WO2023079180A2

WO2023079180A2 - Narrow-band optical filters based on metamaterials and multi-layer coatings

Info

Publication number: WO2023079180A2
Application number: PCT/EP2022/081167
Authority: WO
Inventors: Ragip Pala; Aykutlu Dana; Alireza SAFAEI; Asman Tamang
Original assignee: Meta Materials Inc.
Priority date: 2021-11-08
Filing date: 2022-11-08
Publication date: 2023-05-11
Also published as: WO2023079180A3

Abstract

An optical filter having an asymmetric blocking response and resonant cavity for light migration are provided, a camera, comprising the same, as well as methods of their use, is provided.

Description

Narrow-band Optical Filters Based on Metamaterials and Multi-layer Coatings

FIELD OF THE INVENTION

[0001] The present invention relates to the field of optics. More specifically but not exclusively, the present invention relates to narrow-band optical filters based on metamaterials and/or multi-layer coatings.

BACKGROUND OF THE INVENTION

[0002] Small form-factor, high optical density (OD) color filters with narrow frequency bandwidth response and angle independent performance are needed across the optical frequencies for various applications.

[0003] The CMOS microbolometer technology provides a low-cost, high-sensitivity sensor platform for thermal imaging applications. However, these sensors consist of uncooled focal plane arrays which make them more vulnerable to dazzling and damage from high power CO2 laser attacks. There is a need for low-cost narrowband filter solutions that will effectively block the 10.6 um line to protect the IR imaging systems.

[0004] An optical filter will block external light within a specific optical bandwidth, while transmitting the remaining visible spectrum. If the blocking is only based on reflection (e.g., holographic filters), the filter will effectively reflect back the light coming from either side. The straylight reflection (Narcissus) occurs when there is sufficient indoor light, which will be blocked and reflected back to the observer with an image of the indoors. The effect is significantly stronger when outdoors is dark, and there are indoor light sources emitting within the blocking bandwidth (e.g., green blinking LEDs in the cockpit). A filter with an asymmetric blocking response is needed, i.e., it would reflect and/or absorb light from outdoors, while it would suppress the reflection of light from indoors. The Narcissus effect observed in Holographic Bragg filters is illustrated in FIG. 1. The Bragg Filter works as a green mirror from both sides, i.e., reflecting back a 20nm full width at half maximum (FWHM) portion of the incident spectrum. Observer is indoors; outside view is partially obscured due to reflected light coming from indoors. [0005] A color filter layer is designed to block a given wavelength of laser light (e.g., 532 nm) which blocks ambient light within a certain bandwidth (lOnm - 30nm). This introduces a color imbalance towards the red end of the visible spectrum, and manifests as a pronounced pinkish tinge, which is major obstacle for commercialization. Thus, color balancing technologies must be investigated that allow the rebalance the color pallet transmitted through the material and substantially eliminate the color imbalance effect.

[0006] One solution employed involves a dyed thin-film layer that reintroduces green into the color pallet on the user side of the laser guard. However, this solution is less than ideal because it is sensitive to the angle of incidence. For incident angles that are close to normal to the film surface the solution works well. For all other angles though, the green dye introduces an over-correction making the final product appear green. For practical applications, e.g., a pilot looking through airplane windscreen would perceive a green tinge around the periphery of the field of view. Since pilots very much rely on their peripheral vision and chromatic fidelity (both of these are tested in pilots’ medical exams), peripheral color imbalance is a major handicap.

[0007] It is therefore necessary to explore novel color balancing techniques that are less sensitive to incident angles. One solution described herein involves replacing the thin-film dye technique with a specifically designed metamaterial layer that will be incorporated into the final product laminate in addition to the metamaterial blocking layer as explained below

SUMMARY OF THE INVENTION

[0008] It has been discovered that an optical filter can be achieved using first and second reflective layers with an intermediate absorber layer in between the first and second layers. This discovery has been exploited to provide the present disclosure, which at least in part provides an optical filter that in some examples has an asymmetric blocking response, and that is transparent in the 8pm -14 pm IR imaging window, and that may also in some examples reject a 0.4 pm bandwidth centered at 10.6 pm.

[0009] In addition, it has been discovered that a reflector having a narrowband absorption resonance can be achieved using a cavity-coupled disk array. The cavity coupled hole disk array has cavity-assisted surface plasmon excitation centered at an absorption wavelength for trapping incident photons and absorbing the photon energy as plasma loss. This discovery has been exploited to provide the present disclosure, which at least in part can achieve absorption of incident broadband light in a desired wavelength and reflection of the rest of the light. [0010] It has also been discovered that a resonant cavity for stray light mitigation can be achieved using a vertical Fabry-Perot cavity having a leaky metal film as a top layer; and a lossy thin film cavity. This discovery has been exploited to provide the present disclosure, which at least in part can achieve absorption of incident light within a predetermined bandwidth.

[0011] In one aspect, the present disclosure is directed to an optical filter comprising a top reflective layer, an absorber intermediate layer; and a bottom reflective layer.

[0012] In an example of the optical filter, the optical filter has an asymmetric blocking response.

[0013] In another example of the optical filter, the top reflective layer and bottom reflective layers are distributed Bragg reflector filters.

[0014] In yet another example of the optical filter, the absorber intermediate layer is selected from the group of materials consisting of TetraMethyl Ammonium Iodide, Octachloropropane, Chromium (VI) Oxide, lodoacetonitrile, lodoethane, Potassium Thiocyanate, Tetramethylammonium Fluoride Tetrahydrate, Tetramethylammonium Boronhydride, Tetramethyl ammonium Hydroxide, Tetramethylammonium Fluoride, Tetramethylammonium Bromide, Tetramethylammonium Acetate, bis trichloronitrosylbis ruthenate, tetrakis di-mu-nitrosyl-bis(trichloro(trichlorostannyl)ruthenate(III)). The optical filter is transparent in the 8pm -14 pm IR imaging window.

[0015] In yet a further example of the optical filter, the top reflective layer is partially reflective.

[0016] In an example of the optical filter, the absorber intermediate layer rejects a 0.4 pm bandwidth centered at 10.6 pm.

[0017] In another aspect, the present disclosure is directed to a method of blocking incident light in a predetermined bandwidth and centered at a predetermined wavelength using an optical filter according to the present disclosure.

[0018] In an example of the method, the method comprises transmitting the remaining incident light using the optical filter.

[0019] In a further aspect, the present disclosure is directed to a method of using an optical filter according to the present disclosure to protect a user from laser radiation. The method comprises blocking incident light in a predetermined bandwidth and centered at a predetermined wavelength using the optical filter. [0020] In yet another aspect, the present disclosure is directed to a reflector having a narrowband absorption resonance. The reflector comprises a cavity-coupled hole-disk array having cavity-assisted surface plasmon excitation centered at an absorption wavelength for trapping incident photons and absorbing the photon energy as plasma loss, a sensor disposed within the hole-disk array, and a narrowband absorber disposed within the hole-disk array proximate the sensor and at a first angle with respect to a first surface of the reflector.

[0021] In an example of the reflector, the cavity-coupled hole-disk array absorbs the incident broadband light in a desired wavelength and reflects the rest of the light towards the sensor.

[0022] In another example of the reflector, the first angle is 45 degrees.

[0023] In yet a further aspect, the present disclosure is directed to a method of absorbing incident light in a predetermined bandwidth and centered at a predetermined wavelength using the cavity coupled hole disk array of a reflector according to the present disclosure. [0024] In an example of the method, the method comprises reflecting the remaining incident light towards the sensor of the reflector.

[0025] In another aspect, the present disclosure is directed to method of using a reflector according to the present disclosure to protect a user from laser radiation. The method comprises blocking incident light centered at a predetermined wavelength using the using the cavity coupled hole disk array of the reflector.

[0026] In yet another aspect, the present disclosure is directed to a resonant cavity for stray light mitigation. The resonant cavity comprises a vertical Fabry-Perot cavity having a leaky metal film as a top layer, and a lossy thin film cavity.

[0027] In an example of the resonant cavity, the lossy thin film cavity has a thickness optimized to match a desired reflection band.

[0028] In another example, the resonant cavity is configured to absorb incident light within a predetermined bandwidth.

[0029] In yet another example, the resonant cavity comprises a-SI nanopillars disposed in the top layer. The pillars act as a subtractive color filter.

[0030] In a further example, the resonant cavity is configured to absorb incident light with a predetermined wavelength, wherein the predetermined wavelength is based on the radius of the pillars.

[0031] In yet a further example, the resonant cavity further comprises a coupler grating in the top layer. [0032] In a further aspect, the present disclosure is directed to method of absorbing incident light in a predetermined bandwidth using a resonant cavity according to the present disclosure.

[0033] In yet a further aspect, the present disclosure is directed to a method of using a resonant cavity according to the present disclosure to protect a user from laser radiation. The method comprises blocking incident light in a predetermined bandwidth using the resonant cavity.

[0034] In still a further aspect, the present disclosure is directed to a thermal camera comprising one or more of: an optical filter according to the present disclosure, a reflector according to the present disclosure, and a resonant cavity according to the present disclosure. [0035] In the following Detailed Description, several filter descriptions are provided that rely on resonant absorption and reflection at different parts of the optical spectrum. Although the examples are chosen for 10.6 pm and the visible spectrum, they can be applied to any other optical frequency bands using the same design principles.

[0036] Nanostructured designs can enhance the efficiency of light-related applications.

[0037] For example, nano-structured thin film materials for transparent, ultra thin optical filters are developed for applications in laser protection in aviation.

[0038] A highly efficient flexible solar panel application in aerial vehicles improves the overall efficiency of ultra-thin cells by collecting solar light from all angles and enhancing absorption across the most useful spectral regions.

[0039] An optimized LED emission enhancer (LEE) can be mounted on existing LED sources to substantially improve their luminosity, making them super-bright, dramatically brighter to current LED sources.

[0040] Improvements in medical diagnostics using metamaterials can be achieved by modeling and analyzing a dual-sensor glucose monitor that utilizes a low power, high frequency radio wave transmission system. The device utilizes a wearable thin-film, which makes the skin transparent to radio waves and allows deep enough penetration to reach blood plasma, thereby increasing the device’s glucose measurement precision. This is achieved by metamaterial technology, which enhances the signal transmission through the skin.

[0041] Optimization of electromagnetic interference shielding, optical transparency and haze of metallic mesh or nanoweb is achieved by optimizing a metallic mesh design which provides highest electromagnetic interference (EMI) shielding with high transparency and metallic mesh distributions that minimize haze and obscuration since nanoweb metallic mesh has superior optical and electrical properties.

[0042] In each of these, there is a clear common ground in that properly researched and optimized nanostructured designs can enhance the efficiency and applicability of light in various mediums. Each of the foregoing objectives deals with light, its properties and its interactions with metamaterials. Each objective will further the overall field but will also contribute to other objectives.

DESCRIPTION OF THE DRAWINGS

[0043] 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 in which:

[0044] FIG. l is a graphic representation depicting the Narcissus effect observed with the use of holographic Bragg filters, as known in the art;

[0045] FIG. 2A is a graphus representation illustrating for a vertical cavity resonator the optical transmission as a function of wavelength for an optimized distributed Bragg reflector (DBR) filter design centered at 10.6 pm;

[0046] FIG. 2B is a graphic representation depicting that a top layer of a vertical cavity resonator provides a partial transmission which enables formation of a vertical cavity and absorbs at the resonant wavelength;

[0047] FIG. 2C is a graphic representation depicting that the amount of reflection and absorption in a vertical cavity resonator is controlled by tuning the cavity size;

[0048] FIG. 3 A is a schematic representation depicting a lateral cavity resonator design with 10.6 pm resonance;

[0049] FIG. 3B is a graphic representation illustrating a simulated transmission and reflection of the resonator of FIG. 3 A;

[0050] FIG. 3C is a graphic representation illustrating the optical density of the attenuation of the resonator of FIG. 3 A as a function of wavelength;

[0051] FIG. 4A is a schematic representation depicting a proposed narrowband infrared bolometer absorber that may be fabricated using large-area nanoimprinting;

[0052] FIG. 4B is a graphic representation illustrating simulation results of light absorption for different pitches (P) and diameters (D), with D/P being constant, showing tunability of the resonance wavelengths by changing P and D, with RD being the relief depth that is the distance between the hole and disk arrays;

[0053] FIG. 4C is a graphic representation illustrating illustrates simulation results of light absorption as a function of angle of incidence showing absorption remains within 20% of its maximum and experiences slight blue shift up to 9i < 45°;

[0054] FIG. 4D is a graphic representation of a light absorber placed in the vicinity of a sensor at an angle, e.g., but not limited to, 45°, to absorb the incident light at the desired wavelength and reflect the rest of the photons towards the sensor;

[0055] FIG. 5A is a graphic representation of a vertical Fabry-Perot cavity consisting of a leaky thin metal film on top, and a lossy thin film cavity for a broad range of wavelengths (e.g., but not limited to, 400nm - 700nm);

[0056] FIG. 5B is a graphic representation of a sparse array of a-Si nanopillars acting as a subtractive color filter - by tuning the radius of pillars, light can be selectively coupled into waveguide modes within the pillars and effectively get absorbed;

[0057] FIG. 5C is a graphic representation illustrating that guided mode resonance (GMR) can be utilized for absorbing light coming from the absorber side - GMR consists of a coupler grating and an absorber waveguide;

[0058] FIG. 5D is a graphic representation illustrating that the cavity is transparent, and once incident light is within the reflection band (520nm - 540 nm), a cavity resonance is induced;

[0059] FIG. 5E is a graphic representation illustrating that the cavity effectively absorbs the incident light;

[0060] FIG. 5F is a graphic representation illustrating reflection/transmission calculations showing a significant reduction in the reflection, with only 15% remaining;

[0061] FIG. 6A is a schematic representation illustrating a hexagonal a-Si pillar array;

[0062] FIG. 6B is a graphic representation illustrating that, at resonance, light in the array of FIG. 6A is strongly confined in the high index a-Si pillars which effectively absorb the light coming from indoors;

[0063] FIG. 6C is a graphic representation illustrating that, at off-resonance, light passes through the lossless dielectric of the array of FIG. 6 A; and

[0064] FIG. 6D is a graphic representation illustrates that effective suppression of the indoor reflection can be achieved, while maintaining a high transmission at off-resonance wavelengths. DESCRIPTION

[0065] 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.

[0066] For the purposes of explaining the invention well-known features of optics known to those skilled in the art of optics have been omitted or simplified in order not to obscure the basic principles of the invention. Parts of the following description will be presented using terminology commonly employed by those skilled in the art of optical design. It should also be noted that in the following description of the invention repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment.

[0067] As used herein, the articles “a” and “an” refer to one or to 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, use of the term “including” as well as other forms, such as “include,” “includes,” and “included,” is not limiting.

[0068] 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.

[0069] The systems and devices disclosed herein can be fabricated by using, for example, a large-area nanoimprinting technique or other lithography techniques, as would be apparent to the skilled person. Other techniques that are apparent to the skilled person and that are suitable for manufacturing the disclosed optical filter, reflector and resonant cavity may also be used.

[0070] Reference will now be made to specific examples illustrating the disclosure. It is to be understood that the examples are provided to illustrate exemplary embodiments and that no limitation to the scope of the disclosure is intended thereby.

Optical Filters Based on Metamaterials [0071] The disclosure provides optical filters based on metamaterials. Nano-structured thin film materials are used in transparent, ultra- thin optical filters, e.g., but not limited to, for laser protection in aviation. Adhesive thin film filters that can be applied on any surfaces such as goggles, cockpit windscreens and other clear surfaces. The disclosed optical filters can also be applied to a metallic nanomesh, which may be incorporated into goggles or cockpit windscreens or other clear surfaces for the purpose of electromagnetic interference shielding. These unique filters are designed to be transparent to incoming light in the visible spectrum with the exception of certain predetermined wavelengths (e.g., but not limited to, blue & green laser light) which can be simultaneously attenuated inside the filter, protecting the persons or objects behind the filter. The filter protects from laser radiation at optical wavelengths in a passive manner (i.e., without requiring a power source).

[0072] The two key objectives of this effort are to mitigate angle-dependent color imbalance and the Narcissus Effect. Both are undesirable side effects of prior art laserfiltering film.

Suppression of Color Imbalance

[0073] As discussed above, color imbalance occurs because the metamaterial layer that is designed to block a given wavelength of laser light (e.g., but not limited to, 532 nm) blocks ambient light within a wide bandwidth (20nm - 30 nm) around that particular wavelength. This introduces a color imbalance towards the red end of the visible spectrum, and manifests as a pronounced pinkish tinge, which is major obstacle for commercialization. Thus, color balancing technologies must be investigated that allow the rebalance the color pallet transmitted through the material and substantially eliminate the color imbalance effect.

[0074] A dyed thin-film layer that reintroduces green into the color pallet on the user side of the laser guard has been practiced in the art. However, this solution is less than ideal because it is sensitive to the angle of incidence. For incident angles that are close to normal to the film surface the solution works well. For all other angles though, the green dye introduces an over-correction making the final product appear green. For practical applications, e.g., but not limited to, a pilot looking through airplane windscreen would perceive a green tinge around the periphery of the field of view. Since pilots very much rely on their peripheral vision and chromatic fidelity (both of these are tested in pilots’ medical exams), peripheral color imbalance is a major handicap. [0075] It is therefore necessary to explore novel color balancing techniques that are insensitive to incident angles. One non-limiting solution involves replacing the thin-film dye technique with a specifically designed metamaterial layer that is incorporated into the final product laminate in addition to the metamaterial blocking layer.

Transparent Vertical and Lateral Cavity Resonators

[0076] The present disclosure provides an optical filter that is transparent in the 8 pm - 14 pm IR imaging window. The optical filter has a top reflective layer, an absorber intermediate layer and a bottom reflective layer. In some examples, the optical filter has, but is not limited to, an asymmetric blocking response. In some non-limiting examples, the top reflective layer and bottom reflective layer are distributed Bragg reflector filters.

[0077] In some examples, an optical filter blocks external light within a specific optical bandwidth, while transmitting the remaining visible spectrum. If the blocking is only based on reflection (e.g., but not limited to, holographic filters), the filter effectively reflects back the light coming from either side. The Narcissus Effect occurs when there is sufficient indoor light, which is blocked and reflected back to the observer with a ghost image of the indoors. The effect is significantly stronger when outdoors is dark, and there are indoor light sources emitting within the blocking bandwidth (e.g., but not limited to, green blinking LEDs in the cockpit ). According to some examples, a filter is designed with, but not limited to, an asymmetric blocking response, i.e., it would reflect and/or absorb light from outdoors, while it would transmit and/or absorb light from indoors.

[0078] In some examples, a notch filter design is described, using non-absorbing materials such as chalcogenides reflects a band of wavelengths around 10.6 pm, while making the other wavelengths within 8pm -14 pm imaging band available to the detector. One feature of such a filter is wide angular operation range, where the angular bandwidth is set by the imaging optics.

[0079] FIG. 2A shows the reflectance of a distributed Bragg reflector (DBR) consisting of 60 pairs of quarter wave stacks of high and low refractive index chalcogenides (nH=2 and nL=1.85) with thicknesses adjusted to reject a 0.4 pm BW centered at 10.6 pm. Such a DBR can have ± 20° acceptance angle for OD 3 rejection.

[0080] In this example system, DBR layers are used for the bottom and top reflective layers, which can be replaced with any other type of reflective systems with similar reflection characteristics, e.g., but not limited to, holographic, metamaterial reflectors. In some examples, the top and bottom layers may otherwise be referred to as first and second layers, respectively. An absorber layer in between the first (i.e., top) and second (i.e., bottom) layers may be referred to as an intermediate layer, for example. FIG. 2B shows the schematic for a vertical cavity according to some non-limiting examples, formed by using a partially reflective layer on top and a reflector layer at the bottom. Another layer of absorber inside the cavity enables absorption of the signal inside the cavity, suppressing the reflection > OD 1 while still providing > OD 3 transmission (FIG. 2C). The design can be further optimized for systems that incorporate materials absorbing at the 10.6 pm band to enable an attenuation of > OD 2 in absorption while maintaining the target transmission of > OD 3. Several candidate compounds have strong and narrow, isolated infrared absorption centered around 10.6 um (943 cm-1). Example compounds include TetraMethyl Ammonium Iodide, Octachloropropane, Chromium (VI) Oxide, lodoacetonitrile, lodoethane, Potassium Thiocyanate, Tetramethylammonium Fluoride Tetrahydrate, Tetramethylammonium Boronhydride, Tetramethyl ammonium Hydroxide, Tetramethylammonium Fluoride, Tetramethylammonium Bromide, Tetramethylammonium Acetate, bis trichloronitrosylbis ruthenate, tetrakis di-mu-nitrosyl-bis(trichloro(trichlorostannyl)ruthenate(III)). By using these compounds in a composite layer that has a matrix which is transparent in the 8pm -14 pm IR imaging window, a narrowband high efficiency filter can be realized.

[0081] As a separate design, in some examples, lateral cavities are described in FIG. 3 A. In the simulation plots (FIGs. 3B, 3C) OD 3 transmission attenuation at 10.6 pm using Mie resonances is demonstrated. In the proposed system, this structure/platform can be combined with absorber layers as well as diffusive scattering structures and the design parameters can be further optimized for omnidirectional performance.

LWIR-Reflector Resonant Cavity

[0082] The present disclosure also provides a reflector that has a narrowband absorption resonance. The reflector comprises a cavity-coupled hole-disk array having cavity-assisted surface plasmon excitation centered at an absorption wavelength for trapping incident photons and absorbing the photon energy as plasma loss. In some examples, cavity coupled hole disk array is configured to (but not limited to) absorb incident broadband light in a desired wavelength and reflect the rest of the light towards the sensor. The reflector also comprises a sensor disposed within the hole-disk array; and a narrowband absorber disposed within the hole-disk array proximate the sensor and at a first angle with respect to a first surface of the reflector.

[0083] In some examples, a reflector design is provided with a narrowband absorption resonance at 10.6pm. The remaining reflected signal is directed towards the sensor array using the configuration shown in FIG. 4D.

[0084] In some examples, the absorption mechanism relies on a cavity-assisted surface plasmon excitation on a hole/disk array (FIG. 4A), which is used to trap the incident photons and subsequently gets absorbed as plasma loss. There are two types of interactions between the optical cavity mode and the plasmonic mode: 1. coherent interaction between localized surface plasmon (LSP) mode and the cavity mode, and the 2. fundamental cavity resonance coupling with surface plasmon modes. This strong coupling of the cavity and LSP mode and the very narrow bandwidth of the fundamental cavity resonance induce spectral narrowing of the perfect absorption of light (FIG. 4B). The coupling between two complementary metallic elements can be tuned flexibly to shift the resonance wavelength and strength. Tunability can also be exploited from the cavity phase relation, which can be tuned with cavity thickness. [0085] In some examples, the maximum light absorption in the narrowband peak reaches to an attenuation of > OD 3, which is independent of the light polarization and is tunable by altering the geometry (FIG. 4B). The peak wavelength of absorption of the incident light is almost independent of the angle of incidence and remains within 20% of its maximum (100%) up to 0i < 45°, as shown in FIG. 4C. Increasing the angle effectively reduces the effective pattern period and red shifts the resonance. At the same time, the effective distance between the hole and disk is increased compared to the period, which in turn blue shifts the resonance and the overall coupling strength diminishes as well.

[0086] In some examples, the proposed architecture of the bolometer consists of a narrowband absorber placed in the vicinity of the sensor at an angle, e.g., but not limited to, 45°, as shown in FIG. 4D. According to some examples, The cavity-coupled hole-disk array is configured to, but not limited to, absorb the incident broadband light in the desired wavelength and reflects the rest of the light towards the sensor. Since there is a blueshift in the resonance wavelength at higher angles of incidence, the system can be further tuned to absorb the incident light at desired wavelength in 0i = 45°. The proposed system can be fabricated by using a large-area nanoimprinting technique or other lithography techniques.

Resonant Cavity for Stray Light Mitigation [0087] The present disclosure further provides a resonant cavity for stray light mitigation. The resonant cavity comprises a vertical Fabry-Perot cavity having a leaky metal film as a top layer, and a lossy thin film cavity.

[0088] In some examples, an asymmetric optical response can be realized by using combination of reflective, e.g., but not limited to, holographic, and absorptive layers. Incident light from observer side is reflected from the holographic filter. Using a partially absorbing film on the observer side can suppress the reflection at the observer side. However, existing absorber films, e.g., but not limited to, dyes, have a large bandwidth of absorption, which results in a decrease in the overall transmission. In the described systems below, resonant cavity designs selectively absorb the light coming from the observer side, within a similar bandwidth as the reflective filter, e.g., but not limited to, holographic filter, and provides improved transmission and/or color balance. Each design is based on a lateral or vertical cavity, which effectively absorbs the light and suppresses the back reflection to the observer. [0089] FIGs. 5A-5C provide exemplary, but non-limiting, designs for some of the proposed structures described in this disclosure. One of the designs is based on a vertical Fabry-Perot (FP) cavity (FIG. 5A) consists of a leaky thin metal film (an absorbing film layer with a leaky mirror or Bragg reflector) on top, and a lossy thin film cavity. In some examples, the cavity thickness can be, but is not limited to be, optimized to match the reflection band of the holographic filter. For a broad range of wavelengths (400nm - 700nm), the cavity is transparent, as illustrated in FIG. 5D. Once the incident light is within the reflection band (520nm - 540 nm), a cavity resonance is induced, as illustrated in FIG. 5E, and effectively absorbs the incident light. FIG. 5F shows the calculated reflection observed indoors; reflection/transmission calculations show a significant reduction in the reflection (only 15% remaining). The response can be further optimized by tuning the resonance bandwidth and the absorber material properties.

[0090] Another exemplary, but non-limiting, design involves the use of nanostructure arrays, e.g., but not limited to, a-Si nanopillars (FIG. 5B and FIG. 6A). Incident light is resonantly coupled into waveguide modes propagating along the pillars (FIG. 6B), where the resonant wavelength is determined by the pillar radius. The pillars act as a substractive color filter. By tuning the radius of the pillars, light can be selectively coupled into waveguide modes within the pillars and effectively absorbed. On the other hand, off-resonance wavelengths light propagates along the low-index, lossless dielectric (SiO2). FIG. 6D shows a reduction of 80% in reflection, while a high overall off-band transmission is preserved. [0091] Another exemplary design is based on Guided-Mode-Resonator (GMR) type systems (FIG. 5C). GMR consists of a coupler grating and an absorber waveguide. A 2D grating structure on top resonantly couples incident light into waveguide modes of a higher index absorber layer. The resonance is mainly determined by the periodicity. The angular response can be controlled by tuning the resonance properties, e.g., but not limited to, quality factor.

[0092] Commercial applications of the foregoing include providing solutions to mitigate the Narcissus effect stray-light reflection problem in addition to providing a color-balanced filtering advantage. One specific application among many others includes the thermal camera market. The proposed designs provide filter solutions for various spectrum bands that are not covered by holographic films.

EQUIVALENTS

[0093] Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific embodiments described specifically herein. Such equivalents are intended to be encompassed in the scope of the following claims.

Claims

1. An optical filter transparent in the 8 pm - 14 pm IR imaging window, comprising: a top reflective layer; an absorber intermediate layer; and a bottom reflective layer.

2. The optical filter of claim 1, wherein the optical filter has an asymmetric blocking response.

3. The optical filter of claim 1, wherein the top reflective layer and bottom reflective layers are distributed Bragg reflector filters.

4. The optical filter of any preceding claim, wherein the top reflective layer is partially reflective.

5. The optical filter of any preceding claim, wherein the absorber intermediate layer is selected from the group of materials consisting of TetraMethyl Ammonium Iodide, Octachloropropane, Chromium (VI) Oxide, lodoacetonitrile, lodoethane, Potassium Thiocyanate, Tetramethylammonium Fluoride Tetrahydrate, Tetramethylammonium Boronhydride, Tetramethyl ammonium Hydroxide, Tetramethylammonium Fluoride, Tetramethylammonium Bromide, Tetramethylammonium Acetate, bis trichloronitrosylbis ruthenate, tetrakis di-mu-nitrosyl-bis(trichloro(trichlorostannyl)ruthenate(III)).

6. The optical filter of any preceding claim, wherein the absorber intermediate layer rejects a 0.4 pm bandwidth centered at 10.6 pm.

7. A method of blocking incident light in a predetermined bandwidth and centered at a predetermined wavelength using the optical filter of any preceding claim.

8. The method of claim 7, further comprising transmitting the remaining incident light using the optical filter.

9. A method of using the optical filter of any of claims 1 to 6 to protect a user from laser radiation, the method comprising blocking incident light in a predetermined bandwidth and centered at a predetermined wavelength using the optical filter.

10. A reflector having a narrowband absorption resonance, comprising: a cavity-coupled hole-disk array having cavity-assisted surface plasmon excitation centered at an absorption wavelength for trapping incident photons and absorbing the photon energy as plasma loss; a sensor disposed within the hole-disk array; and a narrowband absorber disposed within the hole-disk array proximate the sensor and at a first angle with respect to a first surface of the reflector.

11. The reflector of claim 10, wherein the cavity coupled hole disk array is configured to absorb incident broadband light in a desired wavelength and reflect the rest of the light towards the sensor.

12. The reflector of claim 10 or 11, wherein the first angle is 45 degrees.

13. A method of absorbing incident light in a predetermined bandwidth and centered at a predetermined wavelength using the cavity coupled hole disk array of the reflector of any of claim 10 to 12.

14. The method of claim 13, further comprising reflecting the remaining incident light towards the sensor of the reflector.

15. A method of using the reflector of any of claims 10 to 12 to protect a user from laser radiation, the method comprising blocking incident light centered at a predetermined wavelength using the using the cavity coupled hole disk array of the reflector.

16. A resonant cavity for stray light mitigation, comprising: a vertical Fabry-Perot cavity having a leaky metal film as a top layer; and a lossy thin film cavity.

17. The resonant cavity of claim 16, wherein the lossy thin film cavity has a thickness optimized to match a desired reflection band.

18. The resonant cavity of claim 16 or 17, wherein the resonant cavity is configured to absorb incident light within a predetermined bandwidth.

19. The resonant cavity of any of claims 16 to 18, further comprising a-SI nanopillars disposed in the top layer, whereby the pillars act as a subtractive color filter.

20. The resonant cavity of claim 19, wherein the resonant cavity is configured to absorb incident light with a predetermined wavelength, wherein the predetermined wavelength is based on the radius of the pillars.

21. The resonant cavity of any of claims 16 to 18, further comprising a coupler grating in the top layer.

22. A method of absorbing incident light in a predetermined bandwidth using the resonant cavity of any of claims 16 to 21.

23. A method of using the resonant cavity of any of claims 16 to 21 to protect a user from laser radiation, the method comprising blocking incident light in a predetermined bandwidth using the resonant cavity.

24. A thermal camera comprising one or more of: the optical filter of any of claims 1 to 6; the reflector of any of claims 10 to 12; and the resonant cavity of any of claims 16 to 21.

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