Surface Plasmon Resonance Creation Method

Abstract

A method of making a security product includes providing a substrate, depositing a discontinuous layer of conductive material on the substrate, and laser annealing the discontinuous layer of conductive material to produce nanoparticles that exhibit a surface plasmon resonance effect.

Description

REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 143,139, filed January 29, 2021, which is incorporated herein by reference. [Technical field]

[0002] The concepts disclosed herein primarily relate to methods for making devices that utilize the surface plasmon resonance effect, and also relate to anti-counterfeiting products incorporating the devices. [Background technology]

[0003] Anti-counterfeiting measures often make use of optical effects that are difficult to reproduce in order to authenticate a product. Recently, complex nanostructures that exhibit surface plasmonic effects have been used. However, to enable mass production of products, anti-counterfeiting measures often need to be both created and replicated. This can lead to problems with complex nanostructures.

[0004] Examples of problems during the production phase include the need for advanced techniques such as electron beam and ion beam techniques for the required nanoscale structures, which can be time consuming and very expensive. In some cases, the physical master image is replicated for use. However, the use of the physical master image carries risks that need to be properly controlled. Furthermore, errors in the physical master or in its replication stage will propagate to subsequent products. Furthermore, changes to the image require the creation of a new master image. A further consideration is that techniques for creating security products, such as embossing, are already well known and can be used for unauthorized replication. These issues and challenges are common in the development of security products for authenticating genuine products. There is a constant need to develop new technologies that address or solve the problems of production and cloning, and that allow products to be manufactured quickly and cost-effectively, and that are difficult for illegal actors to replicate. There is still room for improvement in anti-counterfeiting measures. Summary of the Invention

[0005] According to an aspect of the disclosed concepts, a method of making a security product includes providing a substrate, depositing a discontinuous layer of conductive material on the substrate, and laser annealing the discontinuous layer of conductive material to produce nanoparticles that exhibit a surface plasmon resonance effect. [Brief description of the drawings]

[0006] A complete understanding of the disclosed concepts can be achieved by reading the following description of the preferred embodiments in conjunction with the accompanying drawings. [Figure 1] 1A-1D are diagrams illustrating various stages in creating a security product utilizing the surface plasmon resonance effect in an exemplary embodiment of the disclosed concept. [Diagram 2] FIG. 2 is a flow chart of a method for making a security product utilizing the surface plasmon resonance effect in an exemplary embodiment of the disclosed concepts. [Diagram 3] FIG. 3 shows a discontinuous layer of conductive material before and after laser annealing in an exemplary embodiment of the disclosed concepts. [Figure 4] FIG. 4 shows nanoparticles after laser annealing with a single laser pulse in an exemplary embodiment of the disclosed concepts. [Diagram 5] FIG. 5 shows the nanoparticles after laser annealing with two laser pulses in an exemplary embodiment of the disclosed concepts. [Figure 6] FIG. 6 shows nanoparticles after laser annealing with 150 laser pulses in an exemplary embodiment of the disclosed concepts. [Figure 7] FIG. 7 is a conceptual diagram showing nanoparticles being partially or completely immersed in a substrate by laser annealing in an exemplary embodiment of the disclosed concepts. [Figure 8]1 is a conceptual diagram illustrating the effect of fully and / or partially immersed nanoparticles on optical effects in an exemplary embodiment of the disclosed concepts. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] 1A-1D are diagrams showing various stages of creating a security product utilizing the surface plasmon resonance effect in an exemplary embodiment of the disclosed concepts, and FIG. 2 is a flowchart of a method of creating a security product utilizing the surface plasmon resonance effect in an exemplary embodiment of the disclosed concepts.

[0008] FIG. 1A shows a substrate 10 and a thin layer of discontinuous conductive material 20. In some exemplary embodiments, the substrate 10 is made of polycarbonate. For example, the substrate 10 may be a polycarbonate composition that meets security standards. In some exemplary embodiments, the thickness of the substrate 10 may be about 50 microns. The substrate 10 may be transparent and / or flexible. The substrate 10 may be constructed of other materials without departing from the scope of the disclosed concepts. These other materials include, but are not limited to, polyethylene terephthalate (PET) or polyethylene terephthalate glycol (PETG), polyamide / nylon, polypropylene (PP), cellophane, and polymeric materials such as paper or hybrid paper / polymer substrates used in banknotes. The thickness of the substrate 10 may vary without departing from the scope of the disclosed concepts. In some exemplary embodiments, the thickness may be in the range of about 25 μm to about 100 μm, but may be other thicknesses without departing from the scope of the disclosed concepts. In some exemplary embodiments, the substrate 10 may be rigid and / or opaque.

[0009] A thin layer of discontinuous conductive material 20 is deposited on the substrate 10. An image of an example of a thin layer of discontinuous conductive material is shown in the left portion of FIG. 3. In the example shown in FIG. 3, the conductive material has an "islands and rivers" morphology. The layer is discontinuous in that there are gaps in the layer of conductive material, as opposed to a continuous layer where no such gaps exist. In some exemplary embodiments of the disclosed concepts, the gaps in the layer are not filled with any particular material in the deposition process, but rather are filled depending on the ambient conditions (i.e., by vacuum during vacuum deposition, or by air when exposed to ambient conditions, etc.).

[0010] The conductive material in some exemplary embodiments of the disclosed concepts is silver. Other non-limiting examples of metals, such as gold, aluminum, copper, and metal oxides, may also be used. In some exemplary embodiments, semi-metals or semiconductors may also be used. In some exemplary embodiments, alloys and composites may also be used. For example, but not limited to, alloying or doping silver with other metals may shift the spectral window to more red or blue visible colors. The choice of material generally determines the spectral region of the plasmonic response (i.e., visible, infrared, or ultraviolet). In some embodiments, the thin layer of discontinuous conductive material 20 has a thickness of less than about 100 nm. In some embodiments, the thin layer of discontinuous conductive material 20 has a thickness of less than about 50 nm. In exemplary embodiments where the conductive material is silver, the thin layer of discontinuous conductive material 20 has a thickness in the range of about 4-16 nm. In some exemplary embodiments, the discontinuous conductive material 20 has a thickness of greater than about 100 nm.

[0011] The thin layer of discontinuous conductive material 20 may be formed by depositing a conductive material on the substrate 10. In some exemplary embodiments, the conductive material is deposited on the substrate by vacuum deposition, such as thermal evaporation or magnetron sputtering, such as in a batch or roll-to-roll process. However, the thin layer of discontinuous conductive material 20 may also be formed using other processes, such as, for example, but not limited to, ink printing, solution-based deposition, sol-gel, or other processes.

[0012] FIG. 1B shows a laser annealed thin layer of discontinuous conductive material 20'. In some exemplary embodiments of the disclosed concepts, laser radiation from a KrF / ArF excimer or Nd-doped solid-state laser with harmonics can be used for laser annealing the thin layer of discontinuous conductive material 20 and forming a nanoparticle distribution exhibiting a plasmonic response. In an exemplary embodiment, a KrF excimer laser with a wavelength of 248 nm is used for the laser annealing process. In another exemplary embodiment, a Nd-doped solid-state laser with a harmonics of a wavelength of 266 nm is used for the laser annealing process. Other lasers and wavelengths (e.g., but not limited to, 193 nm (excimer), 248 nm (excimer), 266 nm (solid-state), 355 nm (solid-state), 532 nm (solid-state), 1064 nm (solid-state), 308 nm (XeCl excimer), etc.) can also be used for the laser annealing process without departing from the scope of the disclosed concepts. Although several examples of lasers used for laser annealing are shown, the disclosed concepts are not limited to these examples and other lasers, such as fiber lasers, disk lasers, and other gas lasers, can also be employed without departing from the scope of the disclosed concepts.

[0013] In an exemplary embodiment, the beam energy distribution profile used in the laser annealing process is a spatially uniform distribution ("top hat"). In another exemplary embodiment, the beam energy distribution profile used in the laser annealing process is an M2 / Gaussian-like distribution. The shape of the energy profile of the beam can be manipulated using custom beam shaping optics, and a wide range of configurations are possible. The "top hat" profile and the Gaussian profile are two examples, but other beam profiles can be used in the laser annealing process without departing from the scope of the disclosed concepts.

[0014] Laser annealing of a thin layer of discontinuous conductive material 20 induces the formation of a nanoparticle distribution that results in a plasmonic optical effect. The process parameters can be controlled to produce the desired optical effect.

[0015] One advantage of using laser annealing to form plasmonic nanoparticles is the flexibility afforded by digitally controlled laser scanning systems. Such an approach can operate without the use of physical templates / masks (photolithography) or master images (embossing), with image resolution determined by the size of the laser beam used to induce annealing. Digital control also allows for full customization of the plasmonic image artwork, allowing custom data, barcoding, and serialization to be integrated into individual plasmonic images within a single laser annealing process. With such custom data, readable, identifiable marks with plasmonic optical properties can be used to digitally link to information in a database that allows tracking and tracing of products or production lines, providing an additional barrier against unauthorized duplication of such data.

[0016] A variety of optical components are available to expand the beam to a large area (e.g., but not limited to, 130 mm x 130 mm) or to create beam widths on the micron scale (beam waist of ~25 microns). In some exemplary embodiments, the laser annealing process is performed on the order of nanoseconds (7-25 nanosecond pulse durations) regardless of the size of the beam. However, shorter or longer pulse durations can be used without departing from the scope of the disclosed concepts. There is a tradeoff between scanning speed and resolution, so a higher resolution requires a smaller beam size and slower scanning speed to fill a comparable area, while a lower resolution uses a larger beam size to anneal more image area per unit time (the time required to generate the next laser pulse of sufficient energy ranges from 1-50 Hz for excimer systems and 1-120 kHz for solid-state systems, but this time may vary depending on the system used).

[0017] 1C shows an encapsulation layer 30 disposed over the laser annealed thin layer of discontinuous conductive material 20'. In exemplary embodiments of the disclosed concepts, the encapsulation layer 30 is added after the laser annealing process. It is also possible to omit the encapsulation layer 30 without departing from the scope of the disclosed concepts. However, the encapsulation layer 30 serves to protect the laser annealed thin layer of discontinuous conductive material 20' and, in some exemplary embodiments, is essential to achieve the desired plasmonic response.

[0018] In some exemplary embodiments, the encapsulation layer 30 is a polymer laminate that provides a protective hard coat to the laser annealed thin layer of discontinuous conductive material 20'. The polymer laminate may induce a small red shift in the spectral characteristics of the plasmonic response, depending on the nature of the laminate polymer selected. The red shift of the plasmonic response increases with the refractive index of the material used for the encapsulation layer 30. This effect should be considered in the design process, and in some exemplary embodiments, it is used to achieve a particular effect. For example, in some exemplary embodiments, a high refractive index material is used for the encapsulation layer to achieve a plasmonic response that is difficult to achieve by the selection of materials and laser annealing parameters alone (i.e., the intentional use of a dielectric that red shifts the plasmonic response). The encapsulation layer 30 is not limited to a polymer material, and any other suitable material may be used. In an exemplary embodiment, the encapsulation layer 30 may be composed of zinc sulfide, which acts as a high refractive index layer. The zinc sulfide may be topcoated with a polymer. However, this is only one example of a material that may be used, and other suitable materials may be used without departing from the scope of the disclosed concept.

[0019] In some exemplary embodiments, additional layers of dielectric material can be placed above and below the laser annealed thin layer of discontinuous conductive material 20'. Such structures will produce different optical results (i.e., when placed on a transparent substrate, different colors will be observed when viewed in front side reflective mode, back side reflective mode, and transmission mode). In some exemplary embodiments, the deposition of some dielectric layers may be performed before the deposition of the thin layer of conductive material 20'. However, the encapsulation layer 30 is still deposited after the laser annealing, and the annealing of the thin layer of conductive material 20 is performed open to the ambient environment.

[0020] In some exemplary embodiments, a first dielectric layer may be deposited and patterned (removing or masking portions of the first dielectric layer) using lithographic techniques, and then a second dielectric layer with a different refractive index may also be deposited to provide a pattern of contrasting dielectrics with different refractive indices (e.g., a checkerboard, stripes, etc.). Forming laser annealed nanoparticles on a surface in this manner results in two different plasmonic responses to the same laser annealing conditions, with the nanoparticles on (or immersed in) the higher refractive index dielectric being more red-shifted than the other. The dielectric layers may be polymer layers with different refractive indices.

[0021] 1D shows an example of an additional layer 40 into which the thin layer of laser annealed discontinuous conductive material 20' may be incorporated when the thin layer is incorporated into a security product. The additional layer 40 may be omitted without departing from the scope of the disclosed concepts. The additional layer 40 may provide, for example, but not limited to, other optical or security effects. The location of the additional layer 40 may vary. For example, one or more additional layers 40 may be disposed below the substrate 10 without departing from the scope of the disclosed concepts.

[0022] Additional features can be incorporated into security products in accordance with various exemplary embodiments of the disclosed concepts. For example, some embodiments can use optical spacer layers that act to modify the light absorption and thermal properties of the bond structure to affect the laser annealing process. The optical spacer layers can take the form of layers of silicon or aluminum oxide (or other layers, or combinations thereof and / or otherwise) that are deposited prior to forming a thin layer for laser annealing. Other non-limiting examples of materials that can be used for the optical spacer layers include TiO2, ZnO, and MgF2. However, these are non-exhaustive examples and other materials can be used without departing from the scope of the disclosed concepts.

[0023] Another example of an additive feature is a hidden effect. The laser annealing method is compatible with materials that generate a plasmonic response in infrared (e.g., copper nanoparticles) or ultraviolet (e.g., aluminum nanoparticles). Using such materials, invisible hidden security effects can be created. Such hidden features become visible using appropriate illumination for inspection (e.g., sunlight or IR or UV lamps) and reading tools (e.g., IR or UV cameras). In some exemplary embodiments, hidden and overt plasmonic features can be created during one laser annealing step; i.e., overt and hidden plasmonic optical effects can be created in registration. The creation of hidden features may be customizable and / or personalized.

[0024] The thin layer of laser annealed discontinuous conductive material 20' may provide a zero-order reflection-transmission color switching effect, alone or in combination with an encapsulation layer 30 and / or additional features, in exemplary embodiments. In its simplest form, a monolayer of plasmonic metal clusters on a visually transparent polymer substrate may provide a visible color switch (e.g., from blue to yellow) between direct reflection and transmission. The switched color may be determined by the laser annealing parameters and is fixed after completion of the laser annealing process. The color switch may also be modified by the encapsulation layer 30.

[0025] The optical effects provided by the laser annealed thin layer of discontinuous conductive material 20' may be integrated with additional optical technologies in the final product. For example, the optical effects provided by the laser annealed thin layer of discontinuous conductive material 20' may be performed in juxtaposition with other optical effects. For example, juxtaposition optical effects may enhance contrast effects and be useful for authentication applications (due to the difficulty of counterfeiting multiple technologies, the difficulty of tampering with multiple technologies without damage, and the difficulty of aligning / realigning in good registration).

[0026] As another example, the optical effects provided by the laser-annealed thin layer of discontinuous conductive material 20' may be integrated with microstructures such as micromirrors or diffractive elements that provide kinetic effects or other structures that enhance or amplify the visual impact of the plasmonic particles. For example, thin layers of liquid crystals, dielectrics and / or metals and optical stacks, methods of inducing polarization, and other variations of plasmonic nanostructures (e.g., nanohole arrays) may be utilized to enhance or amplify the visual impact of the plasmonic particles.

[0027] Security products include, but are not limited to, applications such as government issued documents (e.g., ID cards, passports, tax stamps, etc.), currency (banknotes, transaction cards, etc.), and other applications (bottles, blister packs, microelectronics, direct product attachment, downline supply chain applications, etc.).

[0028] The steps shown in the flow chart of Figure 2 correspond to the stages shown in Figures 1A-1D. For example, at 100, a conductive material is deposited on a substrate as shown in Figure 1A. At 102, laser annealing of the conductive material is performed as shown in Figure 1B. At 104, the laser annealed layer is encapsulated as shown in Figure 1C. And at 106, integration into a security product is performed as shown in Figure 1D. Steps 104 and 106 can be omitted without departing from the scope of the disclosed concept.

[0029] Figures 3-8 show the effect of the laser annealing process in some exemplary embodiments of the disclosed concepts when a polymer substrate is used instead of silicon, as will be described below. In Figure 3, a thin layer of discontinuous conductive material is shown in the left image, and laser annealed nanoparticles after a single laser pulse are shown in the right image. The left image is displayed on a 100 nm scale, and the right image is displayed on a 1000 nm scale. Similarly, Figure 4 is an image of laser annealed nanoparticles after a single laser pulse. As shown in Figure 4, the nanoparticles, shown in white, are disposed on a polymer substrate, shown in black.

[0030] Figure 5 shows images of nanoparticles after laser annealing with two laser pulses. The image on the left is a zoomed-in portion of the image on the right. After two laser pulses, it can be seen that some nanoparticles have been removed from the surface, leaving nanoscale indentations in the polymer substrate. Figure 6 shows images of nanoparticles after laser annealing with 150 laser pulses. It becomes clear that with this many laser pulses, the distribution of nanoparticles has become partially and / or completely immersed in the substrate.

[0031] FIG. 7 is a schematic diagram showing nanoparticles 204, 208, 210 immersed in a substrate 200 and a site 206 where the nanoparticles were removed during a laser annealing process. An ambient environment 202 is above the substrate 200 in FIG. 7. As the number of laser pulses increases, more of the remaining nanoparticles 204, 208, 210 become partially or completely immersed in the substrate 200. FIG. 7 shows various immersion states, where the nanoparticles 204 are barely immersed in the substrate 200, the nanoparticles 208 are partially immersed, and the nanoparticles 210 are completely immersed. Immersion is an effect that occurs in some exemplary embodiments of the disclosed concepts using polymer substrates (such as polycarbonate). Although this effect can occur with other types of substrates, such as glass substrates, polymer substrates are preferred due to their flexibility and suitability for mass production of roll-to-roll products. Glass is brittle and prone to shattering, making it difficult to handle in mass production. Additionally, polymer substrates are a popular medium for anti-counterfeiting products, due in part to their flexibility.

[0032] FIG. 8 is a schematic diagram showing an encapsulation layer 302 disposed on nanoparticles 304, 308, 310 at various stages of immersion in a substrate 300 and a site 306 where the nanoparticles were removed during a laser annealing process. As previously mentioned, the encapsulation layer red-shifts the plasmonic optical effect of the nanoparticles 304, 308, 310. However, the magnitude of the red-shift of the plasmonic response induced by the encapsulation layer 302 is proportional to the proportion of the nanoparticle distribution that is immersed in the substrate during the laser annealing stage. Structures with a smaller number of immersed nanoparticles have a larger red-shift of the plasmonic response after encapsulation compared to structures with a larger number of immersed nanoparticles, which have little or no shift in the plasmonic response. Furthermore, if the nanoparticles are fully immersed and enveloped by the substrate, a separate encapsulation for protection purposes can be omitted since the substrate protects the nanoparticles. When creating security products, the proportion of nanoparticles that are immersed in the substrate can be controlled to produce the desired effect. Thus, the desired final structure of the product can be achieved by the selection of polycarbonate polymer and a particular combination of laser annealing parameters such as the selection of laser, pulse profile, number of pulses, power, duration individually or collectively, as well as additional features such as encapsulation, optical spacers, and other additional features.

[0033] The use of a polymer substrate and the process of laser annealing a layer of conductive material such that at least a portion of the generated nanoparticles are at least partially immersed within the substrate can be used in various embodiments of the disclosed invention.

[0034] In some exemplary embodiments of the disclosed concepts, the conductive material is laser annealed such that at least some of the nanoparticles are at least partially immersed in the substrate. The percentage of the nanoparticles that are at least partially immersed in the substrate can vary without departing from the scope of the disclosed concepts. In an exemplary embodiment, at least about 10% of the nanoparticles are at least partially immersed in the substrate. In another exemplary embodiment, at least about 25% of the nanoparticles are at least partially immersed in the substrate. In another exemplary embodiment, at least about 50% of the nanoparticles are at least partially immersed in the substrate. In another exemplary embodiment, at least about 75% of the nanoparticles are at least partially immersed in the substrate. In another exemplary embodiment, between about 25% and about 75% of the nanoparticles are at least partially immersed in the substrate. At least partially immersed nanoparticles include both partially immersed nanoparticles and fully immersed nanoparticles.

[0035] While particular embodiments of the disclosed concepts have been described in detail, various modifications and substitutions to these details will become apparent to those skilled in the art upon consideration of this disclosure as a whole. Accordingly, the particular configurations disclosed are intended to be illustrative only and are not intended to limit the scope of the disclosed concepts, which is set forth in the full scope of the appended claims and equivalents thereof.

Claims

1. 1. A method of making a security product, comprising: Providing a substrate (10) made of a polymeric material; Depositing a discontinuous layer of conductive material (20) on a substrate (10); and laser annealing the discontinuous layer of conductive material (20) to produce nanoparticles exhibiting a surface plasmon resonance effect; The method, wherein a step of laser annealing a conductive material (20) to produce nanoparticles exhibiting a surface plasmon resonance effect includes laser annealing the conductive material (20) such that at least a portion of the nanoparticles are at least partially immersed within a substrate (10).

2. The method of claim 1, wherein the substrate (10) has a thickness in the range of 25 μm to 100 μm.

3. The method of claim 1 , wherein the discontinuous layer of conductive material (20) comprises at least one of silver, gold, copper, and aluminum.

4. The method of claim 1 , wherein the discontinuous layer of conductive material (20) has a thickness of less than 100 nm.

5. The method of claim 1 , wherein the discontinuous layer of conductive material (20) is deposited by vacuum deposition.

6. The method of claim 1 , wherein the discontinuous layer of conductive material (20) is laser annealed by an excimer laser or a solid state laser.

7. The method of claim 1 further comprising depositing an encapsulation layer (30) over the laser annealed discontinuous layer of conductive material (20).

8. The method of claim 7, wherein the encapsulation layer (30) is made of a polymeric material and provides a protective hardcoat over the laser annealed discontinuous layer of conductive material (20).

9. The method of claim 1 further comprising depositing at least one additional layer (40) onto the substrate.

10. 10. The method of claim 9, wherein the at least one additional layer (40) comprises a first dielectric layer having a first refractive index and a second dielectric layer having a second refractive index, the first refractive index and the second refractive index being different.

11. 2. The method of claim 1, wherein the step of laser annealing a conductive material (20) to produce nanoparticles exhibiting a surface plasmon resonance effect comprises laser annealing the conductive material (20) such that at least a portion of the nanoparticles are fully immersed within the substrate (10).

12. The method of claim 1 , wherein the security product exhibits a plasmonic response in the infrared or ultraviolet spectrum.

13. A substrate (10) made of a polymeric material; a discontinuous layer of conductive material (20) deposited on the substrate (10) and including nanoparticles exhibiting a surface plasmon resonance effect; A security product, wherein at least some of said nanoparticles are at least partially immersed within a substrate (10).

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