WO2020180255A1 - Optical security device, methods of forming and using the same - Google Patents

Abstract

Various embodiments may provide an optical security device. The optical security device may include a plurality of plasmonic structures. Each of the plurality of plasmonic structures may include a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure. The plurality of plasmonic structures may show a first image upon illumination of a first light having a first wavelength. The plurality of plasmonic structures may show a second image different from the first image upon illumination of a second light having a second wavelength different from the first wavelength.

Description

OPTICAL SECURITY DEVICE, METHODS OF FORMING AND USING THE

SAME

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application No. 10201902059T filed March 7, 2019, the contents of it being hereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] Various aspects of this disclosure relate to an optical security device. Various aspects of this disclosure relate to a method of forming an optical security device. Various aspects of this disclosure relate to a method of using an optical security device.

BACKGROUND

[0003] Artwork often needs to be verified for authenticity during transactions. Electronic tags and labels are easily spotted and hence counterfeited.

SUMMARY

[0004] Various embodiments may provide an optical security device. The optical security device may include a plurality of plasmonic structures. Each of the plurality of plasmonic structures may include a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure. The plurality of plasmonic structures may show a first image upon illumination of a first light having a first wavelength. The plurality of plasmonic structures may show a second image different from the first image upon illumination of a second light having a second wavelength different from the first wavelength.

[0005] Various embodiments may provide a method of forming an optical security device. The method may include forming a plurality of plasmonic structures. Each of the plurality of plasmonic structures may include a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure. The plurality of plasmonic structures may show a first image upon illumination of a first light having a first wavelength. The plurality of plasmonic structures may show a second image different from the first image upon illumination of a second light having a second wavelength different from the first wavelength.

[0006] Various embodiments may provide a method of using an optical security device. The method may include illuminating the optical security device with a first light having a first wavelength to show a first image. The method may include illuminating the optical security device with a second light having a second wavelength to show a second image different from the first image. The optical security device may include a plurality of plasmonic structures, each of the plurality of plasmonic structures including a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

FIG. 1 is a general illustration of an optical security device according to various embodiments. FIG. 2 is a general illustration of a method of forming an optical security device according to various embodiments.

FIG. 3 is a general illustration of a method of using an optical security device according to various embodiments.

FIG. 4A is a schematic showing a perspective view of an optical security device according to various embodiments.

FIG. 4B is a scanning electron micrograph (SEM) image of an array of plasmonic structures of the device according to various embodiments.

FIG. 5 is a schematic illustrating a method of forming a plasmonic structure of the device according to various embodiments.

FIG. 6A is a cross-sectional transmission electron microscopy (TEM) image of the sample according to various embodiments.

FIG. 6B is a plot of simulated reflectance as a function of wavelength (in nanometer or nm) illustrating the reflectance of a disk array with 240 nm diameter disks (represented by solid continuous line), and a disk array with 80 nm disks (represented by dashed line) according to various embodiments.

FIG. 6C illustrates the simulated electric (E) and magnetic (H) fields for the fundamental, 3rd order and 5th order gap plasmon modes of an array of disks with 240 nm diameter (D) and 70 nm gap (G) between neighboring disks according to various embodiments.

FIG. 7A is a plot of simulated reflectance as a function of wavelength (in nanometer or nm) illustrating the effect of increasing diameter for disk arrays with aluminum oxide (AI2O3) thicknesses of 3 nm, 7 nm and 20 nm on the fundamental resonances according to various embodiments.

FIG. 7B is a plot of the maximum absorptance as a function of wavelength (in nanometer or nm) illustrating the variation of absorptance with diameter for disk arrays with the three oxide thicknesses of 3 nm, 7 nm and 20 nm according to various embodiments.

FIG. 8A is a plot of disk diameter (in nanometer or nm) as a function of wavelength (nm) illustrating variation of measured reflectance spectra of square arrays of 44 nm tall aluminum (Al) nanodisks for wavelengths between 300 nm and 1700 nm and disk diameters between 60 nm and 280 nm according to various embodiments.

FIG. 8B is a plot of disk diameter (in nanometer or nm) as a function of wavelength (nm) illustrating variation of simulated reflectance spectra of square arrays of 44 nm tall aluminum (Al) nanodisks for wavelengths between 300 nm and 1700 nm and disk diameters between 60 nm and 280 nm according to various embodiments.

FIG. 9A is an optical micrograph of gap (in nanometers or nm) as a function of diameter (in nanometers or nm) showing 10 pm X 10 pm arrays of 44 nm tall aluminum (Al) nanodisks according to various embodiments.

FIG. 9B is an infrared micrograph of gap (in nanometers or nm) as a function of diameter (in nanometers or nm) showing 10 pm X 10 pm arrays of 44 nm tall aluminum (Al) nanodisks according to various embodiments.

FIG. 9C is an International Commission on Illumination (CIE) 1931 plot of chromaticity coordinates showing the range of colors obtained from the fabricated nanodisk array according to various embodiments.

FIG. 9D is a plot of reflectance R as a function of wavelength (in nanometers or nm) showing measured reflectance spectra (shifted along the y-axis for clarity) of 4 arrays of nanodisks: (1) diameter (D) - 60 nm, inter-disk gap (G) - 40 nm; (2) diameter (D) - 200 nm, inter-disk gap (G) - 100 nm; (3) diameter (D) - 80 nm, inter-disk gap (G) - 120 nm; and (4) diameter (D) - 230 nm, inter-disk gap (G) - 70 nm according to various embodiments.

FIG. 10A is (left) an optical micrograph of a 240 pm X 240 pm sample according to various embodiments under brightfield illumination showing a Quick Response (QR) code, and (right) the optical micrograph after image processing according to various embodiments.

FIG. 10B is an infrared micrograph of the sample shown in FIG. 10A according to various embodiments showing a bar code (Code 128C): 010203. The micrographs show good reproductions of the original QR code and barcode.

FIG. IOC is an optical micrograph of the sample according to various embodiments under darkfield illumination.

FIG. 10D shows a high-magnification optical micrograph and scanning electron micrograph (SEM) of the sample showing disks of four different disk diameters according to various embodiments.

FIG. 11A is a three-dimensional (3D) schematic of an array of single aluminum disks on aluminum oxide (AI2O3) on aluminum (Al) according to various embodiments, as well as a three-dimensional (3D) schematic of an array of aluminum disk clusters on aluminum oxide (AI2O3) on aluminum (Al) according to various embodiments.

FIG. 11B is a schematic showing light paths in darkfield microscopy according to various embodiments, where the central part of the light from the source is blocked by the central stop. FIG. l lC is an optical brightfield micrograph of an array according to various embodiments taken using a Nikon 20 x /0.45 NA objective.

FIG. 11D is an optical darkfield micrograph of the array according to various embodiments taken using a Nikon 50 x /0.8 NA objective.

FIG. 1 IE is a plot of reflectance (R) (in percent or %) as a function of wavelength (in nanometer or nm) showing the darkfield spectra for 280 nm wide disks and inter-disk gaps between 20 nm and 130 nm according to various embodiments. The threshold wavelengths for the array with a gap of 30 nm are indicated.

FIG. 1 IF is a plot of threshold wavelength (in nanometer or nm) as a function of period P showing fits of the experimental threshold wavelengths to the values obtained from the diffraction grating equation according to various embodiments.

FIG. 11G is a wave vector diagram for blue light at the threshold wavelength for complete collection of first-order diffracted light according to various embodiments. FIG. 11H is a wave vector diagram for red light according to various embodiments.

FIG. 12A shows (left) a schematic of a disk array according to various embodiments; and (right) a schematic showing the electric fields for x-linearly polarized light according to various embodiments.

FIG. 12B is an unpolarized, brightfield image of a plurality of arrays according to various embodiments.

FIG. 12C is an unpolarized, darkfield image of the plurality of arrays according to various embodiments.

FIG. 12D is a x-polarized, darkfield image of the plurality of arrays according to various embodiments.

FIG. 12E is a y-polarized, darkfield image of the plurality of arrays according to various embodiments.

FIG. 12F is a plot of reflectance R (in percent or %) as a function of wavelength (in nanometers or nm) showing the x-polarized darkfield spectra for the rectangular arrays of 200 nm wide disks according to various embodiments.

FIG. 12G is a plot of reflectance R (in percent or %) as a function of wavelength (in nanometers or nm) showing the y-polarized darkfield spectra for the rectangular arrays of 200 nm wide disks according to various embodiments.

FIG. 12H is a plot of reflectance R (in percent or %) as a function of wavelength (in nanometers or nm) showing the darkfield spectra for square arrays of single 200 nm wide disks with different periods according to various embodiments under unpolarized incident light.

FIG. 13 A is a plot of reflectance as a function of wavelength (in nanometers or nm) showing the simulated reflectance spectra from a 63°-polar angle Q, 0°-azimuthal angle f plane wave source for an array of 200 nm wide disks according to various embodiments.

FIG. 13B shows the simulated electric field and the simulated magnetic field for p-polarized illumination at 610 nm wavelength and at 460 nm wavelength according to various embodiments.

FIG. 13C is a plot of intensity (xlO ¹² W/m²) as a function of wavelength (in nanometer or nm) showing the simulated scattered field spectra for a rectangular array of 200 nm wide disks according to various embodiments under p- and .s-po lari zed illumination from a polar angle of 63° (Oin) and an azimuthal angle (f) of 0°. FIG. 13D shows a plot of flux (xlO ¹⁵ W) as a function of wavelength (in nanometer or nm) showing the total scattered light flux that fits into a collection cone with a half-angle of 53° for p-polarized and s- polarized illumination.

FIG. 13E shows a schematic of the setup with the collection cone according to various embodiments.

FIG. 14A shows (left) a schematic of an array of single disks according to various embodiments, and (right) a schematic of an array including a plurality of disk clusters according to various embodiments.

FIG. 14B shows a scanning electron microscope (SEM) image taken of an array including 120 nm wide disks with inter-disk gap gi of 20 nm and inter-disk gap g2 of 120 nm according to various embodiments.

FIG. 14C shows a darkfield micrograph of disk cluster arrays, each array including 80 nm wide disks, an inter-disk gap gi varying from 20 nm to 80 nm, and an inter-cluster gap g2 varying from 80 nm to 200 nm according to various embodiments.

FIG. 14D shows a darkfield micrograph of disk cluster arrays, each array including 100 nm wide disks, an inter-disk gap gi varying from 20 nm to 100 nm, and an inter-cluster gap g2 varying from 80 nm to 200 nm according to various embodiments.

FIG. 14E shows a darkfield micrograph of disk cluster arrays, each array including 120 nm wide disks, an inter-disk gap gi varying from 20 nm to 120 nm, and an inter-cluster gap g2 varying from 120 nm to 240 nm according to various embodiments.

FIG. 14F shows a brightfield micrograph of the disk cluster arrays shown in FIG. 14C according to various embodiments.

FIG. 14G shows a brightfield micrograph of the disk cluster arrays shown in FIG. 14D according to various embodiments.

FIG. 14H shows a brightfield micrograph of the disk cluster arrays shown in FIG. 14E according to various embodiments.

FIG. 141 is a three-dimensional (3D) plot of reflectance (in percent or %) as a function of wavelength l (in nanometer or nm) and inter-disk gap gi (in nanometer or nm) showing darkfield reflectance spectra for 120 nm wide disks and 480 nm period, with gi varied from 20 nm to 120 nm according to various embodiments.

FIG. 14J is a three-dimensional (3D) plot of reflectance (in percent or %) as a function of wavelength l (in nanometer or nm) and period P (in nanometer or nm) showing the darkfield spectra for 120 nm wide disks and gi of 20 nm, with a period varying from 380 nm to 500 nm according to various embodiments.

FIG. 14K is a plot of intensity (x 10 ¹² W/m²) as a function of wavelength (in nanometer or nm) showing scattered field spectra for an array of 120 nm wide disk clusters with gi of 20 nm and a period of 380 nm according to various embodiments, under p- and .s-polari cd illumination from polar angle of 63° and azimuthal angle of 0°.

FIG. 14L is a plot of flux (x 10 ¹⁵ W) as a function of wavelength (in nanometer or nm) showing the total collected light for a disk cluster array according to various embodiments under p- and .s-polarizcd illumination from polar angle of 63° and azimuthal angles of 0°, 15°, 30°, and 45°. FIG. 15A shows a section of the electron beam lithography (EPL) layout of the disks for patterning according to various embodiments.

FIG. 15B shows a scanning electron microscopy (SEM) image of a small section of the fabricated tag according to various embodiments.

FIG. 15C shows a brightfield visible image of the optical micro-tag according to various embodiments.

FIG. 15D shows an infrared image of the optical micro-tag according to various embodiments. FIG. 15E shows a x-polarized darkfield image of the optical micro-tag according to various embodiments.

FIG. 15F shows a y-polarized darkfield image of the optical micro-tag according to various embodiments.

DETAILED DESCRIPTION

[0008] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, and logical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0009] Embodiments described in the context of one of the methods or optical security devices are analogously valid for the other methods or devices. Similarly, embodiments described in the context of a method are analogously valid for an optical security device, and vice versa.

[0010] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0011] The device as described herein may be operable in various orientations, and thus it should be understood that the terms “top”, “bottom”, etc., when used in the following description are used for convenience and to aid understanding of relative positions or directions, and not intended to limit the orientation of the device.

[0012] In the context of various embodiments, the articles“a”,“an” and“the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0013] In the context of various embodiments, the term“about” or“approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.

[0014] As used herein, the term“and/or” includes any and all combinations of one or more of the associated listed items.

[0015] Colors in the visible spectrum are the basis of human vision, a highly developed part of the nervous system, and serve an important role in nature, signaling, technology and the arts. In addition to pigments, metallic and dielectric nanostructures act as“geometry-tunable colors” due to the excitation of surface plasmons and Mie scattering. Color generation has been demonstrated using many types of structures, including metal disks above a perforated back- reflector, dielectric resonators above dielectric substrates and all-metallic structures. These designs have expanded the technology of color printing, as the resolution has been increased to lxlO⁵ dots per inch, and the attainable color gamut has exceeded that of standard Red Green Blue (sRGB). There has also been strong interest in designing structures that generate different visual responses by a change in the polarization of light, e.g. using structures with elliptical or rod shapes.

[0016] Conversely, prints in the infrared (IR) spectrum are less well understood and largely based on IR inks. IR prints would enable a discreet channel of security tagging and encryption, as they are invisible to humans. [0017] Various embodiments may address the issues highlighted above. Various embodiments may relate to a covert and effective tag that is (1) microscopic, (2) camouflaged into the surroundings, and (3) contain multiple sets of information.

[0018] Various embodiments may use plasmon resonances in a thin layer of electrically insulating material, e.g. native aluminum (III) oxide (AI2O3), to create micro-tags that can be viewed under different wavelengths (e.g. under visible and infrared illumination), allowing for two sets of images to be printed onto the same area. The micro-tags may include aluminum (Al) nanostructures that resonate across the ultraviolet (UV) to infrared (IR) spectra with varying colors of similar brightness, and containing two sets of information in the visible and IR. The native AI2O3 on the Al films may be ~4-7 nm thick, enabling resonances to be supported by Al disks with diameters merely ~l/6th of the wavelength at the fundamental mode. The micro-tags may be printed on silicon through accurate modeling of the nanostructures and high-resolution electron-beam lithography. The micro-tag may require image processing to extract a quick response (QR) code in the visible, and 1.2 pm IR illumination (or visible light darkfield imaging) to extract a covert barcode. The compact and multi- spectral encoding of prints demonstrated in embodiments described herein may be particularly suited for discreet tagging of art and high-value merchandise.

[0019] In various embodiments, only the infrared image may be observed when the print is viewed under infrared illumination, and vice versa, only the visible color image may be observable when the print is viewed using visible light. The infrared resonance may arise from the fundamental gap plasmon mode, while both fundamental and higher order modes may contribute to producing resonances in the visible regime.

[0020] FIG. 1 is a general illustration of an optical security device 100 according to various embodiments. The optical security device may include a plurality of plasmonic structures 102. Each of the plurality of plasmonic structures 102 may include a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure. The plurality of plasmonic structures 102 may show or may be configured to show a first image upon illumination of a first light having a first wavelength. The plurality of plasmonic structures may show or may be configured to show a second image different from the first image upon illumination of a second light having a second wavelength different from the first wavelength. [0021] In other words, various embodiments may provide an optical security device 100 including more than one plasmonic structures 102. Each plasmonic structure may include a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure. When a first light of a first wavelength is provided onto the optical security device 100, the plasmonic structures 102 may display a first image, and when a second light of a second wavelength is provided onto the optical security device 100, the plasmonic structures 102 may display a second image different from the first image.

[0022] FIG. 1 serves to illustrate various features of the optical security device 100 according to various embodiments, and is not intended to limit the size, orientation, shape, arrangement etc. of the various features. For instance, the plurality of plasmonic structures 102 may occupy an entire surface of the optical device 100, or may occupy only a portion of the optical device 100, such as a central portion or a corner portion. The optical security device 100 and/or the plurality of plasmonic structures 102 may be rectangular, or may be of any other suitable shapes, e.g. circular, or elliptical.

[0023] In various embodiments, in a plasmonic structure, the electrically insulating structure of the plasmonic structure may be on the first electrically conductive structure of the plasmonic structure, while the second electrically conductive structure of the plasmonic structure may be on the electrically insulating structure. A first surface of the electrically insulating structure may be in contact with the first electrically conductive structure, and a second surface of the electrically insulating structure opposite the first surface may be in contact with the second electrically conductive structure. A thickness of the electrically insulating structure may be equal to a gap between the first electrically conductive structure and the second electrically conductive structure.

[0024] By changing the dimensions of the plasmonic structure, e.g. the lateral dimensions of the top second electrically conductive structure or the gap between neighboring plasmonic structures, the resonant wavelengths may be tuned. At the resonant wavelength, energy may be absorbed, so that there is a dip in reflectance at that wavelength, which may be observed by the eye or detected by an appropriate detector. The plasmonic structure may support up to five modes.

[0025] The different images may arise due to resonant electromagnetic modes, which may occur at ultraviolet, visible and infrared wavelengths. By having plasmonic structures 102 of different dimensions in the optical security device 100 or by having different gaps between neighboring plasmonic structures, different plasmonic structures 102 may exhibit resonance at different wavelengths, thereby giving rise to different images when the optical security device is illuminated under the different wavelengths.

[0026] For instance, by making a specific pattern of differently sized nanostructures in the top layer (i.e. second electrically conductive structures), an image may be produced, as different points formed by different nanostructures may have different reflectances and/or different colors. One image may be observable to the human eye or a visible light detector under a visible light source. The second distinct image may be hidden under normal lighting conditions, but may become observable with an infrared light source and an infrared detector. Only the visible color image may be observed when the print is viewed using visible light, and vice versa, only the infrared image may be observed when the print is viewed using infrared light. Additional images may be made by changing the period of the structures in the top layers or by making the structures rectangular, so that they are asymmetrical in the orthogonal horizontal axes.

[0027] As highlighted above, parameters of the plurality of plasmonic structure 102 may be tuned to give rise to different images under different wavelengths. For instance, the lateral dimensions of the plasmonic structures, the thickness of the electrically insulating layer and/or the gap between neighboring plasmonic structures may be tuned to give rise to different images under different wavelengths.

[0028] For instance, a first diameter of a first plasmonic structure (e.g. of the top second electrically conductive layer of the first plasmonic structure) of the plurality of plasmonic structures 102 may be different from a second diameter of a second plasmonic structure (e.g. of the top second electrically conductive layer of the second plasmonic structure) of the plurality of plasmonic structures. Additionally, or alternatively, a first thickness of a first electrically insulating structure of a first plasmonic structure of the plurality of plasmonic structures may be different from a second thickness of a second electrically insulating layer of a second plasmonic structure of the plurality of plasmonic structure. Additionally, or alternatively, a first gap (or pitch) between a first pair of neighboring plasmonic structures of the plurality of plasmonic structures may be different from a second gap (or pitch) between a second pair of neighboring plasmonic structures of the plurality of plasmonic structures. [0029] In the current context, a“gap” between neighboring plasmonic structures may refer to a spacing between a first point along the circumference or perimeter of a first plasmonic structure and a second point along the circumference or perimeter of a neighboring second plasmonic structure that is closest to the first point. Neighboring plasmonic structures may refer to a pair of plasmonic structures that are in the immediate vicinity of each other amongst the plurality of plasmonic structures 102, or closest to each other amongst the plurality of plasmonic structures 102.

[0030] In the current context, a“pitch” between neighboring plasmonic structures may be a distance between a center of the first plasmonic structure and a center of the second plasmonic structure. A pitch between neighboring plasmonic structures may be equal to the sum of a gap between the neighboring plasmonic structures, a radius of the first plasmonic structure and a radius of the second plasmonic structure.

[0031] In various embodiments, the first light may be visible light, and the second light may be infrared (IR) light. In other words, the optical device 100 may exhibit different images under visible light and under infrared light. Visible light may have a range from 400 nm to 700 nm. Accordingly, when the first light is visible light, the first wavelength may be any wavelength or range of wavelengths selected between 400 nm and 700 nm. Infrared light may have a range from 700 nm to 1 mm. Accordingly, when the second light is infrared light, the second wavelength may be any wavelength or range of wavelengths selected between 700 nm and 1 mm, e.g. between 700 nm to 2100 nm.

[0032] In various other embodiments, the optical device 100 may exhibit different images under any combination pair selected from a group consisting of visible light, infrared (IR) light, and ultraviolet (UV) light. For instance, the first light may be visible light, and the second light may be ultraviolet (UV) light. When the second light is ultraviolet light, the second wavelength may be any wavelength or range of wavelengths selected between 10 nm and 400 nm.

[0033] In another example, the first light may be infrared (IR) light, and the second light may be ultraviolet (UV) light. Accordingly, the first wavelength may be any wavelength or range of wavelengths selected between 700 nm and 1 mm, e.g. between 700 nm to 2100 nm, and the second wavelength may be any wavelength or range of wavelengths selected between 10 nm and 400 nm.

[0034] In various embodiments, two or more plasmonic structures (of the plurality of plasmonic structure 102) may be or may be configured to be of one color (i.e. of the same color) under illumination of visible light, but may be or may be configured to be of different infrared (IR) reflectances under illumination of infrared light, or of different ultraviolet (UV) reflectances under illumination of ultraviolet light. Another two or more plasmonic structures (of the plurality of plasmonic structures 102) may be or may be configured to be of different colors under illumination of visible light, but may be or may be configured to be of one IR reflectance under illumination of infrared light, or of one UV reflectance under illumination of ultraviolet light. The two or more plasmonic structures and the other two or more plasmonic structures may have different dimensions, and/or different gaps with neighboring structures. By arranging or forming the two or more plasmonic structures and the other two or more plasmonic structures at desired locations of the optical security device 100, the plurality of plasmonic structures 102 may be or may be configured to show a first image upon illumination of a first light having a first wavelength, and to show a second image different from the first image upon illumination of a second light having a second wavelength different from the first wavelength.

[0035] In various embodiments, the plurality of plasmonic structures may show more than two images under illumination of light of different wavelengths. For instance, the plurality of plasmonic structures may show or may be configured to show a third image (that is different from the first image and the second image) upon illumination of a third light having a third wavelength (that is different from the first wavelength and the second wavelength). In various embodiments, the different lights may be the same type of light but of different wavelengths. For instance, the plurality of plasmonic structures may show a first image under illumination of infrared light at 900 nm, but may show a second image different from the first image under illumination of infrared light at 1500 nm. The different images may be detected by using a first detector that is responsive to the first wavelength but not to the second wavelength, and using a second detector that is responsive to the second wavelength, but not to the first wavelength.

[0036] In various embodiments, the first electrically conductive structure may include aluminum, the electrically insulating structure may include aluminum oxide, and the second electrically conductive structure may include aluminum. In various other embodiments, the first electrically conductive structure and/or the second electrically conductive structure may include any other suitable electrically conductive material, e.g. a metal such as silver or gold, while the electrically insulating structure may include any suitable insulating material, e.g. silicon dioxide. [0037] In various embodiments, the second electrically conductive structure of each of the plurality of plasmonic structures may be a nanostructure, e.g. a nanodisk. A“nanostructure” may be defined as a structure with at least one dimension in the nanoscale range, i.e. less than 100 nm. In various other embodiments, the second electrically conductive structure may not be nanostructures, i.e. all dimensions of the second electrically conductive structure may each be equal or greater than 100 nm.

[0038] In various embodiments, the first electrically conductive structure of each of the plurality of plasmonic structures may be a portion of a single continuous layer.

[0039] In various embodiments, the electrically insulating structure of each of the plurality of plasmonic structures may be a nanostructure, e.g. a nanolayer with thickness equal to or less than 20 nm, or equal to or less than 7 nm, or equal to or less than 3 nm. The thickness of the electrically insulating structure may be less than 100 nm.

[0040] In various embodiments, the first image and the second image may be different encoded images, such as barcodes. For instance, the first image may be a Quick Response (QR) code, and the second image may be a Universal Product Code (UPC) bar code. In various other embodiments, the first image may be a non-encoded image, e.g. a drawing or a portion of the drawing or a solid color image, while the second image may be an encoded image, such as a bar code.

[0041] Various embodiments may be used for tagging luxury merchandise, beverage bottles, time pieces, art pieces, documents, or currency notes etc. Various embodiments may find applications in the fashion, beverage, watch, art, or printing industries.

[0042] Various embodiments may not rely on polarization or viewing angle effects to generate additional images. Various embodiments may form or print images of very high resolution, e.g. about 80,000 dots per inch (dpi). Various embodiments may form or print images that are much higher in resolution than those formed or generated by other technologies.

[0043] The optical security device 100 may be a micro-tag.

[0044] FIG. 2 is a general illustration of a method of forming an optical security device according to various embodiments. The method may include, in 202, forming a plurality of plasmonic structures. Each of the plurality of plasmonic structures may include a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure. The plurality of plasmonic structures may show a first image upon illumination of a first light having a first wavelength. The plurality of plasmonic structures may show a second image different from the first image upon illumination of a second light having a second wavelength different from the first wavelength.

[0045] In various embodiments, forming the plurality of plasmonic structures may include forming a first electrically conductive layer (e.g. the bottom electrically conductive layer) on a substrate by depositing a suitable electrically conductive material. The first electrically conductive structures of the plurality of plasmonic structures may be parts or portions of the first electrically conductive layer.

[0046] The method may further include exposing the first electrically conductive layer to form a native oxide layer. The native oxide layer may be formed at the exposed surface of the first electrically conductive layer by exposing the first electrically conductive layer to air or oxygen.

[0047] The method may also include depositing resist over the native oxide layer. The method may additionally include patterning the deposited resist to expose portions of the first native oxide layer. The resist may be an electron-beam (e-beam) resist, and patterning the deposited resist may involve using electron beam lithography (EBL) to pattern the e-beam resist.

[0048] The method may additionally include depositing the suitable electrically conductive material and removing the deposited resist in a lift-off process to form a plurality of second electrically conductive structures (e.g. the top electrically conductive structures). A further native oxide layer may form around each of the plurality of second electrically conductive structures. An electrically insulating structure of a plasmonic structure may include a portion of the native oxide layer between a part or portion of the first electrically conductive layer and a corresponding second electrically conductive structure of the plurality of second electrically conductive structures, as well as a portion of the further native oxide layer over the portion of the native oxide layer.

[0049] Accordingly, the plasmonic structure may be formed from the part or portion of the first electrically conductive layer, the corresponding second electrically conductive structure, and the electrically insulating structure.

[0050] FIG. 3 is a general illustration of a method of using an optical security device according to various embodiments. The method may include, in 302, illuminating the optical security device with a first light having a first wavelength to show a first image. The method may include, in 304, illuminating the optical security device with a second light having a second wavelength to show a second image different from the first image. The optical security device may include a plurality of plasmonic structures, each of the plurality of plasmonic structures including a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure.

[0051] For avoidance of doubt, step 302 may occur before step 304, or may occur after step 304.

[0052] In various embodiments, the first light may be visible light, and the second light may be infrared (IR) light. The method may include observing the first image directly or via a visible light detector. The optical security device may be illuminated with a visible light source, which provides the visible light. The method may also include observing the second image via an infrared light detector. The infrared detector may be coupled to a processor for processing the second image. The second image may be shown after detecting via the infrared light detector, and processing via the processor. The second image may be shown via a display coupled to the processor. The optical security device may be illuminated with an infrared light source, which provides the infrared light. Similarly, the visible light detector may be coupled to the processor, and the first image may be shown via a display coupled to the processor.

[0053] In various other embodiments, the optical device may exhibit different images under any combination pair selected from a group consisting of visible light, infrared (IR) light, and ultraviolet (UV) light. For instance, the first light may be visible light, and the second light may be ultraviolet (UV) light. As highlighted above, the first image may be observed directly or via a visible light detector, and a visible light source may be used to provide the visible light. The second image may be observed via an ultraviolet (UV) light detector, and the optical security device may be illuminated with an ultraviolet light source, which provides the ultraviolet light. The UV light detector may be coupled to a processor for processing the second image. The second image may be shown after detecting via the UV light detector, and processing via the processor. The second image may be shown via a display coupled to the processor.

[0054] In various embodiments, the first image and the second image may be different encoded images, such as barcodes. For instance, the first image may be a Quick Response (QR) code, and the second image may be a Universal Product Code (UPC) bar code. In various other embodiments, the first image may be a non-encoded image, e.g. a drawing or a portion of the drawing or a solid color image, while the second image may be an encoded image, such as a bar code.

[0055] Example 1

[0056] Gap-plasmon resonators may be formed when a thin layer of dielectric is sandwiched by two layers of electrical conductors. The sandwich structure may support a localized surface plasmon mode with tight confinement of the electric and magnetic fields. In particular, if the top layer is comprised of nanodisks, a metasurface may be formed that acts as a magnetic mirror and exhibits resonances with desirable features including very high or perfect absorption, high spectral selectivity and strong wavelength tunability. As the gap decreases, the resonance modes may redshift due to the larger capacitive coupling between the top layer and the bottom layer. Hence, plasmon resonances in structures with sub- 10 nm thick dielectric layers may support resonances that span the ultraviolet, visible and infrared spectra while preserving the sub-micron length scale of the nanodisks. Specifically, for a ~7 nm thick aluminum oxide (AI₂O₃) layer, the fundamental gap plasmon resonance may be shown to occur at a wavelength larger than six times the diameter of the nanostructures in the top layer. Thus, the gap plasmons may be used to generate infrared images, in addition to the visible color images.

[0057] Aluminum (Al) may be a suitable material as it supports surface plasmons and may form a self-limiting 3-4 nm thick layer of native oxide, i.e. AI₂O₃, on its surface. Native AI₂O₃ had previously been used as the insulator layer for metal-insulator- metal (MIM) structures; and AI₂O₃ had also been demonstrated to be deposited using atomic layer deposition (ALD) or physical vapor deposition (PVD). However, the absorptances of MIM structures with a 3 nm thick AI₂O₃ layer are low, and strong absorptance at resonance may require at least twice the native oxide thickness. Nanostructures with diameters larger than 120 nm may support higher- order modes at visible wavelengths, in addition to the fundamental resonance at infrared wavelengths, whereas smaller nanostructures with diameters less than 100 nm may only support the fundamental resonance mode. Thus, images may be created at both visible and infrared wavelengths and recorded using appropriate optics. In addition to micro-tags, various embodiments may be used in security printing, anti-counterfeiting, and covert information storage.

[0058] 1.1 Structure of the gap plasmon resonator [0059] FIG. 4A is a schematic showing a perspective view of an optical security device according to various embodiments. The optical security device shown in FIG. 4A includes 44 nm tall (T) A1 nanodisks on a layer of AI2O3 above a 100 nm thick A1 film on a bulk silicon (Si) substrate. The diameter (D) and inter-disk gap (G) may be varied from 60 nm to 280 nm and from 30 nm to 140 nm respectively. Each plasmonic pixel or plasmonic structure may include a nanodisk, an underlying portion of the A1 film, as well as the intervening AI2O3 film. FIG. 4B is a scanning electron micrograph (SEM) image of an array of plasmonic structures of the device according to various embodiments. The A1 disks are 44 nm tall, and the diameter (D) of each disk is 240 nm. The gap (G) between neighboring disks may be 70 nm.

[0060] 1.2 Nanofabrication

[0061] The fabrication procedure may involve an initial A1 deposition step, electron beam lithography, a second A1 deposition step, and lift-off to form A1 disks.

[0062] FIG. 5 is a schematic illustrating a method of forming a plasmonic structure of the device according to various embodiments. First, under (a) a 100 nm thick film of A1 may be evaporated onto a bare Si substrate using an electron beam evaporator (Labline, Kurt J. Lesker Co.). The working pressure and deposition rate may be set to 1 x 10 ⁶ Torr and 2 A/s respectively. Upon exposure to air for one day, a self-limiting ~4 nm thick layer of native AI2O3 may be formed.

[0063] Next, the positive-tone electron-beam resist (polymethyl methacrylate (PMMA) (950K A4, MicroChem Corp.)), may be spin-coated at 6000 revolutions per minute (rpm) for 60 s and then baked at 180 °C for 60 s to a thickness of 170 nm. Electron beam lithography (EBL) may be performed using the eLine Plus, Raith GmbH at 30 kV acceleration voltage and 400 pA beam current (under (b)). In order to produce the color palette (described in more detail below), a write field of 100 pm x 100 pm was used, whereas for the security pattern (also described in more detail below), a write field of 50 pm x 50 pm was used. Proximity effect correction may be performed for the palette due to the larger write field used. The resist may be developed in 1:3 methyl isobutyl ketone/isopropanol (MIBK/IPA) at -15 °C for 30 s, rinsed in IPA for 5 s and blow-dried using a nitrogen gun.

[0064] Thereafter, under (c), a second evaporation step of 44 nm of A1 using the same electron beam evaporator may be performed as before. Finally, a lift-off process may be done to remove the unexposed resist by soaking the samples in acetone at 60 °C to obtain the desired metal-disk-on-oxide-on-metal structures (as shown in (d)). The disk diameter may range from 60 nm to 280 nm, while the inter disk gap may range from 30 nm to 140 nm.

[0065] 1.3 Optical and infrared microscopy

[0066] Brightfield and darkfield optical microscopy, infrared microscopy and reflection spectra measurements were performed on the samples to study their responses to visible and infrared light. The brightfield and darkfield images were taken in reflection mode using an upright compound microscope (Nikon Eclipse LV100ND, Nikon Instruments Inc.) and a digital complementary metal-oxide-semiconductor (CMOS) camera (Nikon DS-Ri2, Nikon Instruments Inc.). The optical images were magnified using a brightfield/darkfield (BD) objective lens (TU Plan Fluor BD 50 x/0.80 NA). Infrared images were taken using a cryogenically cooled infrared analysis system (FEI Meridian IV, FEI Inc.) attached with a 50 x/0.45 NA objective and an indium gallium arsenide (InGaAs) camera (DiamondBack extended, DCG Systems Inc.). The light source was a gallium arsenide (GaAs) light emitting diode (FED) with a central wavelength of 1.2 pm and full- width at half-maximum of 0.05 pm.

[0067] 1.4 Reflectance Spectroscopy

[0068] Reflectance spectra were measured using a UV-visible-NIR microphoto spectrometer (CRAIC QDI 2010, CRAIC Technologies Inc.) with a 75 W xenon lamp at normal incidence. The light was passed through a 7.1 pm x 7.1 pm aperture and an objective lens (36 x/0.5 NA) and the reflected light (0.3-1.7 pm) was collected by charge- coupled device (CCD) detectors. The two detectors are a silicon detector (working range of 200-950 nm) and an InGaAs detector (850-2300 nm). Finear interpolation was done for data points between 1.37 to 1.42 pm to remove the noise in the spectra occurring at -1.4 pm due to absorption in the optical fiber connecting the lamp to the spectrometer.

[0069] 1.5 Finite-difference time-domain simulations

[0070] A commercial finite-difference time-domain (FDTD) package (Fumerical FDTD Solutions, Fumerical Inc.) was used to calculate the electromagnetic fields and reflectance of the structures. A bulk layer of A1 was used as the substrate, while a 7 nm thick layer of AI2O3 was placed atop the A1 substrate. The disk was modelled as a 44 nm tall tapered cylindrical core of A1 with a thin 4 nm coating of AI2O3 on the top and sides. The diameter of the disk was set to a value between 60 nm and 280 nm with 5 nm increments. The side wall of the disk was tapered with a gradient of 2.2, while the edge connecting the top face of the disk and the side wall was curved with a 10 nm radius of curvature. The permittivity data for A1 and AI2O3 were obtained from a reference text ( Edward D. Palik, Handbook of Optical Constants of Solids {Academic, 1998)), while the background of the simulation region was taken to be vacuum (refractive index of 1).

[0071 ] A rectangular simulation region was used with the z-axis perpendicular to the surface of the substrate. The simulation region extended in the x- and y-directions. The lengths of the simulation region in the x- and y-directions were both equal to the period of the disk array. Periodic boundary conditions were used for the x-min, x-max, y-min and y-max faces of the boundary, whereas perfectly matched layer boundary conditions were used for the z-min and z-max planes. A uniform mesh of 1 nm step size was used for a rectangular region around the disk, while a non-uniform conformal mesh was used for the other regions. A plane wave source parallel to the xy-plane was placed 700 nm above the surface of the substrate. It was linearly polarized in the x-direction and had a wavelength range of 300 nm to 1700 nm. Field monitors were respectively placed in the xy-plane 100 nm above the source and in the xz -plane through the center of the disk to measure the reflected power and the electric and magnetic fields.

[0072] 2. Characterization of resonators

[0073] The physical dimensions of the disks and substrate were measured by taking a cross- sectional transmission electron microscopy (TEM) image of the sample. FIG. 6A is a cross- sectional transmission electron microscopy (TEM) image of the sample according to various embodiments. The A1 disk is 44 nm tall, and the diameter of the disk is 240 nm. The thickness of the AI2O3 in the regions of the substrate that are free of disks was measured as 4 nm, while the thickness of the AI2O3 below the disk was 7 nm. The TEM image also shows a 3-4 nm thick AI2O3 layer coating the upper surface, side walls and bottom surface of the disk. Plausibly, a second layer of native oxide may form around the disk after the second deposition leading to a total thickness of AI2O3 below each A1 disk to be 7 nm. The AI2O3 may have been formed in two steps: (1) after the base A1 layer was deposited onto the Si substrate using an electron beam evaporator, a ~4 nm thick layer of native AI2O3 may be formed on the surface; (2) during the lift-off process, an additional ~3 nm layer of AI2O3 may be formed all around the disks.

[0074] 2.1 Finite-difference time-domain (FDTD) simulations and experimental reflectance spectra

[0075] Small A1 disks with diameters < 100 nm may support fundamental resonance modes in the visible range, between 400 nm and 800 nm. FIG. 6B is a plot of simulated reflectance as a function of wavelength (in nanometer or nm) illustrating the reflectance of a disk array with 240 nm diameter disks (represented by solid continuous line), and a disk array with 80 nm disks (represented by dashed line) according to various embodiments. Both disk arrays have 70 nm spacing and have 7 nm AI2O3 thickness. Light is incident normally at both disks. Based on the FDTD simulation results in FIG. 6B, the fundamental resonance wavelength for a disk array with a diameter of 80 nm and a period of 150 nm is -580 nm. There is a strong absorption peak at resonance, corresponding to a low reflectance of -0.25. Inter-band transitions between the W2’ and W1 symmetry points in the electronic band structure of A1 manifest as a dip in the reflectance spectra at -800 nm wavelength. The fundamental, 3^rd order and 5^th order modes for the 240 nm disk array may occur at near-IR, visible and UV wavelengths respectively.

[0076] As the diameters of the disks increase, the fundamental resonance may redshift into the infrared region, and higher order resonances may appear at visible and ultraviolet wavelengths. Thus, for a disk array with a diameter of 240 nm and a period of 310 nm, the fundamental resonance may occur at 1700 nm, while the third and fifth order resonances may occur at 600 nm and 360 nm respectively. The tunability of strong resonances across both infrared and visible wavelengths suggests that it may be possible to control the infrared and visible appearances of the disks by changing the disk sizes and array periods, and to design specific patterns observable at either visible or infrared wavelengths.

[0077] FIG. 6C illustrates the simulated electric (E) and magnetic (H) fields for the fundamental, 3^rd order and 5^th order gap plasmon modes of an array of disks with 240 nm diameter (D) and 70 nm gap (G) between neighboring disks according to various embodiments. The period of the disk array is thus 310 nm. A gap plasmon may be excited in the AI2O3 layer when electromagnetic radiation, polarized along the x-axis, is normally incident onto the AI2O3 layer. For normal incidence, only the odd anti- symmetric modes may be excited. Opposite charges may accumulate at opposite edges of the base of the A1 disk, and may be balanced by charges at points on the surface of the A1 substrate directly below the edges of the base of the disk. Thus, the electric field may be strongest at points along the vertical direction, i.e. within the AI2O3 gap layer, between the base of the A1 disk and the A1 substrate. The charges may produce a displacement current loop, and a strong magnetic field may be formed in the central region of the AI2O3 layer, parallel to the plane of the substrate and perpendicular to the polarization of the plane wave light source. Electromagnetic energy may be absorbed efficiently and dissipated in the A1 disks and substrate, producing the reflectance minima shown in FIG. 6B. For the third and fifth order modes, the number of displacement current loops may increase to three and five respectively, and the electromagnetic energy that was formerly concentrated in the AI2O3 gap layer may extend into the air surrounding the A1 disk, i.e. the mode may become less localized.

[0078] The oxide thickness may affect both the tunable range of wavelength shifts, and the strength of the absorptance (Absorptance = 1 - Reflectance) at resonance. Tunability may be the highest for the thinnest oxides, while the strength of the absorptance may follow a broad envelope with the thinnest oxides having the strongest absorptances at longer wavelengths. The reflectances obtained from FDTD simulations of disk arrays on oxide layers of different thicknesses (3 nm, 7 nm and 20 nm) are presented in FIG. 7A.

[0079] FIG. 7A is a plot of simulated reflectances as functions of wavelength (in nanometer or nm) illustrating the effect of increasing diameter for disk arrays with aluminum oxide (AI2O3) thicknesses of 3 nm, 7 nm and 20 nm on the fundamental resonances according to various embodiments. The height of the A1 disks is kept constant at 44 nm, and the gap between neighboring disks is kept constant at 70 nm.

[0080] In this range of diameters, the thinnest oxide of 3 nm shows the largest resonance shifts, Dl = 1740 nm (620 nm to 2360 nm). Increasing the oxide thickness may blueshift the resonances and may decrease the tunability. For instance, Dl is halved to -880 nm for an oxide thickness of 20 nm.

[0081] FIG. 7B is a plot of the maximum absorptances as functions of wavelength (in nanometer or nm) illustrating the variation of absorptance with diameter for disk arrays with the three oxide thicknesses of 3 nm, 7 nm and 20 nm according to various embodiments. Each data point is taken from one disk diameter, and the interval for the disk diameter is 10 nm for all three oxide thicknesses. The smallest and largest diameters for the data set of each oxide thickness are indicated in FIG. 7B. The inter-band transition region is indicated with dotted lines.

[0082] For the disk diameters considered, the maximum absorptance occurs at the fundamental resonance. A clear trend may be observed in the maxima of the absorptances, i.e. that the disks on 3 nm thick oxide have weak absorptances in the visible but good absorptances in the IR. On the other hand, the disks on 20 nm thick oxide have stronger absorptances in the visible, and may hence be useful in producing plasmonic colors. The 7 nm thick oxide may provide a desirable trade-off between tunability and strong absorptances spanning both visible and IR, since the maximum absorptance is at least 0.55 and Dl is 1220 nm for disk diameters from 60 nm to 240 nm.

[0083] The experimental reflectance data from square arrays of disks with diameters between 60 nm and 280 nm and a fixed gap of 30 nm are presented in FIG. 8A. FIG. 8A is a plot of disk diameter (in nanometer or nm) as a function of wavelength (nm) illustrating variation of measured reflectance spectra of square arrays of 44 nm tall aluminum (Al) nanodisks for wavelengths between 300 nm and 1700 nm and disk diameters between 60 nm and 280 nm according to various embodiments. The pitch size ranges from 90 nm to 310 nm. The lines indicate the wavelength scaling of resonances fit. There are three minima lines observed, i.e. the fundamental, third and fifth order modes as labelled. The fundamental resonance is present for all the disk diameters studied, while the third and fifth order modes are observed for disk diameters larger than 120 nm. The largest shift is for the fundamental resonance, as the resonance wavelength starts at 350 nm for 60 nm wide disks and redshifts to 1650 nm for 280 nm wide disks. In contrast, the third-order and fifth-order resonances only redshift by 250 nm and 150 nm respectively. Inter-band transitions in Al lead to a reduction of 0.1 in the reflectance around 800 nm, and cause a broadening of the fundamental resonance for disk diameters between 120 nm and 150 nm.

[0084] FIG. 8B is a plot of disk diameter (in nanometer or nm) as a function of wavelength (nm) illustrating variation of simulated reflectance spectra of square arrays of 44 nm tall aluminum (Al) nanodisks for wavelengths between 300 nm and 1700 nm and disk diameters between 60 nm and 280 nm according to various embodiments. The reflectance data in FIG. 8B were obtained from FDTD simulation, for comparison with the experimental data and to verify the accuracy of the structural and material models used. There is good agreement between the two sets of data, as the trend lines for the fundamental, third and fifth order resonances and inter-band transitions are similar for the simulated and experimental reflectance data. However, the simulated fundamental resonances are blue- shifted by about 100 nm compared to the experimental results. The reflectance minima for the fundamental and fifth order resonances measured experimentally are larger than those of the simulated data, as the experimental minima were between 0.2 and 0.3, while the simulated minima were between 0 and 0.2. This discrepancy may be explained by disk diameter variations due to fabrication errors and proximity effects in the EBL process that cause dose reduction at the boundaries of the pixels relative to their centers. The actual oxide thickness may also be slightly larger than the thickness determined from the TEM image. Nonetheless, the linear dependence of resonance wavelength on disk diameter may be evidence of the lack of coupling between disks due to the tightly confined modes in the gap, despite the close spacing of the disks fixed at 30 nm. By fitting the data to the gap plasmon resonator formula, w ⁿeff ^{= mn ~}

F - the effective refractive index n_e ^ of the resonator at ~ 3 may be obtained, which is a large value that enables the large wavelength scaling of ~6x. In the equation, w is the width of the resonator, l is the resonance wavelength, m is the order of the resonance mode, and f is the phase change due to reflection at the edges of the resonator.

[0085] 2.2 Optical and infrared micrographs

[0086] FIG. 9A is an optical micrograph of gap (in nanometers or nm) as a function of diameter (in nanometers or nm) showing 10 pm X 10 pm arrays of 44 nm tall A1 nanodisks according to various embodiments. FIG. 9A shows the brightfield optical image of the fabricated nanodisk array, illustrating the response of the array to visible light.

[0087] The disk diameter and inter-disk gap in each array were varied from 60 nm to 280 nm and from 30 nm to 140 nm respectively. The increment for each parameter was 10 nm, except for disk diameters between 60 nm and 100 nm, for which the increment was 5 nm. Under visible light, the pixels exhibit a broad range of colors. The colors are largely determined by the disk diameter, as the hues are similar for disks of the same diameter, but the intensity decreases when the gap increases. The discontinuity in color variation for the columns with disk diameters between 60 nm and 90 nm was due to intra-field dose variation during the lithography process.

[0088] The colors for disk diameters between 60 nm and 150 nm are caused by the reflectance minimum associated with the fundamental resonance mode described in FIGS. 8A- B. When the diameter increases, the colors turn from yellow to blue. This apparent blueshift in color arises due to the subtractive nature of plasmonic colors. The redshift of the resonances with increasing diameter causes the remaining wavelengths that are reflected to consist of shorter wavelength components. The colors are the most saturated for disk diameters between 65 nm and 100 nm, because the fundamental resonances are strong and centered at a wavelength between 400 nm and 800 nm. For even larger disks, the fundamental resonance shifts into the IR regime and no longer affects the optical response at visible wavelengths. The colors are a result of the third and fifth order resonances instead. [0089] FIG. 9B is an infrared micrograph of gap (in nanometers or nm) as a function of diameter (in nanometers or nm) showing 10 pm X 10 pm arrays of 44 nm tall aluminum (Al) nanodisks according to various embodiments. An illumination source of l=1 .2 pm was chosen as it is sufficiently far from the inter-band transition wavelength. The infrared image of the arrays is markedly different from the visible light image. Only disk diameters between 180 nm and 240 nm appear dark (black), whereas the smaller disks appear bright (grey). The appearance of the dark arrays is due to absorption by the disks at their fundamental resonance, as can be observed from FIG. 9A, which reduces the reflectance at the source wavelength. Similarly, the bright pixels are due to strong reflectance at the source wavelength. The pixels with disk diameters between 110 nm and 170 nm and between 250 nm and 280 nm appear dark grey, as they partially absorb light at the source wavelength. Some inter-disk coupling of plasmon modes is also evident as a general darkening of the arrays in disks patterned closer than a separation of -50 nm and with diameters of 140 nm to 190 nm.

[0090] FIG. 9C is an International Commission on Illumination (CIE) 1931 plot of chromaticity coordinates showing the range of colors obtained from the fabricated nanodisk array according to various embodiments. The triangle indicates the standard Red Green Blue (sRGB) gamut.

[0091] The coordinates were calculated from the reflectance data according to standard equations. The colors encompass most hues, including orange, yellow, cyan, blue, purple and magenta; unsaturated green was also obtained. While the color gamut achieved only encompasses -25% of the area of the sRGB triangle, it is comparable to that previously described for Al-based gap plasmonic nanostructures and significantly broader than that for gold nanostructures.

[0092] 3. Micro-tag for authentication

[0093] The disparity between the visible and infrared responses of the nanodisks may allow for the design of a security tag that hides a covert infrared image within a colorful visible image in the following manner. Firstly, two pairs of disk diameters and gaps may be selected from the color and IR palettes of FIGS. 9A-B. One pair needs to exhibit the same color but different IR reflectance, while another needs to have different colors but the same IR reflectance. As the visible colors of the disk arrays appear to repeat when the disk diameter is increased, while the infrared responses do not, pairs of arrays that have similar colors but different IR reflectance intensities may be carefully selected. As an additional requirement, the disks should have similar color lightness (or brightness), quantified using the CIELAB model, so that the security tag will be difficult to spot. For instance, two yellow disks can be chosen - one with small disk diameters around 60 nm and a second with large disk diameters around 200 nm (labelled respectively as 1 and 2 in FIGS. 9A-B). A magenta color of similar lightness to yellow but different hue is chosen for the second pair of disks. The magenta disks, with disk diameters of 80 nm and 230 nm, are also indicated in FIGS. 9A-B (labelled respectively as 3 and 4 in FIGS. 9A-B).

[0094] FIG. 9D is a plot of reflectance R as a function of wavelength (in nanometers or nm) showing measured reflectance spectra (shifted along the y-axis for clarity) of 4 arrays of nanodisks: (1) diameter (D) - 60 nm, inter-disk gap (G) - 40 nm; (2) diameter (D) - 200 nm, inter-disk gap (G) - 100 nm; (3) diameter (D) - 80 nm, inter-disk gap (G) - 120 nm; and (4) diameter (D) - 230 nm, inter-disk gap (G) - 70 nm according to various embodiments. Under illumination of infrared light of 1200 nm wavelength, the arrays with large disk diameters will appear black while the arrays with small disk diameters will appear grey, as expected from the reflectance data in FIG. 9C. Thus, there are four types of disks used, each with one unique combination of IR appearance (bright/dark) and visible color appearance (yehow/magenta). The exact disk diameters, gaps and periods are listed in Table 1.

Table 1. Physical Parameters of Disks Used to Make Micro-Tag

IR Diameter Gap Pitch (nm)

Number Visible (1.2 pm) (nm) (nm)

1 Yellow Bright 60 40 100

2 Yellow Dark 200 100 300

3 Magenta Bright 80 120 200

4 Magenta Dark 230 70 300

[0095] The data for two binary images are input into a computer algorithm using MATFAB (MathWorks Inc.) to generate the layout file for input to the EBF tool. The algorithm allocates the correct disk diameter and gap for each pixel in the image field based on the pair of binary values for the two images, and specifies an integral number of disks to fill up a square pixel. The size of the pixel may be a common multiple of the periods of the disks to avoid gaps between pixels. For potential security applications, a bar code may be used as the infrared image, while a quick response (QR) code may be used as the visible color image (in yellow/magenta) . [0096] FIG. 10A is (left) an optical micrograph of a 240 pm X 240 pm sample according to various embodiments under brightfield illumination showing a Quick Response (QR) code, and (right) the optical micrograph after image processing according to various embodiments. FIG. 10B is an infrared micrograph of the sample shown in FIG. 10A according to various embodiments showing a bar code (Code 128C): 010203. The sample shows good reproductions of the original QR code and barcode.

[0097] The QR code appears as a magenta pattern on a yellow background, whereas the barcode appears as black lines on a grey background. For better legibility of the codes, the image files (JPEG) of the visible color micrographs taken of the fabricated sample were enhanced in PowerPoint (Microsoft Corp.) by increasing the contrast and thresholding to produce a black-and-white image. The following steps carried out for FIG. 10A. First, the contrast of the image is increased by 80% and its saturation increased to 200%. Next, the image was recolored to obtain a grayscale image and saved as a new image. Finally, the new image was recolored using the“Black and White: 50%” setting to obtain a black-and-white image. This set the threshold for designating white pixels to the 50th percentile of the luminance of all pixels in the image.

[0098] The low crosstalk between the visible color and infrared images may prevent the infrared image from being easily deciphered from the visible color micrograph. FIG. IOC is an optical micrograph of the sample according to various embodiments under darkfield illumination. The barcode appears as blue lines on a black background. The large disks that form the lines diffract blue light due to their correspondingly large periodicity of 300 nm. As their periodicities are identical, they generate the same hue, saturation and luminance. On the other hand, the small disks do not diffract light due to their correspondingly small periodicities of 100 nm and 200 nm. FIG. 10D shows a high-magnification optical micrograph and scanning electron micrograph (SEM) of the sample showing disks of four different disk diameters according to various embodiments. The box in the optical image shows the area from which the SEM image is taken.

[0099] The observability of an infrared image, encoded in an otherwise ordinary-looking colorful pattern, may have several potential applications in security printing, law enforcement, military and industry. For example, valuable artwork can be tagged with the pattern for authentication before any transactions or loans to other institutions. A portable universal serial bus (USB) infrared microscope with a filter can then be used to detect the covert infrared image. Current security prints and labels are either easily counterfeited, e.g. holograms can be duplicated optically or mechanically, or difficult to embed into existing art pieces, e.g. security threads have to be inserted during the production of the canvas or paper. Radio-frequency identification (RFID) tags can also be easily hacked. However, the micro-tag according to various embodiments may have the advantages of containing two sets of visual information - (1) a visible color image, (2) a covert infrared image - as well as being small enough to be easily embedded into an existing art piece, and requiring sophisticated nanofabrication tools to replicate.

[00100] Various embodiments may relate to a micro-tag including metal-insulator-metal nanostructures, so that a visible color image and an infrared image are patterned onto the same area. Various embodiments may include a layer of native oxide of A1 as the insulator. In experiments carried out, an insulator thickness of ~7 nm was obtained as measured from TEM cross-sectional images. This oxide thickness may suitably allow for strong absorptances spanning the visible to infrared regimes. Due to the tight confinement of modes in this ultra- thin oxide, the disk resonators may exhibit little coupling between disks and may retain a small footprint < 300 nm diameter while supporting resonances up to 1.8 pm wavelength. A design process may be developed for encoding two images into a tag with an array of disks using four types of pixels, each with a different disk diameter and separation. The two images may be observable using visible light and a narrow-wavelength infrared source respectively, with low crosstalk between the two. Such a tag may be used for authentication, anti-counterfeiting and cryptography. In various other embodiments, UV inspection may be an additional channel for encryption for artwork not susceptible to damage by UV-irradiation.

[00101] Example 2

[00102] Various embodiments may relate to an optical security device. The optical security device may include a plurality of plasmonic structures. Each of the plurality of plasmonic structures may include a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure. The plurality of plasmonic structures may show a first image under brightfield illumination. The plurality of plasmonic structures may show a second image different from the first image under darkfield illumination.

[00103] In various embodiments, the plurality of plasmonic structures may be grouped in clusters. Neighboring plasmonic structures in the same cluster may be separated by an inter- disk gap. Neighboring plasmonic structures in two different clusters may be separated by a further inter-disk gap (alternatively referred to as inter-cluster gap) which is different from the inter-disk gap. Clusters having a first inter-disk gap (and/or a first inter-cluster gap) may show a first color under darkfield illumination, while clusters having a second inter-disk gap (and/or a second inter-cluster gap) different from the first inter-disk gap may show a second color under darkfield illumination that is different from the first color.

[00104] In various embodiments, the plurality of plasmonic structure may further show the first image under brightfield illumination, the second image under darkfield x-polarized illumination, and a third image under darkfield y-polarized illumination. The plurality of plasmonic structure may further show a fourth image under infrared light.

[00105] Various embodiments may relate to a method of forming an optical security device. The method may include forming a plurality of plasmonic structures. Each of the plurality of plasmonic structures may include a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure. The plurality of plasmonic structures may show a first image under brightfield illumination. The plurality of plasmonic structures may show a second image different from the first image upon illumination of a second light under darkfield illumination.

[00106] The method may include forming the plurality of plasmonic structures so that the plurality of plasmonic structures is grouped in clusters.

[00107] Various embodiments may relate to a method of using an optical security device. The method may include illuminating the optical security device under brightfield illumination to show a first image. The method may include illuminating the optical security device under darkfield illumination to show a second image. The optical security device may include a plurality of plasmonic structures, each of the plurality of plasmonic structures including a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure.

[00108] In various embodiments, the method may include illuminating the optical security device under brightfield illumination to show the first image, illuminating the optical security device under darkfield x-polarized illumination to show the second image, and illuminating the optical security device under darkfield y-polarized illumination to show a third image. The method may additionally include illuminating the optical device under infrared light to show a fourth image.

[00109] Though plasmonic colors can be viewing-angle independent, increasing the periodicity of the arrays beyond -250 nm may result in diffraction with illumination and/or viewing at glancing angles. This sparser arrangement may allow for the structures to be observable under darkfield illumination, as they would diffract certain wavelengths of light that satisfy the grating condition. Darkfield colors have been previously reported for individual plasmonic nano-resonators, while vibrant darkfield colors have been demonstrated by arrays of such resonators. However, a clear understanding of the intensity of the diffracted light from these resonator arrays and its dependence on the array geometry is currently lacking. Furthermore, decreasing the density of the resonators may result in changes in their brightfield color. Various embodiments may relate to an array of structures, e.g. disks, that exhibit unchanging brightfield colors while varying darkfield colors due to small variations in the positioning of disks in the array. A few previous studies had also discussed the optical response of fixed clusters of resonators, but the focus was on simple metallic nanostructures rather than on gap plasmonic resonators.

[00110] Various embodiments may relate to an array of aluminum disk gap plasmon resonators to produce colors viewable in darkfield illumination. In darkfield imaging, the colors of arrays of single disks may be primarily controlled by the array periods, while those for arrays of disk clusters may be affected by both the period and the inter-disk gap within the clusters. Polarization-tunable darkfield colors may also be achieved by varying the periods of the array in the x and y axes. This effect may provide the possibility of hiding two darkfield images in a featureless brightfield color print and of making an optical micro-print that can encode up to four images using only circular disk gap plasmon resonators that have different sizes and periods. The covertness of the darkfield images may have potential applications for currency anti-counterfeiting, document and artwork authentication, and information encryption.

[00111] 4.1 Structure of the gap plasmon resonator

[00112] The structure of the plasmonic pixel may include a square array of A1 disks lying on an AhOs-Al-bulk silicon substrate, as shown in FIG. 11 A. FIG. 11A is a three-dimensional (3D) schematic of an array of single aluminum disks on aluminum oxide (AI2O3) on aluminum (Al) according to various embodiments, as well as a three-dimensional (3D) schematic of an array of aluminum disk clusters on aluminum oxide (AI2O3) on aluminum (Al) according to various embodiments.

[00113] The single disks have diameter d, inter-disk gap g, and period P. The disk clusters have disks of diameter d, inter-disk gap gi between disks of the same disk cluster, gap g2 between neighboring disks belonging to different disk clusters and period P.

[00114] The height of the disks is 44 nm, while the diameter d is between 100 nm and 280 nm. The thickness of the AI2O3 layer is 7 nm between the disk and the Al film and 4 nm elsewhere, while the thickness of the underlying Al layer is 100 nm. A ~3 nm thick layer of native AI2O3 is present all around the top and sides of the disks due to oxidation by air.

[00115] The fabrication may be similar to the process as described under Example 1. Briefly, the first step is the deposition of a 100 nm layer of Al onto a silicon wafer using an electron- beam evaporator. After waiting for 1 day, the Al oxidizes and forms a 3 nm thick layer of AI2O3. The second step is the spin-coating of poly(methyl methacrylate) resist onto the wafer and patterning using electron beam lithography (EBL). The next step is the evaporation of 44 nm of Al, and the final step was lift-off of the resist in N-methyl-2-pyrrolidone solvent.

[00116] 4.2 Darkfield microscopy

[00117] In optical microscopy, there are two main modes of illuminating a sample - brightfield and darkfield. In brightfield microscopy, light from the source is collected and sent onto the sample in a solid cone that is perpendicular to the sample, and the reflected or transmitted light is collected by lenses and directed to the observer. The image contrast and colors are a consequence of the absorption of certain wavelengths of light by the sample. However, in darkfield microscopy, the central circular region of the light from the source is blocked, so that a hollow cone of light is sent onto the sample instead. The darkfield image and its colors are formed by scattered and diffracted light that is directed to the central acceptance cone of the microscope objective. FIG. 1 IB is a schematic showing light paths in darkfield microscopy according to various embodiments, where the central part of the light from the source is blocked by the central stop. Light may be sent onto the sample as a hollow cone. The paths followed by the two threshold wavelengths are marked as ‘a’ and ‘b’ respectively. Scattering of light may be performed by a single particle, whereas diffraction may require a large periodic array of particles.

[00118] The incident and collected light in a microscope may be characterized in terms of their wave vectors and angles. The equation Q = sin^-1 NA is used to calculate the half- acceptance angle of the acceptance cone, where NA is the numerical aperture of the objective. For a darkfield microscope, the wave vector of the incident light may make an angle that is bounded by upper and lower limits, which are associated with the outer and inner circumferences of the ring of light, respectively. The lower limit may necessarily be greater than the half- acceptance angle. The incident wave vector thus has both large in-plane and out- of-plane components, and in order for the light to be scattered and diffracted off the sample to be collected by the objective, an in-plane wave vector may need to be added in the opposite direction to the incident wave vector to force the resultant wave vector to fall within the acceptance cone of the objective. In practice, many modem optical microscopes have integrated components for performing brightfield and darkfield measurements, so the microscope body has a retractable central stop piece, and special microscope objectives can be used for both brightfield and darkfield imaging.

[00119] FIG. l lC is an optical brightfield micrograph of an array according to various embodiments taken using a Nikon 20 x /0.45 NA objective. FIG. 11D is an optical darkfield micrograph of the array according to various embodiments taken using a Nikon 50 x /0.8 NA objective. Each square in the micrographs is 10 pm by 10 pm in size. The disk diameter range is 60 nm to 280 nm, while the inter-disk gap range is 30 nm to 140 nm. Saturated blue colors appear when the period increases above 250 nm.

[00120] 4.3 Diffraction by gratings

[00121] The wave vector of a one-dimensional simple grating, ki in_g, has a magnitude given by 2p/R, where P is the period, and a direction that is perpendicular to the axis of the grating and in-plane with the grating. For any wavelength of light l, the input light from a microscope has a wave vector in air, ki_n, whose magnitude is given by 2 p/l, while the direction is determined by the light path through the optical components. Thus, the shortest blue wavelengths of light (-400 nm) have the largest wave vectors, while the longest red wavelengths (-700 nm) have the smallest wave vectors. When light is reflected by a perfect reflector, the wave vector of the reflected light retains its original magnitude, but the vertical or z-component of the vector reverses its sign.

[00122] FIG. 11E is a plot of reflectance (R) (in percent or %) as a function of wavelength (in nanometer or nm) showing the darkfield spectra for 280 nm wide disks and inter-disk gaps between 20 nm and 130 nm according to various embodiments. The threshold wavelengths for the array with a gap of 30 nm are indicated. FIG. 1 IF is a plot of threshold wavelength (in nanometer or nm) as a function of period P showing fits of the experimental threshold wavelengths to the values obtained from the diffraction grating equation according to various embodiments.

[00123] FIG. 11G is a wave vector diagram for blue light at the threshold wavelength for complete collection of first-order diffracted light according to various embodiments. Light may be incident onto the grating in a wedge of angles, where Omi and 0m2 correspond to the minimum and maximum polar angles, respectively. All the output angles of the diffracted light from the grating lie within the collection cone of the objective. FIG. 11H is a wave vector diagram for red light according to various embodiments. The output angle of the diffracted light lies just outside the collection cone, so no diffracted light is collected.

[00124] Wave vector diagrams can be used to explain the diffraction of the input light, as shown in FIGS. 11G-H, for the case of a one-dimensional grating. The grating wave vector, incident and output wave vectors, and incident and collection cones are as indicated. (The right half of the incident cone is omitted for clarity.) We consider the case where the grating vector k_grating has a larger magnitude than the wave vectors of the input light, as the period of the disk array is smaller than the wavelength of violet light, which is 380 nm. The incident cone for a 50 x /0.8 NA objective has incident angles between -53° and 73°. The input and output wave vectors lie on a circle because of conservation of energy. To find the direction of the output wave vector for a particular input angle, in-plane phase matching along the x-direction is used:

[00125] The diffraction order, m, is taken as positive, as negative values would give a diffraction angle that lies outside the collection cone. Oi_n and 0_out are measured with respect to the normal to the surface and are limited to the upper half-plane (z > 0). By definition, if the output wave vector lies in the same quadrant as the input vector, then 0_out is negative, whereas Oout is positive if the output and input wave vectors lie in different quadrants. From Equation (3), Oout is negative when the magnitude of ki_n,_x is smaller than that of m kgrating. With small periodicities, no light is collected as k_grating is too large. However, with increasing periodicity, k _ratin decreases, and we start to observe some light. [00126] In FIG. 11G, the specific condition where the input light is blue and has an incident angle equal to the smallest angle of the incident light cone (9mi) is considered. This illumination with wave vector km.biuei is diffracted and deflected in the opposite direction such that the output light kout.biuei just fits into the collection cone (I0_out I < 0_Coiiection/2). For larger incident angles, e.g. for km,biue2, the input x-component increases, so the output light k_out,biue2 has a smaller x- component and easily fits into the collection cone. Thus, all the first-order diffracted blue light, regardless of incident angle, is collected by the objective. The threshold (maximum) wavelength at a particular incident angle for which light will be first-order diffracted into the collection cone of the objective occurs when 9_0ut is equal to -sin^-1 NA, or sin 9_0ut = -NA. 9_0ut has a negative sign as it lies in the same quadrant as 9m.

[00127] Thus, the following equations apply, where 9mi < 9m2 and kthreshoidi < kthreshom:

[00128] FIG. 11H shows the wave vector diagram for red light. Since the red light has a longer wavelength than blue light, its wave vector is shorter and the x-component of the diffracted light is larger, producing a larger output angle that falls outside of the collection cone, e.g. for km,redi . This condition applies for all the incident angles of red light smaller than 9i_n2, which is the largest angle of the incident light cone. Therefore, in contrast to the case for blue light, almost no red light is collected by the objective. The resultant spectrum collected by the objective will mostly consist of short blue wavelengths of light, and thus the image will appear blue. Since the disk arrays are two-dimensional, the diffraction grating vector is not fixed along one axis and can point in various directions. In fact, there are two grating vectors, kg_rating.x and k_graung.y, and two boundary conditions to be satisfied. For the disk arrays in this section, the x-period P_x and y-period P_y are the same (P_x = P_y = P), so the magnitudes of kg_rating.x and kgrating,_y are the same. In general,

[00129] When the array period is small, only the first-order diffracted light in either the x- or y-direction can be collected, so the disk array can be analyzed in terms of a one-dimensional (ID) grating.

[00130] The brightfield and darkfield images shown in FIGS. 11C-D are taken using Nikon Eclipse LV 100ND microscope. A 50 x /0.8 NA objective is used, which corresponds to a half acceptance angle of 53°. In FIG. 11D, saturated blue and cyan colors are observed for arrays with large periods greater than 250 nm. These colors form bands of similar color along the diagonal, as the array periods are constant along each diagonal line from the upper left to the lower right. These colors are caused by diffraction, as explained in the previous section. For periods around 250 nm, only short wavelengths of light are diffracted by the array and collected by the objective, so the arrays appear blue. When the period increases, longer green wavelengths are also collected, so the arrays appear cyan.

[00131] For arrays with smaller disk diameters (<240 nm), the central part of the array appears black but the square borders are colorful due to enhanced plasmon scattering by the individual disks, spanning blue, green, yellow, orange, and red. These colors are size- dependent and remain the same throughout each column of arrays and had been demonstrated previously using gold nanoparticles separated from a gold film by a sub-nanometer spacer.

[00132] The reflectance spectra for arrays of 280 nm wide disks were measured with a CRAIC 508 PV spectrophotometer and showed a clear peak-and-side-lobe profile (FIG. 11E). The inter-disk gap g may be varied from 20 nm to 130 nm; thus, the periods P may be from 300 nm to 410 nm. The peaks in the spectra occur around 400 to 450 nm and increase in amplitude when the period increases. The reflectance gradually decreases to nearly 0 at a characteristic threshold wavelength. This threshold wavelength is the maximum wavelength at which the diffracted light from the array falls into the collection cone of the objective. Since the incident light cone has a spread of polar angles, there are two threshold wavelengths, k_threshoidi and k_threshoid2, corresponding to the minimum and maximum polar angles, respectively. Both threshold wavelengths may redshift when the inter-disk gap increases. The threshold wavelengths are also compared to the theoretical prediction in FIG. 11F and show good agreement. Here the theoretical relationship between the threshold wavelength and the array period may be obtained from the diffraction grating equation, where the diffraction order m is +1:

threshol_{d ~} P(NA + Si7l0j_n). (9) [00133] 5.1 Polarization-dependent darkfield images

[00134] Polarization-dependent brightfield colors have been previously demonstrated using asymmetric plasmonic structures, e.g. rods, ellipses, and crosses, so that the plasmon resonances along one principal axis have a different frequency from those along the orthogonal axis. However, with darkfield colors, instead of changing the shape of the resonator, polarization dependence may be introduced by having different array periods along the x- and y-axes, which causes the grating vectors to have different magnitudes. As the resonators themselves are circular, the brightfield colors may not have polarization dependence, as the plasmon resonance frequencies are determined by the resonator geometry and less influenced by rearrangement of the disks.

[00135] One type of multi-periodic disk array was fabricated - a rectangular array of 200 nm wide disks with dissimilar x- and y-periods. The arrays were observed using a darkfield microscope with a linear polarizer placed in the light path between the source and the sample. When the direction of the polarizer axis is 0°, in other words parallel to the x-axis, the electric field of the wave that passes through the polarizer is pointed along the x-axis. Now, if the azimuthal angle f is 0° or 180°, the incident electromagnetic wave is -po lari zed relative to the sample. When the polarizer is rotated by 90°, the electromagnetic wave becomes s- polarized for an azimuthal angle of 0° or 180°. The above results about the polarization properties of the incident light are summarized in Table 2.

Table 2. Dependence of polarization of incident light on azimuthal angle and orientation of linear polarizer

Azimuth f x-polarizer y-polarizer

0°, 180° (east, west) p-polarization 5-polarization

90°, 270° (north,

south) 5-polarization -polarization

[00136] The diffraction efficiency of the disk grating array may be higher for p-polarized incident light than for s-polarized light, since the -polari/cd light couples better to the surface plasmons, as will be discussed below via simulation results.

[00137] FIG. 12A shows (left) a schematic of a disk array according to various embodiments; and (right) a schematic showing the electric fields for x-linearly polarized light according to various embodiments. The x-period and y-period are indicated by P_x and P_y respectively. P_x and P_y have different values.

[00138] Further investigations were carried out by varying the P_x and P_y values. The disk diameter was 200 nm, while both P_x and P_y were varied from 220 nm to 320 nm. Thus, the inter-disk gap was varied from 20 nm to 120 nm. FIG. 12B is an unpolarized, brightfield image of a plurality of arrays according to various embodiments. FIG. 12C is an unpolarized, darkfield image of the plurality of arrays according to various embodiments. FIG. 12D is a x-polarized, darkfield image of the plurality of arrays according to various embodiments. FIG. 12E is a y- polarized, darkfield image of the plurality of arrays according to various embodiments. The different arrays in each of FIGS. 12B-E have different P_x and P_y values.

[00139] From FIGS. 12D-E, the x-polarized and y-polarized images appear to be reflections of each other about the diagonal axis, while the unpolarized image shown in FIG. 12C is a sum of the x- and y-polarized images. The colors of the x-polarized darkfield image are arranged according to columns, which implies that the colors are affected more by the x-period than by the y-period.

[00140] The bottom rows of the arrays shown in FIGS. 12D-E are indicated with dashed boxes. The y-period of the bottom rows is constant at 220 nm, while the x-period is varied from 220 nm to 320 nm. Approximating the disk array as a ID grating, for light incident from an azimuthal angle of 0° or 180°, the period of the grating is the x-period of the disk array (220 nm to 320 nm). Similarly, the period of the ID grating encountered by light incident from an azimuthal angle of 90° or 270° is the y-period. From Table 2, when the incident light passes through an x-linear polarizer, the polarization states for 0°/180° and for 90°/270° are p and s, respectively, and since the diffraction efficiency is greater for p-polarization than for s- polarization, the intensity of diffracted light is stronger for the x-polarized configuration than for the y-polarized configuration.

[00141] FIG. 12F is a plot of reflectance R (in percent or %) as a function of wavelength (in nanometers or nm) showing the x-polarized darkfield spectra for the rectangular arrays of 200 nm wide disks according to various embodiments. FIG. 12G is a plot of reflectance R (in percent or %) as a function of wavelength (in nanometers or nm) showing the y-polarized darkfield spectra for the rectangular arrays of 200 nm wide disks according to various embodiments. For both FIGS. 12F-G, P_y is kept at 320 nm, while P_x is varied from 220 nm to 320 nm. [00142] The spectra shown in FIGS. 12F-G provide support for the colors shown by the darkfield images, as the spectra for y-polarization are compressed to the left and the amplitudes of the peaks are 30% of those for x-polarization. As diffraction occurs independently of the polarization of the incident light, the threshold wavelengths for the x-polarized and y-polarized spectra may be similar.

[00143] FIG. 12H is a plot of reflectance R (in percent or %) as a function of wavelength (in nanometers or nm) showing the darkfield spectra for square arrays of single 200 nm wide disks with different periods according to various embodiments under unpolarized incident light. The period is varied from 220 nm to 320 nm.

[00144] The reflectance spectra for 200 nm wide disks in square arrays are shown in FIG. 12H for comparison and have narrower peaks than for the x- polarized spectra for the rectangular arrays. Thus, diffraction is more efficient when there is a smaller inter-disk gap in the axis perpendicular to the plane of the incident light, i.e. when the disk arrays resemble line gratings.

[00145] In order to further investigate the effect of plasmon resonances on the polarization dependence of the spectra, finite-difference time-domain method (FDTD) simulations were performed using a commercial program (FDTD Solutions) for a rectangular array of disks with 200 nm diameter, 320 nm x-period, and 220 nm y-period. FIG. 13 A is a plot of reflectance as a function of wavelength (in nanometers or nm) showing the simulated reflectance spectra from a 63°-polar angle Q, 0°-azimuthal angle f plane wave source for an array of 200 nm wide disks according to various embodiments. The array had a P_x of 320 nm and a P_y of 220 nm. Periodic boundary conditions were used, and the source light of 300-800 nm was delivered at a 63° angle to the sample surface from the right. The total reflected and diffracted light was collected by the field monitor placed above the sample. One point to note is that this configuration does not represent experimental conditions as the monitor collects all light coming off the sample. When the source light is incident at an angle, the reflectance for the disk array may be highly polarization-dependent - the spectrum for s-po lari zed incident light may be relatively flat, whereas the spectrum for -po lari zed incident light may have a sharp dip. The dip may mark the wavelength at which the incident light is coupled to in-plane surface plasmons. This difference in the spectra may also suggest that the diffraction efficiency for p-polari cd light is higher than that for s- polarized light. [00146] FIG. 13B shows the simulated electric field and the simulated magnetic field for p- polarized illumination at 610 nm wavelength and at 460 nm wavelength according to various embodiments. At the wavelength of 460 nm, the electric field and the magnetic field may correspond to the fourth-order resonance mode. The even resonance modes may only be stimulated when light is incident at an angle. The electric field is highest at the center of the base of the disk, with smaller regions of high field intensity at the extreme ends of the base of the disk. The magnetic field forms four anti-nodes at the edges of the base of each disk and is also high in the region between the disks. The smaller extent of the field maxima for this fourth- order mode may cause the absorption to be weaker than for the fundamental mode. At a wavelength of 610 nm, the electric and magnetic fields of the disk array may be markedly different and may correspond to the grating mode. The electric field is highest in the gap below the disk and at the comers of the top surface of the disk. The magnetic field formed three maxima at the base of the disk, and the extreme right maximum lobe extends into the surrounding air. There is high absorption at the top circumference of the metal disk, so reflectance is low. The coupling of the incident light to the surface plasmon causes energy to propagate along the surface of the substrate, so little energy is reflected back in the vertical direction.

[00147] FIG. 13C is a plot of intensity (xlO ¹² W/m²) as a function of wavelength (in nanometer or nm) showing the simulated scattered field spectra for a rectangular array of 200 nm wide disks according to various embodiments under p- and .s-po lari zed illumination from a polar angle of 63° (O_m) and an azimuthal angle (f )of 0°.The array had a P_x of 320 nm and a P _y of 220 nm. The incident plane wave had a polar angle O_m of 63° and azimuthal angle f of 0°. Far- field projection was carried out to calculate the diffracted light intensity at various points at a distance of 1 m from the sample.

[00148] Diffracted light was collected in the same vertical plane (i.e. azimuthal angle cp fixed at 0°) at three different polar angles 0_out of -15°, -30°, and -45°. The negative signs mean that the light was diffracted backwards towards the source. The scattering peak observed at a polar angle of 0° occurred at a wavelength of 370 nm and redshifted to 450 nm and 510 nm when the polar angle increased, as predicted by the diffraction grating equation.

[00149] FIG. 13D shows a plot of flux (xlO ¹⁵ W) as a function of wavelength (in nanometer or nm) showing the total scattered light flux that fits into a collection cone with a half-angle of 53° for -po lari zed and .s-po lari zed illumination. FIG. 13E shows a schematic of the setup with the collection cone according to various embodiments. Both the /^-polarized and the s-po lari zed spectra have a threshold wavelength of 550 nm, but the amplitude of the p- polarized spectrum is 2.5 to 10 times that for the s-polarized case, which may show that the diffraction efficiency is higher for the -polarizcd light. The scattering spectra results may support the results shown in FIGS. 12F - G, as the shapes and peak ratios of the spectra are similar.

[00150] 5.2 Disk clusters for increasing color gamut

[00151] In order to increase the range of hues obtained from the A1 disk arrays, the layout of the disks was modified into clusters of disks, which causes the formation of an additional resonance mode by surface plasmon coupling. FIG. 14A shows (left) a schematic of an array of single disks according to various embodiments, and (right) a schematic of an array including a plurality of disk clusters according to various embodiments. The diameter d, periods along the x-direction P_x, P_x', periods along the y-direction P_y, P_y inter-disk gap g i, and inter-cluster gap g2 are indicated.

[00152] As shown in FIG. 14A, the unit cell (basis) of the disk cluster array is a two-by-two cluster of disks with the same diameter and height, arranged in a square configuration. In other words, the array is a biperiodic two-dimensional grating, where the inter-disk gaps in the array have two fixed values. The smaller inter-disk gap is defined as gi, while the larger inter-disk or inter-cluster gap is g2. FIG. 14B shows a scanning electron microscope (SEM) image taken of an array including 120 nm wide disks with inter-disk gap gi of 20 nm and inter-disk gap g2 of 120 nm according to various embodiments. The total period is 380 nm.

[00153] Darkfield and brightfield micrographs were taken of arrays of clusters of 80 nm wide, 100 nm wide, and 120 nm wide disks, as shown in FIGS. 14C-H. The inter-disk gap, gi, is kept constant for each row, while the inter-cluster gap, g2, is kept constant for each column.

[00154] FIG. 14C shows a darkfield micrograph of disk cluster arrays, each array including 80 nm wide disks, an inter-disk gap gi varying from 20 nm to 80 nm, and an inter-cluster gap g2 varying from 80 nm to 200 nm according to various embodiments. FIG. 14D shows a darkfield micrograph of disk cluster arrays, each array including 100 nm wide disks, an inter disk gap gi varying from 20 nm to 100 nm, and an inter-cluster gap g2 varying from 80 nm to 200 nm according to various embodiments. FIG. 14E shows a darkfield micrograph of disk cluster arrays, each array including 120 nm wide disks, an inter-disk gap gi varying from 20 nm to 120 nm, and an inter-cluster gap g2 varying from 120 nm to 240 nm according to various embodiments. FIG. 14F shows a brightfield micrograph of the disk cluster arrays shown in FIG. 14C according to various embodiments. FIG. 14G shows a brightfield micrograph of the disk cluster arrays shown in FIG. 14D according to various embodiments. FIG. 14H shows a brightfield micrograph of the disk cluster arrays shown in FIG. 14E according to various embodiments.

[00155] The darkfield colors may be greatly expanded in hue compared to the colors for single-disk arrays. For the 80 nm wide disks, the darkfield colors are blue and cyan, indicating that the reflectance profiles for the various arrays are similar. For the 100 nm wide disks, the darkfield colors are redshifted to cyan and yellow, while for the 120 nm wide disks, the darkfield colors span yellow, pink, purple, and blue. This redshift may occur because the reflectance profiles become broader due to the increase in array periodicity and threshold wavelength. The brightfield colors for the 80 nm disks, 100 nm disks, and 120 nm disks are purple, dark cyan, and light green, respectively, and do not change with the inter-disk gaps. Thus, one may be able to print arrays with a uniform brightfield color with varying darkfield colors and vice versa. The top left array for each set (marked with a grey asterisk) has gi equal to g2 and is identical to a single-disk array with an inter-disk gap equal to the disk diameter.

[00156] For the single-disk arrays, the darkfield colors for large disks may be determined primarily by the period of the array, as the fundamental gap plasmon mode occurs at wavelengths larger than the threshold wavelength for collection by the darkfield objective. However, for the cluster arrays, the darkfield colors may be dependent on both the period and the inter-disk gap. For instance, for 120 nm wide disks with a period of 480 nm, there are six possible values of gi = 20 nm, 40 nm, 60 nm, 80 nm, 100 nm, and 120 nm (marked with white asterisks in FIG. 14E). The colors start at yellow for gi = 20 nm and become dark purple for gi = 80 nm. FIG. 141 is a three-dimensional (3D) plot of reflectance (in percent or %) as a function of wavelength l (in nanometer or nm) and inter-disk gap gi (in nanometer or nm) showing darkfield reflectance spectra for 120 nm wide disks and 480 nm period, with gi varied from 20 nm to 120 nm according to various embodiments. The normalized Fourier coefficients are also shown as open circles connected by vertical lines.

[00157] The reflectance spectra have two peaks - one at a wavelength around 420 nm and another at a wavelength of about 640 nm. The reflectance minimum between the peaks may be caused by the gap plasmon mode at around a wavelength of 480 nm, which also decreases the amplitude of the first reflectance peak. The wavelength position of the minimum shifts from 470 nm for gi = 20 nm, to 510 nm for gi = 80 nm, as the resonance energy of the plasmon has decreased. The main reflectance peak may be attributable to first-order diffraction and is largest at 16% for gi = 20 nm, decreasing as gi becomes larger (1% for gi = 100 nm). The change in the relative amplitudes of the two peaks causes the perceived color to change from yellow for the smallest gi to pink and purple as gi increases.

[00158] The effects of a varying pitch on constant basis may also be investigated. The reflectance spectra of the last row of disk arrays shown in FIG. 14E may be determined. The inter-disk gap gi is kept constant at 20 nm while the inter-cluster gap g is between 120 nm and 240 nm. FIG. 14J is a three-dimensional (3D) plot of reflectance (in percent or %) as a function of wavelength l (in nanometer or nm) and period P (in nanometer or nm) showing the darkfield spectra for 120 nm wide disks and gi of 20 nm, with a period varying from 380 nm to 500 nm according to various embodiments. The normalized Fourier coefficients are also shown as open circles connected by vertical lines.

[00159] The spectral profile includes a small peak around 420 nm and a larger, broader peak at a wavelength larger than 500 nm. The first peak is partially caused by the second-order and (l,l)th-order diffraction from the disk array, as the period is larger than the wavelength of incident light, which makes the grating vector shorter than the wave vector of light. The initial portion of the reflectance for wavelengths between 400 nm and 500 nm remains at an amplitude of 7%, as the contributions from the first-order and higher-order diffraction sum to a constant value. The inter-disk gap gi is fixed, so the plasmon resonance has the same energy and wavelength, and the absorbed energy by the resonance remains the same. Since the period increases from 380 nm to 500 nm, the threshold wavelength in the reflectance also increases from 680 nm to 810 nm. This broadening of the acceptance range of wavelengths also causes the amplitude of the second peak to increase, thus increasing the brightness of the colors.

[00160] In far-field diffraction, the Fraunhofer equation applies and the electric field amplitude of diffracted light may be given by the Fourier transform of the diffracting aperture or object. Assume that the first term of the Fourier series for an infinite ID square grating gives a good approximation to the Fourier transform of the disk array, the Fourier amplitude for the nth order component of a square grating with width d, height A, and period P may be calculated to be

whereas the amplitude for the nth order component of a biperiodic square grating with width d, height A, inter-disk gap gi, and period P is found to be

[00161] The normalized Fourier amplitudes are plotted as stems in FIGS . 141- J and give good agreement to the trends for the spectra. The main peaks are due to the first-order diffraction. They increase with gi for the arrays of 120 nm wide disks with 440 nm period and increase with the period for the arrays of 120 nm wide disks with a fixed gi of 20 nm and periods between 380 nm and 440 nm. The Fourier amplitudes for the short-wavelength peaks in FIG. 14J are algebraic sums of the squares of the amplitudes for the first-order and higher-order diffraction.

[00162] The experimental reflectance spectra may be verified by plotting the simulated scattering spectra for an array of clusters of 120 nm wide disks with a period of 380 nm and gi of 20 nm in FIGS. 14K - L.

[00163] FIG. 14K is a plot of intensity (x 10 ¹² W/m²) as a function of wavelength (in nanometer or nm) showing scattered field spectra for an array of 120 nm wide disk clusters with gi of 20 nm and a period of 380 nm according to various embodiments, under p- and s- polarized illumination from polar angle of 63° and azimuthal angle of 0°. Light was collected at four different polar angles 0_out of 0°, 15°, 30°, and 45°, with the azimuthal angle fixed at 0°. The scattering peak observed at a polar angle of 0° occurs at a wavelength of 340 nm and shifts to longer wavelengths when the polar angle increases. For a polar angle of 45°, two peaks are observed, at 310 nm and 610 nm, as second-order diffraction also occurs for short wavelengths. The scattering amplitudes for the -polari/cd and .s-po lari zed incident light are similar, with the -po lari zed amplitude being about 20% higher for polar angles of 30° and 45°.

[00164] FIG. 14L is a plot of flux (x 10 ¹⁵ W) as a function of wavelength (in nanometer or nm) showing the total collected light for a disk cluster array according to various embodiments under p- and .s -polarizcd illumination from polar angle of 63° and azimuthal angles of 0°, 15°, 30°, and 45°. The total collected light flux spectra for a 53°-half angle collection cone are shown for incident light with azimuthal angles f of 0°, -15°, -30°, and -45°. The threshold wavelength decreases from 660 nm for f of 0° to 440 nm for f of -45°. As the incident light for sources with non-zero f is no longer parallel to an axis of the disk array, two grating equations have to be satisfied, one for each axis of the disk array, which increases the polar angle of the output wave vector. For f = 0°, the p-polarized spectrum has a higher amplitude than the s-polarized spectrum for wavelengths between 450 nm and 660 nm and has a peak at 640 nm. This peak at 640 nm contributes to the main broad peak in FIG. 14J. Also, the p- polarized spectrum has a dip at about 420 nm that is caused by the coupled plasmon resonance mode, which shows up as a small dip in the experimental spectrum in FIG. 14J. When f is -15° or -30°, the spectra are similar in profile to that for f of 0°, as the p-polarized spectra exhibit a small peak close to the threshold wavelength and are larger than the .s-po lari zed spectra for wavelengths larger than 470 nm and 420 nm, respectively.

[00165] 5.3 Multi-level optical security tag

[00166] Various embodiments may relate to a multi-level print that serves an optical security tag by making use of the optical characteristics of the A1 gap plasmon resonators. The print may encode an image only viewable under darkfield illumination while simultaneously displaying colors under brightfield illumination, so two images can be stored with low cross talk. The darkfield image may be in the form of a barcode or other machine-readable identification code, which can be uniquely generated for a particular document, while the brightfield image may be a simple logo to aid the observer in locating the print on the document. The print may thus serve as an authentication and anti-counterfeiting tool. The capability of encoding multiple images may increase the security level of the tag, as the counterfeiter would need to duplicate all the images accurately.

[00167] Alternatively, the print can also be made small enough to be indiscernible to the naked eye, to blend into a surrounding photograph or painting, but with sufficiently high resolution for viewing with a microscope. Besides their diffractive properties, the A1 resonators have also been shown to support infrared resonances when the disk diameter is above 120 nm, so they can absorb infrared light. Thus, an extra infrared image may also be encoded for a particular excitation wavelength (1.2 pm). The infrared image may increase the difficulty of replicating the tag. Three pairs of disk arrays may be necessary to encode the three images, which are namely a yellow-magenta brightfield visible color image, a blue-cyan-black darkfield visible color image, and a bright-dark infrared image. The parameters for the disks are shown in Table 3.

Table 3. Physical parameters of disks used to make micro-tags Number Visible IR (1.2pm) Darkfield Diameter Gap (nm) Pitch (nm)

_ (nm) _

1 Yellow Bright Black 60 40 100

2 Yellow Dark Blue 200 100 300

3 Yellow Dark Cyan 200 160 360

4 Magenta Bright Black 80 120 200

5 Magenta Dark Blue 230 70 300

6 _ Magenta _ Dark _ Cyan _ 230 _ 130 _ 360 _

[00168] Finally, a fourth image can be encoded by utilizing the polarization-dependent two- dimensional grating property of rectangular arrays of disks. As shown previously, a disk array can have blue darkfield color under x-linear polarization and cyan darkfield color under y- linear polarization if the x-period and y-period are different. Thus, four more types of disk arrays were used to create two darkfield images, each visible under x or y linearly polarized light. The four additional arrays are (1) d = 200 nm, P_x = 300 nm, P_y = 360 nm; (2) d = 200 nm, P_x = 360 nm, P_y = 300 nm, (3) d = 230 nm, P_x = 300 nm, P_y = 360 nm, and (4) d = 230 nm, P_x = 360 nm, P_y = 300 nm.

[00169] A code is written in Matlab to read four image files and generate the layout file for performing electron-beam lithography, so that the appropriate disk diameter and pitch are defined for each pixel depending on the colors of the image file at the same pixel location. The pixel size is 1.8 pm, and the image size is 100 x 100 pixels. The security tag may then be fabricated using EBL, electron beam evaporation, and lift-off. FIG. 15A shows a section of the electron beam lithography (EPL) layout of the disks for patterning according to various embodiments. FIG. 15B shows a scanning electron microscopy (SEM) image of a small section of the fabricated tag according to various embodiments. The different types of disk sizes and pitches are visible from FIGS. 15A-B. The visible brightfield, infrared, and two visible darkfield images of the physical security tag are displayed in FIGS. 15C-F. FIG. 15C shows a brightfield visible image of the optical micro-tag according to various embodiments. FIG. 15D shows an infrared image of the optical micro-tag according to various embodiments. FIG. 15E shows a x-polarized darkfield image of the optical micro-tag according to various embodiments. FIG. 15F shows a y-polarized darkfield image of the optical micro-tag according to various embodiments. The quality of the images is good, as the QR code, barcode, and letters “S”,“U”,“T”,“D”,“I”,“M”,“R”, and“E” can be clearly discerned.

[00170] As highlighted above, the darkfield reflectance spectra for arrays of aluminum disk MIM gap plasmon resonators have been measured and studied in terms of a two-dimensional diffraction grating. When the unit cell of the array is a single disk, the spectrum has the shape of a single peak with a side-lobe, giving rise to blue and cyan colors. However, when the unit cell is a two-by-two cluster of disks, the spectrum has two peaks due to absorption by the disks at their coupled plasmon resonance mode. This may allow for the generation of a wide range of colors, including green, yellow, orange, pink, and purple, when the disk diameter and inter disk gaps are adjusted.

[00171] To further investigate the physics of the darkfield response of the disk arrays, the periodicities of the arrays are modified so that their x- and y-periods are dissimilar. The arrays thus have two different reflectance spectra depending on the linear polarization of the input light. That the source of the polarization dependence in the spectra is the arrangement of the disks is advantageous for fabrication, as the geometry of the disk resonators is kept constant. The reflectance peak for the polarization corresponding to p-polarized incidence is 2 to 3 times larger than that corresponding to s-polarized incidence, as its diffraction efficiency is higher.

[00172] The broad range of darkfield colors and the feasibility of designing them via the disk diameter and inter-disk gaps may make the A1 gap plasmon structures promising for security printing and data storage. Various embodiments may relate to a four- level security prints that encode information in the visible wavelength, infrared wavelength, darkfield x-polarized, and darkfield y-polarized modes. Further extension may be expected with the use of asymmetric geometries.

[00173] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. An optical security device comprising:

a plurality of plasmonic structures, each of the plurality of plasmonic structures comprising a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure;

wherein the plurality of plasmonic structures shows a first image upon illumination of a first light having a first wavelength; and

wherein the plurality of plasmonic structures shows a second image different from the first image upon illumination of a second light having a second wavelength different from the first wavelength.

2. The optical security device according to claim 1,

wherein a first diameter of a first plasmonic structure of the plurality of plasmonic structures is different from a second diameter of a second plasmonic structure of the plurality of plasmonic structures.

3. The optical security device according to claim 1,

wherein a first thickness of a first electrically insulating structure of a first plasmonic structure of the plurality of plasmonic structures is different from a second thickness of a second electrically insulating layer of a second plasmonic structure of the plurality of plasmonic structure.

4. The optical security device according to claim 1,

wherein a first gap between a first pair of neighboring plasmonic structures of the plurality of plasmonic structures is different from a second gap between a second pair of neighboring plasmonic structures of the plurality of plasmonic structures.

5. The optical security device according to any one of claims 1 to 4, wherein the first light is visible light;

6. The optical device according to any one of claims 1 to 5,

wherein the second light is infrared light.

7. The optical security device according to any one of claims 1 to 6,

wherein the first electrically conductive structure comprises aluminum;

wherein the electrically insulating structure comprises aluminum oxide; and wherein the second electrically conductive structure comprises aluminum.

8. The optical security device according to any one of claims 1 to 7,

wherein the second electrically conductive structure of each of the plurality of plasmonic structures is a nanostructure.

9. The optical security device according to any one of claims 1 to 8,

wherein the first electrically conductive structure of each of the plurality of plasmonic structures is a portion of a single continuous layer.

10. The optical security device according to any one of claims 1 to 9,

wherein the electrically insulating structure of each of the plurality of plasmonic structures is a nanostructure.

11. A method of forming an optical security device, the method comprising:

forming a plurality of plasmonic structures, each of the plurality of plasmonic structures comprising a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure;

wherein the plurality of plasmonic structures shows a first image upon illumination of a first light having a first wavelength; and wherein the plurality of plasmonic structures shows a second image different from the first image upon illumination of a second light having a second wavelength different from the first wavelength.

12. The method according to claim 11,

wherein forming the plurality of plasmonic structures comprises:

forming a first electrically conductive layer on a substrate by depositing a suitable electrically conductive material; exposing the first electrically conductive layer to form a native oxide layer;

depositing resist over the native oxide layer;

patterning the deposited resist to expose portions of the native oxide layer;

depositing the suitable electrically conductive material and removing the deposited resist in a lift-off process to form a plurality of second electrically conductive structures.

13. A method of using an optical security device, the method comprising:

illuminating the optical security device with a first light having a first wavelength to show a first image; and

illuminating the optical security device with a second light having a second wavelength to show a second image different from the first image; wherein the optical security device comprises a plurality of plasmonic structures, each of the plurality of plasmonic structures comprising a first electrically conductive structure, a second electrically conductive structure and an electrically insulating structure between the first electrically conductive structure and the second electrically conductive structure.

14. The method according to claim 13,

wherein the first light is visible light.

15. The method according to claiml4, further comprising observing the first image directly or via a visible light detector.

16. The method according to claim 14 or claim 15,

wherein the optical security device is illuminated with a visible light source, which provides the visible light.

17. The method according to any one of claims 13 to 16,

wherein the second light is infrared light.

18. The method according to claim 17, further comprising:

observing the second image via an infrared light detector.

19. The method according to claim 17 or claim 18,

wherein the optical security device is illuminated with an infrared light source, which provides the infrared light.

20. The method according to any one of claims 13 to 19,

wherein the first image is a Quick Response (QR) code; and

wherein the second image is a Universal Product Code (UPC) bar code.