolet dye.

26. The substrate of any one of claims 1-25, wherein the absorptive layer further comprises a recognition element, such as a protein (e.g., an antibody), an enzyme, DNA, RNA, or any combination thereof.

27. An article comprising the substrate of any one of claims 1-26.

28. The article of claim 27, wherein the article is a bank note, passport document, driver’s license, government ID, or other article with a security film.

29. The article of claim 27, wherein the article is sensor.

30. The article of claim 27, wherein the article is a display.

31. A substrate that exhibits an interference pattern upon reflection of incident electromagnetic radiation, the substrate comprising: a first material having a first refractive index; a second material having a second refractive index disposed on the first material; and an absorptive layer comprising an inorganic, optically active species disposed between and abutting the first material and the second material; wherein the substrate comprises a plurality of microstructures, each of which comprises the first material, the second material abutting the first material, an interface between the first material and the second material, and the absorptive layer disposed at the interface; wherein the interface is configured such that at least a portion of electromagnetic radiation incident a surface of the substrate at least one illumination angle interacts with the interface such that the substrate exhibits the interference pattern upon reflection of the incident electromagnetic radiation.

32. The substrate of claim 31, wherein the interface comprises an arcuate interface.

33. The substrate of claim 32, wherein the arcuate interface is concave relative to the incident electromagnetic radiation.

34. The substrate of claim 32, wherein the arcuate interface is convex relative to the incident electromagnetic radiation.

35. The substrate of claim 31, wherein the interface comprises a plurality of sides.

36. The substrate of claim 35, wherein the interface comprises a truncated arcuate interface.

37. The substrate of any one of claims 31-36, wherein electromagnetic radiation reflected by the plurality of TIR microstructures exhibits variable intensity based on the illumination angle.

38. The substrate of any of claims 31-37, wherein electromagnetic radiation reflected by the plurality of TIR microstructures exhibits variable intensity based on an angle of observation relative to the surface.

39. The substrate of any of claims 31-38, wherein electromagnetic radiation reflected by the plurality of TIR microstructures exhibits structural color.

40. The substrate of any of claims 31-39, wherein the first material comprises a polymer.

41. The substrate of claim 40, wherein the polymer comprises a thermoplastic, such as a polyester, a polyolefin, acrylic, acrylonitrile butadiene styrene (ABS), a polyamide, or any combination thereof.

42. The substrate of claim 40, wherein the polymer comprises a thermoset, such as an epoxy, a polyurethane, Bakelite, a polyimide, or any combination thereof.

43. The substrate of any of claims 31-42, wherein the second material comprises a curable resin.

44. The substrate of claim 43, wherein the second material comprises a UV curable resin.

45. The substrate of claim 43, wherein the second material comprises a thermosetting resin.

46. The substrate of any of claims 31-42, wherein the second material comprises an inorganic material, such as MgF2, SiCh, TiCh, or AI2O3.

47. The substrate of any one of claims 31-42, wherein the second material comprises a fluid.

48. The substrate of any of claims 31-47, wherein the difference between the first refractive index and the second refractive index is at least 0.01, such as from 0.05 to 1.5.

49. The substrate of any of claims 31-48, wherein the plurality of microstructures are disposed in a regular 2-dimensional array.

50. The substrate of any of claims 31-49, wherein the plurality of microstructures are disposed in a regular 3-dimensional array.

51. The substrate of any one of claims 31-50, wherein the absorptive layer has a thickness of less than 250 nm, such as from 5 nm to 225 nm.

52. The substrate of any one of claims 31-51, wherein the inorganic, optically active species comprises plasmonic nanoparticles or nanostructures.

53. The substrate of any one of claims 31-52, wherein the inorganic, optically active species comprises a metallic thin film, such as Al, Cr, Au, Ag, or any combination thereof.

54. The substrate of any one of claims 31-53, wherein the absorptive layer further comprises a recognition element, such as a protein (e.g., an antibody), an enzyme, DNA, RNA, or any combination thereof.

55. An article comprising the substrate of any one of claims 31-54.

56. The article of claim 55, wherein the article is a bank note, passport document, driver’s license, government ID, or other article with a security film.

57. The article of claim 55, wherein the article is sensor.

58. The article of claim 55, wherein the article is a display.

59. A method of manufacturing the substrate of any one of claims 1-26 or 31-54, comprising: forming the plurality of microstructures with the first material; coating the plurality of microstructures with the absorptive layer; and coating the absorptive layer with the second material.

60. The method of claim 59, wherein forming the plurality of microstructures with the first material comprises injection molding, cast molding, embossing, or reel-to-reel processing.

61. The method of any one of claims 59-60, wherein coating the plurality of microstructures with the absorptive layer comprises deposition, spray coating, spin coating, blade coating, solvent or thermal evaporation, dip coating, sputtering, layer-by-layer assembly, or digital or analog printing processes.

62. The method of any one of claims 59-60, wherein coating the plurality of microstructures with the absorptive layer comprises embossing, thermoforming, or UV casting.

63. A substrate, comprising: a first component and a second component disposed on the first component; and a curved, microscale interface between the first component and the second component, the interface configured such that at least a portion of electromagnetic radiation incident to a surface of the interface undergoes total internal reflection between the first component and the second component; an absorptive layer comprising an optically active species disposed between and abutting the first component and the second component; wherein the first component has a first refractive index greater than a second refractive index of the second component; and wherein the electromagnetic radiation undergoes a change in amplitude during total internal reflection.

64. The substrate of claim 63, wherein the curved, microscale interface is concave relative to the incident electromagnetic radiation.

65. The substrate of claim 63, wherein the curved, microscale interface is convex relative to the incident electromagnetic radiation.

66. The substrate of any one of claims 63-65, wherein reflected electromagnetic radiation exhibits variable intensity based on illumination angle.

67. The substrate of any of claims 63-66, wherein reflected electromagnetic radiation exhibits variable intensity based on an angle of observation relative to the surface.

68. The substrate of any of claims 63-67, wherein reflected electromagnetic radiation exhibits structural color due to the total internal reflection.

69. The substrate of any of claims 63-68, wherein the first component comprises a polymer.

70. The substrate of claim 69, wherein the polymer comprises a thermoplastic, such as a polyester, a polyolefin, acrylic, acrylonitrile butadiene styrene (ABS), a polyamide, or any combination thereof.

71. The substrate of claim 70, wherein the polymer comprises a thermoset, such as an epoxy, a polyurethane, Bakelite, a polyimide, or any combination thereof.

72. The substrate of any of claims 63-71, wherein the second component comprises a curable resin.

73. The substrate of claim 72, wherein the second component comprises a UV curable resin.

74. The substrate of claim 72, wherein the second component comprises a thermosetting resin.

75. The substrate of any of claims 63-71, wherein the second component comprises an inorganic material, such as MgF2, SiCh, TiCh, or AI2O3.

76. The substrate of any one of claims 63-71, wherein the second component comprises a fluid.

77. The substrate of any of claims 63-76, wherein the difference between the first refractive index and the second refractive index is at least 0.01, such as from 0.05 to 1.5.

78. The substrate of any one of claims 63-77, wherein the absorptive layer has a thickness of less than 250 nm, such as from 5 nm to 225 nm.

79. The substrate of any one of claims 63-78, wherein the optically active species comprises Sudan yellow, quinoline yellow, solvent blue 59, phthalocyanine green, or any combination thereof.

80. The substrate of any one of claims 63-79, wherein the optically active species comprises a non-visible dye.

81. The substrate of claim 80, wherein the non-visible dye comprises an infrared dye, such as IR-813 toluenesulfonate.

82. The substrate of any one of claims 80-81, wherein the non-visible dye comprises an ultraviolet dye.

83. The substrate of any one of claims 63-82, wherein the optically active species comprises plasmonic nanoparticles or nanostructures.

84. The substrate of any one of claims 63-83, wherein the optically active species comprises a metallic thin film, such as Al, Cr, Au, Ag, or any combination thereof.

85. The substrate of any one of claims 63-84, wherein the absorptive layer further comprises a recognition element, such as a protein (e.g., an antibody), an enzyme, DNA, or any combination thereof.

86. An article comprising the substrate of any one of claims 63-85.

87. The article of claim 86, wherein the article is a bank note, passport document, driver’s license, government ID, or other article with a security film.

88. The article of claim 86, wherein the article is sensor.

89. The article of claim 86, wherein the article is a display.

90. A substrate, comprising: a plurality of domed structures formed on a surface, the surface comprising a first material; a second component adjacent the plurality of domed structures and comprising a second material; an absorptive layer comprising an optically active species disposed between and abutting the first material and the second material; wherein the first material has a refractive index greater than a refractive index of the second material, such that incident electromagnetic radiation undergoes total internal reflection at a curved surface of each domed structure at a microscale interface between the first material and the second material.

91. The substrate of claim 90, wherein the microscale interface is concave relative to the incident electromagnetic radiation.

92. The substrate of claim 90, wherein the microscale interface is convex relative to the incident electromagnetic radiation.

93. The substrate of any one of claims 90-92, wherein reflected electromagnetic radiation exhibits variable intensity based on an illumination angle.

94. The substrate of any of claims 90-93, wherein reflected electromagnetic radiation exhibits variable intensity based on an angle of observation relative to the surface.

95. The substrate of any of claims 90-94, wherein reflected electromagnetic radiation exhibits structural color due to the total internal reflection.

96. The substrate of any of claims 90-95, wherein the first material comprises a polymer.

97. The substrate of claim 96, wherein the polymer comprises a thermoplastic, such as a polyester, a polyolefin, acrylic, acrylonitrile butadiene styrene (ABS), a polyamide, or any combination thereof.

98. The substrate of claim 96, wherein the polymer comprises a thermoset, such as an epoxy, a polyurethane, Bakelite, a polyimide, or any combination thereof.

99. The substrate of any of claims 90-98, wherein the second material comprises a curable resin.

100. The substrate of claim 99, wherein the second material comprises a UV curable resin.

101. The substrate of claim 99, wherein the second material comprises a thermosetting resin.

102. The substrate of any of claims 90-98, wherein the second material comprises an inorganic material, such as MgF2, SiCh, TiCh, or AI2O3.

103. The substrate of any one of claims 90-98, wherein the second material comprises a fluid.

104. The substrate of any of claims 90-103, wherein the difference between the first refractive index and the second refractive index is at least 0.01, such as from 0.05 to 1.5.

105. The substrate of any of claims 90-104, wherein the plurality of domed structures are disposed in a regular 2-dimensional array.

106. The substrate of any of claims 90-105, wherein the plurality of domed structures are disposed in a regular 3-dimensional array.

107. The substrate of any one of claims 90-106, wherein the absorptive layer has a thickness of less than 250 nm, such as from 5 nm to 225 nm.

108. The substrate of any one of claims 90-107, wherein the optically active species comprises Sudan yellow, quinoline yellow, solvent blue 59, phthalocyanine green, or any combination thereof.

109. The substrate of any one of claims 90-108, wherein the optically active species comprises a non-visible dye.

110. The substrate of claim 109, wherein the non-visible dye comprises an infrared dye, such as IR-813 toluenesulfonate.

111. The substrate of any one of claims 109-110, wherein the non-visible dye comprises an ultraviolet dye.

112. The substrate of any one of claims 90-111, wherein the optically active species comprises plasmonic nanoparticles or nanostructures.

113. The substrate of any one of claims 90-112, wherein the optically active species comprises a metallic thin film, such as Al, Cr, Au, Ag, or any combination thereof.

114. The substrate of any one of claims 90-113, wherein the absorptive layer further comprises a recognition element, such as a protein (e.g., an antibody), an enzyme, DNA, RNA, or any combination thereof.

115. An article comprising the substrate of any one of claims 90-114.

116. The article of claim 115, wherein the article is a bank note, passport document, driver’s license, government ID, or other article with a security film.

117. The article of claim 115, wherein the article is sensor.

118. The article of claim 115, wherein the article is a display.