FIELD

[0001] The present disclosure relates to cryptographic security and more specifically physically unclonable function (PUF).

BACKGROUND

[0002] Counterfeit products exist in virtually every industry sector, including food, beverages, apparel, accessories, footwear, pharmaceuticals, cosmetics, electronics, auto parts, toys, and currency. Counterfeit products have grown very fast in the last two decades and have spread worldwide.

[0003] Optical security features, such as holograms, may be difficult to replicate by counterfeiters. The optical security features are commonly used on products and currency. However, optical security features may still be cloneable. Visual inspection can be circumvented by a sufficiently well-produced counterfeit hologram in many cases. The visual inspection of optical security features may not be sufficient to discriminate between a genuine hologram and a counterfeit hologram.

[0004] Thus, there is a need for unclonable optical security features and methods for verifying them.

SUMMARY

[0005] It has been discovered that plasmonic composite surfaces of the present disclosure address the above need. This discovery has been exploited to develop the present disclosure, which, in part, is directed to plasmonic physically unclonable function surfaces, an imaging system therefor and methods of making and validating them.

[0006] In some aspects, a plasmonic physically unclonable function (PPUF) surface comprises a plasmonic composite surface, the plasmonic composite surface having a boundary and having a spectral response. The plasmonic composite surface comprises a first resin layer comprising a substantially flat surface on one side and a corrugated surface on the other side, the corrugated surface having height variations across the first resin layer, a plasmonic metal layer disposed over the corrugated top surface of the bottom resin layer, a dielectric layer disposed over the plasmonic metal layer; a plurality of nanoparticles randomly disposed over the dielectric layer, the plurality of nanoparticles comprising one or more of the group consisting of metallic nanoparticles, dielectric nanoparticles, quantum dot fluorophores, or fluorescent nano-diamonds, and a second resin layer disposed over the dielectric layer and the plurality of nanoparticles. The second resin layer comprises a corrugated surface matched to the geometry of the corrugated surface of the first resin layer on one side facing the first resin layer and a substantially flat surface on the other side.

[0007] In some examples, the plasmonic metal layer comprises one of silver, gold, or aluminum. The plasmonic metal layer has a thickness of about 50 nm, in some examples. In some examples, the dielectric layer has a thickness ranging from about 10 nm to about 50 nm. [0008] The PPUF surface, in some examples, comprises a chemical modifier layer between the dielectric layer and the plurality of nanoparticles.

[0009] In some examples, the PPUF surfaces comprises alignment marks disposed within the boundary of the plasmonic composite surface.

[0010] In some examples, the first and second resin layers are optically transparent.

[0011] The corrugated surface of the first resin layer, in some examples, has one or more characteristic spatial periods ranging from about 200 nm to about 500 nm. In some examples, the one or more characteristic amplitudes of the corrugated surface range from about 20 nm to about 100 nm.

[0012] In some examples, the PFUF surface comprises a plurality of plasmonic composite surfaces configured as described above.

[0013] In some further aspects, an imaging system for measuring the optical response of the PPUF comprises a fluorescence excitation light source for ultraviolet light, an angular control component between the fluorescence excitation light source and a PPUF surface when held for measurement in the imaging system, a scattering excitation light source for white light; an angular control between the scattering excitation light source and the PPUF surface when held for measurement in the imaging system, and a camera for collecting the scattering and fluorescence lights from the PPUF surface when held for measurement in the imaging system. [0014] In some examples, the imaging system comprises a numerical aperture between the camera and the PPUF surface when held in the imaging system. The imaging system, in some examples, comprises a polarization control component between the fluorescence excitation light source and the PPUF surface when held in the imaging system. In some examples, the imaging system comprises a polarization control component between the scattering excitation light source and the PPUF surface when held in the imaging system.

[0015] In some examples, the optical response is measured in a reflective mode or a transmissive mode.

[0016] In yet some further aspects a method for fabricating the PPUF surface comprises imprinting an optically transparent resin to have a quasi-random surface corrugation, depositing a metal layer over the imprinted quasi-random surface corrugation; [0017] depositing a dielectric layer over the meta layer by sputtering or low- temperature atomic layer deposition, randomly depositing metallic nanoparticles, dielectric nanoparticles, or fluorophores over the dielectric layer to form a PPUF surface, and

[0018] encapsulating the PPUF surface with an optically transparent resin.

[0019] In some examples, the method comprises functionalizing the PPUF surface with a chemical modifier.

[0020] In some further aspects, a method for PPUF verification comprises measuring a first optical response from a PPUF surface of a product based on a subset of parameters using the imaging system. The method comprises sending the measured first optical response to a database and receiving a second set of parameters for measurements from the database. The method further comprises measuring a second optical response based on the second set of parameters, sending the measured second optical response to the database and receiving a response indicating that the second optical response matches data which was recorded during the fabrication of the PPUF. The authenticity of the PPUF surface is validated in response to receiving the response.

[0021] In some examples, the method comprises verifying the measured second optical response by the database to validate the authenticity of the PPUF surface of the product.

[0022] In some examples, the parameters comprise one or more of a plurality of wavelengths, a plurality of excitation or emission angles, and a plurality of polarizations.

[0023] In some examples, the quasi-random surface corrugation has one or more characteristic spatial periods ranging from about 200 nm to about 500 nm. One or more characteristic amplitudes of the quasi-random surface corrugation range from about 20 nm to about 100 nm, in some examples.

DESCRIPTION OF THE DRAWING

[0024] The foregoing and other objects of the present disclosure, the various features thereof, as well as the disclosure itself, may be more fully understood from the following description, when read together with the accompanying drawings which:

[0025] FIG. 1 is a schematic representation showing an exemplary hyperspectral imaging system used to interrogate the optical response of plasmonic physically unclonable optical function (PPUF) surface;

[0026] FIG. 2 is a schematic representation showing a side view of a nanostructure having a PPUF surface;

[0027] FIG. 3 A is a diagrammatic representation depicting an exemplary layout for a PPUF surface having divisions and boundaries ; [0028] FIG. 3B is a diagrammatic representation depicting an exemplary layout for one of the divisions of FIG. 3 A including alignment marks or registration features, nanoparticles and fluorophores randomly distributed;

[0029] FIG. 4 is a flow chart illustrating the steps for fabricating the PPUF surface; and

[0030] FIG. 5 is a flow chart illustrating the steps for verification of an Physical Unclonable

Function (PUF).

DETAILED DESCRIPTION

[0031] The disclosures of these patents, patent applications, and publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art as known to those skilled therein as of the date of the invention described and claimed herein. The instant disclosure will govern in the instance that there is any inconsistency between the patents, patent applications, and publications and this disclosure.

[0032] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The initial definition provided for a group or term herein applies to that group or term throughout the present specification individually or as part of another group unless otherwise indicated.

[0033] To explain the disclosure well-known features of plasmon-enhanced fluorescence technology and cryptographic security known to those skilled in the art of plasmon-enhanced fluorescence technology and cryptographic security have been omitted or simplified in order not to obscure the basic principles of the disclosure. Parts of the following description will be presented using terminology commonly employed by those skilled in the art of plasmon- enhanced fluorescence. It should also be noted that in the following description repeated usage of the phrase “in one example” does not necessarily refer to the same example.

Definitions

[0034] “ Surface plasmons” are coherent delocalized electron oscillations that exist at the interface between two materials where the real part of the dielectric function changes sign across the interface (e.g., a metal-dielectric interface, such as a metal sheet in air or metal-water interface). Surface plasmons have lower energy than bulk (or volume) plasmons which quantize the longitudinal electron oscillations about positive ion cores within the bulk of an electron gas (or plasma). The charge motion in a surface plasmon creates electromagnetic fields outside and inside the metal. The total excitation, including both the charge motion and associated electromagnetic field, is called either a surface plasmon polariton at a planar interface or a localized surface plasmon for the closed surface of a small particle. For example, at optical frequencies, plasmons can couple with a photon to create another quasiparticle called a plasmon polariton.

[0035] A “plasmon-enhanced fluorescence” is referred to as metal-enhanced fluorescence (MEF), which represents an attractive method for shortening detection times and increasing the sensitivity of various fluorescence-based analytical technologies. Surfaces of metallic films and metallic nanoparticles can confine electromagnetic fields through coupling to propagating or localized surface plasmons. The interaction is associated with an enhancement of the field intensity and local optical density of states which provides means to increase excitation rate, raise quantum yield and control the far-field angular distribution of fluorescence light emitted by organic dyes and quantum dots. Such emitters are commonly used as labels in assays for the detection of chemical and biological species. Their interaction with surface plasmons allows amplifying fluorescence signal (brightness) that accompanies molecular binding events by several orders of magnitude. The plasmon-enhanced fluorescence can be in conjunction with interfacial architectures for the specific capture of the target analyte on a metallic surface.

[0036] A “fluorescence spectroscopy” with emitters is a tool with for detection and imaging down to the single-molecule level. The plasmon-enhanced fluorescence not only offers enhanced emissions and decreased lifetimes but also allows an expansion of the field of fluorescence by incorporating weak quantum emitters, avoiding photobleaching, and providing the opportunity for imaging with resolutions better than the diffraction limit. It also opens the window to a new class of photostable probes by combining metal nanostructures and quantum emitters. These new developments are based on the coupling of the fluorophores in their excited states with localized surface plasmons in nanoparticles, where local field enhancement leads to improved brightness of molecular emission and higher detection sensitivity.

[0037] “Fluorescent nanoparticles” have demonstrated much potential in research due to their brightness and longer photostability compared to traditional fluorescent dyes and labels. The fluorescent nanoparticles can provide increased fluorescence signal and higher quantum yield and lower background and higher signal-to-noise ratios. The fluorescent nanoparticles can also provide multiple wavelengths for multiplex imaging of various targets using different fluorophores.

[0038] “Hyperspectral imaging” collects and processes information from across electromagnetic spectra. Hyperspectral imaging obtains the electromagnetic spectra for each pixel in the image of a scene, to find objects, identify materials, or detect processes. There are three general branches of spectral imagers. There are push broom scanners and the related whisk broom scanners (spatial scanning), which read images over time, band sequential scanners (spectral scanning), which acquire images of an area at different wavelengths and snapshot hyperspectral imaging, which uses a staring array to generate an image in an instant. The human eye sees the color of visible light in mostly three bands (red, green, and blue). However, spectral imaging divides the spectra into many more bands and may extend beyond the visible wavelengths. Hyperspectral imaging records spectra with fine wavelength resolution and covers a range of wavelengths. Hyperspectral imaging measures continuous spectral bands, as opposed to multiband imaging which measures spaced spectral bands.

[0039] “Physical Unclonable Function (PUF)” can be any physical object that produces specific measurements. The PUF is a physical object that provides a physically defined digital fingerprint output (or response) that serves as a unique identifier for a given input and conditions (or challenge). The PUF may be based on physical variations which occur naturally during semiconductor manufacturing. The PUF is a physical entity embodied in a physical structure. The PUFs may also be implemented in integrated circuits (IC) and are used in applications with high-security requirements, such as cryptography. Due to submicron manufacturing process variations, every transistor in an IC has slightly different physical properties. These variations lead to small but measurable differences in terms of electronic properties, such as transistor threshold voltages and gain factors. Since these process variations are not fully controllable during manufacturing, these physical device properties cannot be copied or cloned. By utilizing these variations, PUFs are very valuable for use as a unique identifier for any given IC.

[0040] “Plasmonic physically unclonable function (PPUF) surface” includes a plasmonic surface, fluorophores, metallic nanoparticles, and/or dielectric particles randomly distributed on the plasmonic surface. A PPUF surface is also referred to as plasmonic structure.

[0041] “Cryptography” is used to protect digital data. Cryptography provides for secure communication in the presence of a third party. An example of basic cryptography is an encrypted message in which letters are replaced with other characters. The encryption uses an algorithm and a key to transform an input (i.e., plaintext) into an encrypted output (i.e., ciphertext).

[0042] The “dielectric constant (c)” is defined as the ratio of the electric permeability of the material to the electric permeability of free space (i.e., vacuum).

[0043] “ Transmittance” is the fraction of incident light that is transmitted through a material.

It is defined as the intensity ratio of the transmitted light over the incident light.

[0044] “Absorption” is the fraction of incident light that is absorbed in a material. It is defined as the intensity ratio of the absorbed light over the incident light.

[0045] “Reflection” is the fraction of incident light that is reflected through a material. It is defined as the intensity ratio of the reflected light over the incident light. [0046] “Diffraction” is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle or aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wave.

[0047] A “diffraction grating” includes a regular pattern. The form of the light diffracted by the diffraction grating depends on the structure of the elements and the number of elements present, but all gratings have intensity maxima at angles 9 which are given by the grating equation: d sin!) = n A where d is the separation of grating elements, is the wavelength, and n is an integer.

[0048] As used herein, the articles “a” and “an” refer to one or more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element. Furthermore, the use of the term “including” as well as other forms, such as “include,” “includes,” and “included,” is not limiting.

[0049] As used herein, the term “about” will be understood by persons of ordinary skill in the art and will vary to some extent on the context in which it is used. As used herein when referring to a measurable value such as an amount, a temporal duration, and the like, the term “about” is meant to encompass variations of ±20% or ±10%, including ±5%, ±1%, and ±0.1% from the specified value, as such variations are appropriate to perform the disclosed methods.

[0050] Reference will now be made to specific examples illustrating the disclosure. It is to be understood that the examples are provided to illustrate exemplary examples and that no limitation to the scope of the disclosure is intended thereby.

[0051] In the following description the terms “light”, “ray”, “beam”, and “direction” may be used interchangeably and in association with each other to indicate the direction of propagation of electromagnetic radiation along rectilinear trajectories.

[0052] The terms “light” and “illumination” may be used with the visible and infrared bands of the electromagnetic spectrum.

Overview

[0053] The disclosure provides systems and methods for cryptographic security using imprinted plasmonic structures to provide unclonable optical security features. The disclosure provides a plasmonic physically unclonable function (PPUF) surface, which is also referred to as a plasmonic structure in the disclosure. The PPUF surface or plasmonic structure includes fluorescent labels, metallic nanoparticles, and/or dielectric nanoparticles, which can be randomly distributed on a plasmonic surface, such as a quasi-random plasmonic surface. The plasmonic surface is used to enhance the scattering and fluorescence signals from the fluorescent labels (e.g., quantum dot fluorophores or fluorescent nano-diamonds), metallic nanoparticles, and/ or dielectric nanoparticles. The metallic or dielectric nanoparticles have variations in resonance wavelengths and positions. The fluorescent labels also have variations in emission wavelengths and positions. The plasmonic surfaces also have variations due to the limitations of the production process so it is very difficult to replicate the plasmonic surfaces.

[0054] Plasmonic structures have an optical response that depends on polarization, wavelength, or angle of incidence. The optical response can be made in a reflective mode or a transmissive mode. The optical response can be achieved by using a combination of fluorophores, metallic nanoparticles, and/or dielectric nanoparticles, and imprinted plasmonic surfaces. Because of the strong dependence of the optical response on the geometry, it is very difficult to replicate a sufficiently large portion of the plasmonic or fluorescent surface, due to the exponential growth of the complexity of the optical response.

[0055] The disclosure also provides imaging systems for obtaining the optical response from the PPUF surface. The optical response from the plasmonic structure or a PPUF surface can be measured by using an imaging system or a microscope equipped with polarizers, light sources having various wavelengths and/or a controllable angle of incidence for angle-dependent excitations to provide for a range of parameters determining the optical response for a given PPUF. The measured optical response from the PPUF surface provides initial characterization data that is stored in a database.

[0056] The disclosure also provides methods for fabricating the PPUF surface, including fabricating plasmonic structures by imprinting and depositing a thin metal layer, a thin dielectric layer, metal nanoparticles, dielectric nanoparticles, and fluorophores over the imprinted plasmonic structures.

[0057] The disclosure further provides methods for verifying the PPUF of a product. In a verification step, a subset of the optical response is measured from the PPUF surface of a product. The measured optical response is communicated to the database for verification. The verification can be used with existing cryptographic algorithms, such as public key encryption, to strengthen the security level.

[0058] The PPUF is a function, which is very difficult to reproduce, due to manufacturing limits. A PPUF surface relies on the fact that tiny variations in nm on optical surfaces can have strong effects on the optical properties, which are measurable in the far field, but very difficult to replicate. For example, optical discs are produced by imprinting from a metal master. Due to process fluctuations, the bits of a compact disc (CD) have a jitter that can be extracted from the signals in a CD reader. The jitter can be used mathematically to extract a fingerprint identification (ID), which can be used for cryptographic encryption purposes.

Hyperspectral Imaging System

[0059] A hyperspectral imaging system can be used to record the optical response from a spatially quasi -random plasm onic surface, for example, to measure a number of bits of information for each given excitation or each imaging parameter. The information may include multiple wavelengths in the visible range, for example, divide the range from about 400 nm to about 700 nm into 10 nm spectral bins, s and p polarizations, a 1024 x 1024 image may produce 62,914,560 distinct bits or values, or more depending on the discretization of intensity measurements. Fluorescent particles, excitations, and emissions can be individually controlled. For example, the information space can be increased to 256 (8 bits intensity binning) multiplied by 62,914,560 for emission in the visible wavelength range and 256 multiplied by 62914,560 for fluorescence excitation assuming the same intensity and wavelength binning, giving (256x62, 914, 560) ^A 2 possible combinations of values including fluorescence excitation and emission at each pixel. This large information space can provide a highly secure way to authenticate a product, that may be realistically unclonable.

[0060] An exemplary hyperspectral imaging system used to interrogate the optical response of plasmonic physically unclonable optical function (PPUF) surface is depicted in FIG. 1. As illustrated, a hyperspectral imaging system 100 includes a camera 110 and an optical lens or a numerical aperture 112 for focusing and collecting scattering signals 106 or fluorescence signals 104 having different wavelengths, from a PPUF surface 102.

[0061] The hyperspectral imaging system 100 includes a fluorescence excitation light source 114 for emitting ultraviolet light, which is used to excite fluorophores 108 in the PPUF surface 102 to generate fluorescence signals 104 having different wavelengths. The hyperspectral imaging system 100 also includes an angular control component 116 between the fluorescence excitation light source 114 and the PPUF surface 102 for controlling the angle of incidence of the ultraviolet light on the PPUF surface 102. The hyperspectral imaging system 100 also includes a polarization control component 117 between the fluorescence excitation light source 114 and the PPUF surface 102 for controlling s o p polarization, for example, using a liquid crystal for controlling 5 or p polarization.

[0062] The hyperspectral imaging system 100 also includes a scattering excitation light source 118 for emitting white light, which is used to cause the nanoparticles in the PPUF surface 102 to generate scattering signals 106 having different wavelengths. In this example setting, the scattering signals 106 are scattered from the PPUF surface and collected by the camera. The imaging system 100 operates in a reflective mode in this specific example but the disclosure is not so limited.

[0063] In some variations, the imaging system 100 can be configured to operate in a transmissive mode.

[0064] The hyperspectral imaging system 100 also includes an angular control component 120 between the scattering excitation light source 118 and the PPUF surface 102 for controlling the angle of incidence of the white light on the PPUF surface 102. The hyperspectral imaging system 100 also includes a polarization control component 121 between the scattering excitation light source 118 and the PPUF surface 102 for controlling s ox p polarization, for example, using a liquid crystal for controlling s ox p polarization. The light from the scattering excitation light source 118 is used for scattering from metallic nanoparticles or dielectric nanoparticles 109.

[0065] The hyperspectral imaging system 100 may be useful to obtain detailed information from the PPUF surface 102. The hyperspectral imaging system 100 may also include focusing and spatial alignment features. The imprinted patterning of the PPUF surface 102 can include alignment features, such as described below and shown in FIG. 3B.

[0066] The PPUF surface 102 can be exhaustively imaged using the hyperspectral imaging system 100, for example, covering all polarizations, excitation, and scattering or emission wavelengths and angles, over all of the PPUF surface. The resulting data set can be mathematically processed by a computing system or a processor to allow extraction of repeatable features, which can be stored in a database.

PPUF Surface (Plasmonic Structure)

[0067] The mechanism for plasmon-enhancement or metal-enhancement of fluorescence can be through several paths. First, the excitation of a fluorophore can be enhanced due to field concentration near a plasmonic structure. The emission from the fluorophore can also be enhanced through increased local optical mode density. The energy of the excited fluorophore can be transferred to the plasmon mode and then outcoupled in a very directional way. This directionality of the emission allows higher collection efficiency by a low numerical aperture lens, thereby increasing the observed fluorescence.

[0068] Plasmonic resonances may occur on plasmonic surfaces, which include gratings or two-dimensional arrangements of protrusions, and can be fabricated through imprinting.

Plasmonic resonances may occur for metallic nanoparticles. Plasmon resonances are sensitive to the surrounding environment’s refractive index and any changes in the refractive index of a dielectric coating layer or local dielectric constant changes can affect the coupling angle. Therefore, the resonant coupling of the excitation is sensitively dependent on the dielectric particles or metal particles that are present on a corrugated metal layer.

[0069] Gratings, metal-insulator-metal type sandwich resonators, and trenches with localized plasmon resonances are examples of plasmonic structures, which can exhibit spectral responses that depend on the geometry of the plasmonic structures. For the gratings or two-dimensional arrangements of protrusions, the plasmonic coupling can be understood in terms of analytical expressions.

[0070] For metallic nanoparticles, the plasmonic resonances can also be approximated by analytical expressions. However, multiple resonant modes, which arise from the placement of a metallic nanoparticle on a metal ground plane, are typically computationally analyzed. When the sizes of metallic nanostructures reach the range from about 10 nm to about 200 nm or the gaps between nanoparticles and a ground plane are in the tens of nm range, the resonance frequencies can strongly depend on the geometry.

[0071] For fluorophores, as the separation distance between one fluorophore and another fluorophore changes, the fluorescent spectrum may change due to near-field energy transfer. Such an effect is strongly dependent on the separation distance, with a characteristic length scale on the order of about 5 nm. Similarly, plasmonic modes can couple due to near-field or far-field interactions, so that mode frequency crossover or shifts can be observed.

[0072] A side view of a nanostructure providing a plasmonic physically unclonable function (PPUF) surface is depicted in FIG. 2. The PPUF surface 102 includes a bottom resin layer 212A having a corrugated top surface 214A and a flat bottom surface 216A. A plasmonic metal layer 204 is disposed over the corrugated surface 214A, such as silver (Ag), gold (Au), or aluminum (Al), among others. Then, a dielectric layer 202 is disposed over the plasmonic metal layer 204 as a spacer.

[0073] Metal nanoparticles 206, dielectric nanoparticles of high refractive index 208, and/or quantum dot fluorophores or fluorescent nano-diamond 210, are randomly disposed over the dielectric layer 202.

[0074] A top resin layer 212B has a flat top surface 216B and the corrugated bottom surface matched to the geometry of the top surface 214A of the bottom resin layer 212A to encapsulate the metal nanoparticles 206, dielectric nanoparticles of high refractive index 208, and/or quantum dot fluorophores or fluorescent nano-diamond 210 and the dielectric layer 202. The top resin layer 212B and bottom resin layer 212A may be optically transparent. For example, the resin layers may have a refractive index of about 1.5.

[0075] An exemplary layout for a PPUF surface having divisions providing spectral labeling with a distinct spectral response and bound by respective boundaries is depicted in FIG. 3 A. The PPUF surface 102 includes multiple divisions of respective quasi -random plasmonic composite surfaces 302, each of which has a boundary 304 and a distinct spectral response. In some examples, a PPUF surface 102 may have a single division, that is the PPUF may have a single quasi-random plasmonic composite surface 302.

[0076] In some variations, each quasi-random plasmonic composite surface or division 302 may have a square shape. It will be appreciated by those skilled in the art that the shape of the division or quasi-random plasmonic composite surface may vary.

[0077] An exemplary layout for one of the divisions of FIG. 3 A including random nanoparticles and fluorophores for spectral labelling and alignment marks as well as a boundary is depicted in FIG. 3B. Each quasi-random plasmonic composite surface 302 has nanoparticles 306 and/or fluorophores 308 disposed on it, which are randomly distributed within the boundary 304. Each quasi-random plasmonic composite surface 302 has a distinct spectral response associated with the combination of nanoparticles 306 and/or fluorophores 308 so that the spectral response is different for the various divisions or quasi-random plasmonic composite surfaces. Each quasi-random plasmonic composite surface 302 also includes alignment marks 310 positioned near some or all of its comers and/or center.

[0078] It will be appreciated that the function of the PPUF surface 102 is not dependent on its orientation relative to gravity, so that reference to “top” and “bottom” surfaces and layers is made for convenience to improve understanding of the figures but is not limiting on the disclosure.

Fabrication of PPUF Surface (Plasmonic Structure)

[0079] The PPUF surface 102 may be fabricated as described below. A flow chart illustrating the steps for fabricating the PPUF surface is depicted in FIG. 4. In step 410, method 400 includes imprinting an optically transparent resin to have a quasi-random surface corrugation. For example, a two-dimensional grating can be imprinted with randomized periods and amplitudes, with characteristic spatial periods from about 200 nm to about 500 nm and characteristic amplitudes ranging from about 20 nm to about 100 nm. The imprinted quasi- random surface corrugation can exhibit absorption resonances within the visible range. The imprinting is irregular but to some extent reproducible and is hence quasi-random rather than truly random.

[0080] In step 420, method 400 also includes depositing a metal layer over the imprinted quasi-random surface corrugation. The imprinted quasi-random surface corrugation may be metalized with a thin layer of metal (e.g., about 50 nm), such as silver, aluminum, or gold. Both silver and aluminum may provide a wider wavelength range and sharper resonances than gold. When the imprinted quasi-random surface corrugation is metalized with a metal, such as silver, gold, or aluminum, multiple orders and localized modes can be present.

[0081] In step 430, method 400 also includes depositing a dielectric layer over the meta layer by sputtering or low-temperature atomic layer deposition. The thickness of the dielectric layer may range from about 10 nm to about 50 nm. The dielectric layer acts as a spacer.

[0082] In step 440, method 400 may optionally include functionalizing the PPUF surface with a chemical modifier. For example, the chemical modifier may be a dilute solution of Biotinylated Bovine Serum Albumin. Chemical modifier can be a self-assembling monolayer or thiolated linker. When the chemical modifier is used, metallic nanoparticles, dielectric nanoparticles, or fluorophores with streptavidin, such as quantum dots or fluorescent nano-diamonds, which are both immune to photobleaching, can be passed over the PPUF surface using a flow channel and a buffer to sparsely adsorb nanoparticles onto the PPUF surface to form a sparse coating, which may help highlight the effects of individual nanoparticles in far field imaging.

[0083] In step 450, method 400 may include randomly depositing metallic nanoparticles, dielectric nanoparticles, or fluorophores over the dielectric layer to form a PPUF surface. The deposition may be in solution as described above, with or without use of Biotin / Streptavidin complexes, or otherwise, for example using spinning, electro-spraying or droplet printing technology.

[0084] In step 460, method 400 also includes encapsulating the PPUF surface with an optically transparent resin, which may have a low refractive index, for example, a refractive index of about 1.5.

Verification of the PPUF Surface on a Product

[0085] PUF optical security is achieved based on verification of the PUF surface on a product. The verification is to extract an optical response from the PPUF surface of a product and to verify the optical response against a database. A flow chart illustrating the steps for verification of PPUF is illustrated in FIG. 5.

[0086] A verification method 500 includes measuring an initial optical response from the PPUF surface of a product for a subset of parameters by using the imaging system of claim 11 at block 510. A user records the optical response from the PPUF surface of the product using the imaging system 100 with a subset of parameters for rapid measurements. The subset of parameters is received from a database. The parameters may include wavelengths, excitation/emission angles, and polarizations, which affect the optical response or optical spectra from the surface of the product. The rapid measurements ensure that the imaging system 100 works with the product properly. [0087] Verification method 500 includes sending the measured initial optical response to the database at block 520. The user communicates, in some examples in encrypted form, the measured data including the optical response or optical spectra to the database, which is encrypted in some examples. Various encryption methods, for example public key encryption can be implemented to achieve the desired level of security.

[0088] Verification method 500 includes receiving a subsequent set of parameters for measurements from the database to validate the initially stored data at block 530. The database sends a subsequent set of parameters to the user to validate the initially stored data. The subsequent set of parameters may include wavelengths, excitation/emission angles, and polarizations. The subsequent set of measurements are then compared to the initially stored data to verify that the PPUF surface reproduces a desired response.

[0089] Verification method 500 also includes measuring a subsequent optical response or spectra based on the subsequent set of parameters at block 540. Verification method 500 also includes sending the measured subsequent optical response or optical spectra to the database at block 550. Verification method 500 includes verifying the measured subsequent optical response against the database to validate the authenticity of the PPUF surface of the product at block 560. The database is used to verify the measured subsequent optical response to validate the authenticity of the PPUF surface of the product.

[0090] For example, when a banknote with the PPUF surface is provided to a bank, a, for example hand-held, hyperspectral imaging system is used to interrogate the PPUF surface at a subset of parameters including the angles or polarizations or excitation and scattering or emission wavelengths. The subset of parameters is sent to the hyperspectral imaging system from the database. Measurements of the initial optical response from the PPUF surface are based on the subset of parameters. Then, the measured optical response or data is shared with the database. Based on the initial optical response, a subsequent query is generated and communicated back to the cloned hyperspectral imaging system to do the subsequent set of measurements of the optical response. The subsequent set of measurements is reported to the database. As such, authenticity can be verified against the original characterization data stored in the database.

EQUIVALENTS

Those skilled in the art will recognize or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific examples described specifically herein. Such equivalents are intended to be encompassed in the scope of the following claims.