Plasmonic metasurface optical filter and imaging sensor including the optical filter

Abstract

A plasmonic metasurface optical filter is disclosed. The optical filter comprises a planar array of unit cells. Each unit cell contains a plurality of optically transmissive nanoholes having a non-circular, substantially elliptical cross-section. Each unit cell is individually tunable for the frequency and polarization of light to be filtered by varying the geometry of the respective nanohole.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to UK Patent Application No. 2202821.1, filed March 1, 2022, and U.S. Patent Application No. 17 / 936095, filed September 28, 2022, the contents of which are incorporated by reference in their entireties as if set forth herein.

[0002] The present invention relates to a plasmonic metasurface optical filter that is particularly suitable for use in imaging sensors. [Background technology]

[0003] Single-photon imagers have been of interest in the fields of depth profile or three-dimensional imaging, astrophysics, and fluorescence imaging due to their capabilities of low-photon imaging and high acquisition rate imaging. By construction, these imaging sensors can only detect the intensity of light. However, this light intensity is only a small part of the information in the bright field scattered from the object being imaged. Valuable information about the object can be obtained from the wavelength, polarization, phase, and temporal characteristics of the light. To achieve color acquisition with a camera, it is common to use dichroic films and Bayer pattern dye filters. To obtain polarized light, wire grid polarizers have been developed that are sensitive to one polarization of light. For a complete polarization imaging analysis, various polarization directions are required, and a combination of movable plates is required to change the polarization. These movable plates are always broadband and therefore not wavelength specific. More importantly, bulk polarizers make it impossible to measure multiple polarization states at once.

[0004] But in recent years, there has been growing interest in using metamaterials, and their two-dimensional equivalents, metasurfaces, to manipulate light, both in single-photon imagers and in more traditional CMOS and similar types of image sensors.

[0005] Metasurfaces are subwavelength nanostructures. Plasmonic metasurfaces exploit the free electron oscillations at the interface between a material with a negative dielectric constant -ε (e.g., a metal layer) and a material with a positive dielectric constant +ε (e.g., a glass substrate). The metal layer is deposited and fabricated with the nanostructures to provide a planar mechanism for modulating incident light. However, while there are many compelling reasons to attempt plasmonic metasurfaces, they suffer from a significant drawback: the efficiency of current plasmonic metasurfaces at visible and near-infrared frequencies can be ≦10%. Summary of the Invention [Means for solving the problem]

[0006] According to an aspect of the present invention, a plasmonic metasurface optical filter is provided that comprises a planar array of unit cells, each unit cell containing a plurality of optically transmissive nanoholes having a substantially elliptical cross-section, and each unit cell is individually tunable for the frequency and polarization of light to be filtered by varying the geometry of the respective nanohole.

[0007] Preferably, the tuning of the unit cell is optimized for extinction ratio.

[0008] Preferably, each nanohole of a unit cell has its major and minor elliptical axes oriented parallel to the major axes of the ellipses of the other nanoholes of the unit cell and selected according to the polarization of light to be filtered by the respective unit cell.

[0009] The size of the nanoholes in the unit cells is preferably selected according to the wavelength of light to be filtered by the respective unit cell.

[0010] Preferably, the array corresponds in size to the image sensor, and each of the unit cells is positioned to align with a pixel of the image sensor when the array is placed over the image sensor.

[0011] The metasurface may comprise an aluminum layer into which the nanoholes are etched.The metasurface may comprise an annealed gold film into which the nanoholes are etched.

[0012] The metasurface may be capped.

[0013] Embodiments of the present invention relate to plasmonic metasurfaces, and in particular to 2D arrays of plasmonic metasurface unit cells, each unit cell in the array having geometric properties that are selected by the array element depending on the wavelength and / or polarization of light to be filtered.

[0014] In a preferred embodiment, a single layer of nanostructures etched into a thin metal film (nm thick) can simultaneously filter specific wavelengths (colors) and polarizations to form a spectropolarimeter for single photon avalanche diodes (SPADs) and CMOS cameras. Due to the implementation as an array, the embodiment can be used in pixelated imaging sensor formats that can be scaled from blue (450 nm) to SWIR (1-2 μm). Thus, not only can higher transmission of the designed color be observed per pixel, but the embodiment can incorporate efficient extinction ratios of the selected polarization per pixel to distinguish it from the orthogonal polarization of the nanostructured device.

[0015] In a preferred embodiment, the geometry and arrangement of elliptical nanoholes in a metal thin film provides the simultaneous dual function of arbitrary color and polarization selectivity without the use of any bulky optical components with moving parts. The interplay between various optical phenomena results in high transmission and polarization discrimination.

[0016] Preferred embodiments can be used for optimized spectral polarizing filters, such as filters for multispectral colors with polarized light. The embodiments can be realized in a single platform of nm-thick nanostructured metal layers by utilizing optical phenomena due to light-matter interaction.

[0017] The metasurfaces in embodiments of the present invention are critically positioned relative to pixels of an image sensor such that when placed over an imager and aligned with that pixel, the filters individually and simultaneously control the amplitude, color, and polarization of the filtered color of light. No other optical components are known to provide such versatility in manipulating optical fields.

[0018] An advantage of the plasmonic metasurface according to the embodiments of the present invention is that it is significantly thinner and easier to fabricate compared to competing technologies such as dichroic films or polymer dye filters, while only requiring a single lithography step to filter any wavelength. In the case of dichroic filters, multiple films of selective refractive index are required, the layer thickness of which depends on the wavelength of light selected to be filtered. Thus, for example, to design an array of filters each filtering a different wavelength of light, multiple lithography steps are required, along with additional coatings to block infrared light. Neither dye filters nor dichroic films can provide polarization selectivity. To achieve spectroscopic polarimetry, not only color and polarization selectivity is essential, but also sufficient out-of-band rejection for other colors and polarizations to obtain better color contrast and polarization extinction ratio. The embodiments of the present invention seek to address this need.

[0019] Currently, there are engineering obstacles to realizing a compact spectroscopic and polarimetric imaging system. Current optical setups are too bulky using centimeter-thick conventional optical components. However, metasurfaces according to embodiments of the present invention overcome these obstacles and provide greater flexibility compared to current approaches. Advantageously, metasurfaces can be integrated directly on top of CMOS camera pixels, single-photon camera pixels, and SPAD arrays. Rather than the need for complex engineering tasks of stacking different metasurfaces, metasurfaces according to embodiments of the present invention allow for the incorporation of different functions on one platform (array) by using unit cells to selectively change the geometric properties of the elliptical nanoholes of each unit cell in the array. Thus, embodiments open up the possibility of multispectral imaging and polarimetric metrology without significant engineering or cost consequences.

[0020] The ability to simultaneously sense the polarization and wavelength of light on a single metasurface layer, combined with its compactness and compatibility with conventional nanofabrication processes, makes metasurfaces versatile candidates for the miniaturization of easily integrated optical devices for many applications, including but not limited to, spectropolarimeters, Earth observation, target identification (defense), biosensing, oceanography, research in academia and industry, and commercial applications such as polarimetry for face recognition on mobile devices.

[0021] The preferred embodiment allows the fabrication of filters from one unit cell design of plasmonic metasurfaces on a single layer that can be scaled from 450 nm to 2 μm (blue to SWIR region). The design requires only one lithography step, which makes it easy to manufacture in a pixelated format that can be directly integrated on top of a camera chip. The design is ultra-thin and has no moving parts. The design also has high out-of-band rejection for color selectivity and appreciable extinction ratio for orthogonal polarization states.

[0022] In the described embodiment, a unit cell refers to a general lattice arrangement of eight nanoholes. The dimensions, orientation, and relative distance between the nanoholes may vary depending on the frequency and polarization being filtered. There may be variation in the size of the unit cell, but this is typically small. For example, a pixel size of 10 μm may require 21 repeats of blue unit cells, 16 repeats of green unit cells, or 14 repeats of red unit cells to be covered. This is taken into consideration when designing the metasurface for a particular sensor.

[0023] The dual function of frequency and polarization filtering provided by the presently claimed invention is not easily achieved. For example, higher electric field confinement and therefore higher transmission can be achieved by a design that is completely polarization insensitive. However, taking polarization into account while still maintaining good transmission is not straightforward. The inventors of the present application have identified a unit cell design, and preferably one that uses only elliptical nanoholes, that seeks to address this. It is understood that unit cells with nanohole sizing and geometry as described in the following embodiments (or calculated using the principles described) are specifically optimized in this regard.

[0024] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0025] [Figure 1a] FIG. 1 is a schematic diagram of a plasmonic metasurface optical filter according to an embodiment. [Figure 1b] FIG. 2 is a schematic plan view of a unit cell according to an embodiment. [Figure 1c] FIG. 1 illustrates a cross-sectional view of a portion of a metasurface according to an embodiment. [Figure 1d] FIG. 1 is a schematic diagram illustrating the integration of an embodiment with an image sensor.

[0026] [Diagram 2] FIG. 2 is a schematic diagram of a unit cell suitable for use in the embodiment of FIG. 1.

[0027] [Diagram 3] FIG. 2 is a schematic diagram of another unit cell suitable for use in the embodiment of FIG. 1.

[0028] [Figure 4] FIG. 2 is a schematic diagram of another unit cell suitable for use in the embodiment of FIG. 1.

[0029] [Diagram 5] 1 is a simulated transmission curve of an embodiment. [Figure 6] 4 is a simulated transmission curve of an embodiment. [Figure 7] 4 is a simulated transmission curve of an embodiment. [Figure 8] 1 is a simulated transmission curve of an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] In a preferred embodiment of the present invention, a single lithographic layer plasmonic metasurface optical filter is produced. The metasurface of the preferred embodiment has a low level of fabrication complexity and allows for the manipulation of transmitted light for simultaneous wavelength selection and polarization control. The metasurface design of the preferred embodiment includes elliptical nanoholes etched into a nanometer thick metal layer.

[0031] As shown in FIG. 1a, a plan view of a plasmonic metasurface optical filter 10 according to an embodiment of the present invention is shown. The metasurface 10 includes a planar array of unit cells 20 (in this example, the planar array is n×m unit cells). Each unit cell 20 includes a plurality of optically transparent nanoholes (discussed in more detail below) having a non-circular, substantially elliptical cross section. Each unit cell is individually tunable for the frequency and polarization of light to be filtered by varying the geometry of each nanohole. In the example shown in FIG. 1a, there are unit cells for red (R), green (G) and blue (B) wavelength filtering, as well as different polarizations of light for each of these wavelengths. The arrows in the figure indicate the polarization angles to be filtered: 0° (arrow from top to bottom of the cell), 90° (arrow from left to right of the cell), 45° (arrow from bottom left to top right of the cell), and 135° (arrow from bottom right to top left of the cell).

[0032] Shown in Figure lb is a plan view of a unit cell 20 according to an embodiment of the invention. The unit cell contains elliptical nanoholes 30, the geometry of which can be varied during fabrication depending on the function of the particular unit cell to tune the filtering of the unit cell.

[0033] A cross-sectional view of a portion of a metasurface according to an embodiment is shown in FIG. 1c. The thickness of the substrate is about 500 μm. The aluminum metal layer has a thickness of 70 nm and is made of SiO 2 The figure shows a cross-section of the metasurface with a fused silica substrate. Light passes through the substrate and is deposited on the PECVD SiO 2 The beam is incident on a metal layer of aluminum that is covered by a cap layer of aluminum.

[0034] To integrate the metasurface with a sensor such as a camera chip, a UV resin adhesive can also be used, and the metasurface was designed to take this into account, as can be seen in the cross-section with the adhesive layer on top of the cap layer.

[0035] Fig. 1d is a schematic diagram showing the integration of a spectroscopic polarizing metasurface

[0005] with the active sensing area of ​​a camera chip

[0004] such that each pixel of the camera chip is simultaneously sensitive to a specific wavelength and polarization. This metasurface can be fabricated in a large area format

[0005] and directly integrated with the camera chip

[0004] . This integration allows each pixel to see a specific wavelength and polarization of light, providing additional functionality beyond just intensity.

[0036] 2-4 are schematic plan views of unit cells according to various embodiments. These particular unit cells are green light (metasurface filter green, MSF-G) and 90° polarization state (Figure 2); red light (metasurface filter red, MSF-R) and 90° polarization state (Figure 3); Blue light polarized at 90° (metasurface filter blue, MSF-B) (Figure 4) has been adjusted to filter out

[0037] The size and spacing of the nanoholes determine the frequency that is filtered, and the orientation of the elliptical axis (which is the same for all nanoholes in a unit cell) determines the polarization filtering. There is a performance relationship between these two geometric attributes of the unit cell, which is taken into account during tuning in preferred embodiments to achieve optimized performance for the desired frequency and polarization. This relationship is discussed in detail below.

[0038] In FIG. 2 (filtering green light (MSF-G) and 90° polarization state), the unit cell has dimensions λ / 6 and 0.6

[0039]

number

[0001] .

[0040] In FIG. 3 (filtering red light (MSF-R) and 90° polarization state), the ellipse

[0003] has a minor axis b of about λ / 6 and a

[0041]

number

[0002] has a minor axis b of about λ / 5.75 and a minor axis b of 0.6

[0042]

number

[0043] In Figure 4 (filtering blue light (MSF-B) and 90° polarization state), for the dimensions of the [00x2] elliptical nanoholes,

[0044]

number

[0045]

number

[0046]

number

[0047] FIG. 5 shows a simulated transmission curve for the embodiment of FIG. 2, with a peak transmission of 27.9% and a FWHM of λ peak At 550 nm, it is 55 nm. Like MSF-B, the PER is effectively -17.5 dB (170:1).

[0048] A simulated transmission curve for the embodiment of FIG. 3 is shown in FIG. 6. The filter has a transmission of 20.6% and a FWHM of λ peak At 648 nm it is 90 nm. The red transmittance is lower than the blue and green due to the reduced effect of the QCW.

[0049] Unlike prior known metasurface designs, the nanohole configuration and its geometry in the preferred embodiment enhances the light transmittance by the generation of plasmons and advantageously also by the generation of quasi-cylindrical waves (QCWs). In contrast to known plasmonic metasurfaces, the metasurface according to the preferred embodiment generates QCWs, which explains the increased transmittance compared to a perfect plasmonic metal film. As a result, the preferred embodiment shows higher transmittance for certain wavelengths and polarizations. Congruently, this device structure is compatible with all CMOS fabrication techniques known to the inventors that are used to manufacture chips for imaging devices.

[0050] When light is coupled into the nanohole, the evanescent field is confined to the metal / substrate and metal / cap interfaces. The evanescent field then interacts with the guided modes in the nanohole, resulting in localized surface plasmon resonance (LSPR). The strong electric fields at both interfaces, together with the QCW emitted from the nanohole and surface plasmon polaritons (SPPs), λ peakThis contributes to the high transmission observed by these devices at λ. This differs from standard metasurface designs, where only resonant coupling between the incident light and the SPPs contributes to the extraordinary optical transmission (EOT). The QCWs are evanescent waves in the form of Bessel functions, which arise from the coupling of the incident light with the guided modes confined within the nanoholes and then decay exponentially. The total electric field scattered by the nanohole at the interface does not incorporate a pure SPP mode, but a QCW traveling along the interface, and its amplitude decays with increasing distance from the nanohole, i.e., x. The decay of the QCWs is much faster than that of the SPPs for x>λ. On the other hand, for x<λ, the two waves contribute almost equally to the total electric field. The interaction of the QCWs and SPPs with the LSPR at the interfaces between the metal layer and the substrate and between the metal and the glass cap layer makes the scattered electric field dependent on the nanohole geometry.

[0051] Equations (1) and (2) represent the y component of the H field and the z component of the E field, respectively, taking into account that the elliptical nanohole is arranged around the origin (z = 0),

[0052]

number

[0053] The design principle of the unit cell of the metasurface according to the preferred embodiment is as follows: 1) Wavelength selectivity with high transmittance along with polarization sensitivity with efficient extinction ratio; 2) suppressing higher order modes such that the sensor pixel underlying each unit cell detects its designated color and polarization without transmission signatures for other colors and polarizations in the visible spectrum; It is.

[0054] In one embodiment, these principles are addressed by using variations in the geometric design of the unit cell. The unit cell is the building block of the metasurface device, which is repeated throughout the region of interest in a manner that achieves the function of that particular design, aligning the unit cell to the pixel when the filter is in place over the sensor (the mapping of unit cell to pixel can be 1:1 or many:1). The geometry and period p of the unit cell can be modified to scale to any wavelength within the visible and SWIR spectrum (450 nm to 2 μm).

[0055] The resonant coupling between the evanescent field and the incident light occurs along the minor axis (b) of the ellipse. Thus, the orientation of the ellipse (denoted as φ in Fig. 1b) makes this design sensitive to the polarization of the incident light, and together with other elliptical nanohole configurations that form the unit cell

[0001] , wavelength selectivity is obtained. The period p is proportional to the dip in the transmittance before EOT, λ SPP Determine the location of , which is given by Equation 3.

[0056]

number

[0057] Therefore, if we change the period p, the λ at which EOT occurs peak Change λ SPP, which changes p. Thus, p can be scaled to make the metasurface selective to any desired wavelength in the visible spectrum. The dimensions of the elliptical nanoholes are similarly modified to achieve higher transmission and better wavelength selectivity while maintaining the same polarization extinction ratio.

[0058] FIG. 7 shows simulated transmission curves for the embodiment of FIG. 4. The unit cells denoted [00x1] comprise elliptical nanoholes of varying dimensions. The period p was calculated according to Eq. 3. As shown above, for the dimensions of the [00x2] elliptical nanoholes, the ratio

[0059]

number

[0060]

number

[0061]

number

[0062]

number

[0063] An example of this is shown in FIG. 8, where φ is rotated to 135° and the horizontal x-axis is 0°, so this unit cell design exhibits selectivity for the 45° polarization state while filtering blue light.

[0064] For optimal performance, the spectral overlap between the colors of a particular polarization is important. It is beneficial for imaging that a particular pixel detects with high certainty the color and polarization state of the metasurface it is designed to integrate on it, and therefore it is desirable to have as low a spectral overlap (i.e., crosstalk) between the various filters as possible. The PER between the three colors of orthogonal polarization states is summarized in Table 1. The table shows the PER in dB between the colors and the polarization states (subscripts).

[0065] Preferably, the difference between the two colors should be as large as possible, well above 50%, since the transmission intensity of the orthogonal pair of 45° and 135° will be 50% of the 90° value for the same color of light being filtered (as shown in FIG. 7). In other words, the PER for any polarization state and color should be much greater than -3 dB. That way, there is no drawback in detecting one particular color and polarization state.

[0066] [Table 1]

[0067] The above embodiments have focused on visible light wavelengths and filtering. However, embodiments can also obtain the same dual function (frequency filtering and polarization filtering) for other wavelengths, such as the SWIR region. In such embodiments, the same design principles are used, with the nanohole dimensions scaled with respect to wavelength, except that annealed gold is used instead of aluminum, and the nanoholes are formed in a nm-thick annealed gold layer.

[0068] Varying the period shifts the color selectivity to any desired wavelength in the visible spectrum, near infrared (NIR), and mid-infrared (SWIR). Scaling the dimensions results in higher transmission and polarization sensitivity.

Claims

1. A plasmonic metasurface color light filter comprising a planar array of red, green, and blue unit cells forming a color filter composed of pixels, each unit cell containing a plurality of optically transparent nanoholes having a non-circular substantially elliptical cross-section, each unit cell being individually tunable to the red, green, or blue frequency and polarization of light to be filtered by varying the geometry of the respective nanohole, whereby each pixel of the filter corresponds to a unit cell and is individually tunable, each pixel simultaneously filtering selected red, green, or blue light wavelengths and polarizations to form a color filter composed of said pixels.

2. The plasmonic metasurface color light filter of claim 1 , wherein the tuning of the unit cells is optimized for extinction ratio.

3. 2. The plasmonic metasurface color light filter of claim 1, wherein each nanohole in a unit cell has its major and minor elliptical axes oriented parallel to the major and minor elliptical axes of each of the other nanoholes in the unit cell, and the orientations of the major and minor axes of the unit cells are selected according to the polarization of light to be filtered by each of the unit cells.

4. The plasmonic metasurface color light filter of claim 1 , wherein the size of the nanoholes in the unit cells is selected according to the wavelength of light to be filtered by each of the unit cells.

5. 2. The plasmonic metasurface color light filter of claim 1, wherein the array corresponds in size to an imaging sensor, and each of the unit cells is positioned to align with a pixel of the imaging sensor when the array is placed over the imaging sensor.

6. The plasmonic metasurface color light filter of claim 1 , wherein the metasurface comprises an aluminum layer in which the nanoholes are etched.

7. The plasmonic metasurface color light filter of claim 1 , wherein the metasurface comprises an annealed gold film into which the nanoholes are etched.

8. The plasmonic metasurface color light filter of claim 6 or 7, wherein the metasurface is capped.

9. A plasmonic metasurface color light filter comprising a planar array of red, green, and blue unit cells forming a pixelated color filter, each unit cell containing a plurality of optically transparent nanoholes having a substantially elliptical cross section, each unit cell being individually tunable to the red, green, and blue frequencies and polarizations of the light to be filtered by varying the geometry of the respective nanoholes; the size of the nanoholes in the unit cells is selected according to the wavelength of light to be filtered by each of the unit cells; The plasmonic metasurface color light filter has each nanohole in the unit cell oriented with its major and minor elliptical axes parallel to the major and minor axes of the ellipses of each of the other nanoholes in the unit cell, and the orientation of the major and minor axes of the unit cell selected according to the polarization of light to be filtered by the unit cell, whereby each pixel of the filter simultaneously filters a selected red, green, or blue wavelength and polarization of light before the light reaches the array of pixels of an imaging sensor.

10. The plasmonic metasurface color light filter of claim 9 , wherein the metasurface comprises an aluminum layer in which the nanoholes are etched.

11. The plasmonic metasurface color light filter of claim 9 , wherein the metasurface comprises an annealed gold film into which the nanoholes are etched.

12. The plasmonic metasurface color light filter of claim 9 or 10, wherein the metasurface is capped.

13. an array of pixels; an imaging sensor comprising: a plasmonic metasurface color light filter disposed between the array of pixels and an imaging target, the plasmonic metasurface comprising a planar array of red, green, and blue unit cells forming a pixelated color filter, each unit cell of the color filter corresponding in size and position to a pixel of the imaging sensor; and the planar array of unit cells disposed over the imaging sensor, such that each of the unit cells is aligned with a pixel of the imaging sensor; each unit cell comprising a plurality of optically transmissive nanoholes having a non-circular substantially elliptical cross-section, each unit cell being individually tunable for red, green, and blue frequencies and polarizations of light to be filtered by varying the geometry of the respective nanohole; the size of the nanoholes in the unit cells is selected according to the wavelength of light to be filtered by each of the unit cells; an imaging sensor, wherein each nanohole of the unit cell has its major and minor elliptical axes oriented parallel to the major and minor axes of the ellipses of each of the other nanoholes of the unit cell, and the orientations of the major and minor axes of the unit cell are selected according to the polarization of light to be filtered by the unit cell, whereby each pixel of the filter simultaneously filters a selected red, green, or blue wavelength and polarization of light before the light reaches the array of pixels of the imaging sensor.

14. 14. The imaging sensor of claim 13, wherein the unit cells are adjusted according to a predetermined pattern across the array of unit cells, whereby filtering of one or more of light wavelength and light polarization is varied according to the predetermined pattern.

15. 15. The imaging sensor of claim 13 or 14, wherein the imaging sensor comprises a single photon avalanche diode (SPAD) imaging sensor.

16. 15. The imaging sensor of claim 13 or 14, wherein the imaging sensor comprises a CMOS imaging sensor.

17. The imaging sensor of claim 13 , wherein the metasurface comprises an aluminum layer into which the nanoholes are etched.

18. The imaging sensor of claim 13 , wherein the metasurface comprises an annealed gold film into which the nanoholes are etched.

19. The imaging sensor of claim 13 , wherein the metasurface is capped.

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