Abstract:
A device for color imaging including an optical sensor having light sensitive pixels with a metal film disposed over the light sensitive pixels. The metal film has a group of nano-holes arranged over the pixels according to a periodic lattice formation and is configured to pass light of a preselected first range of wavelengths. The group of nano-holes arranged over an adjoining group of pixels is configured to pass light having ranges of wavelengths different from the first range of wavelengths.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority to U.S. Provisional Patent Application No. 61/749,123 filed on Jan. 4, 2013, which is incorporated herein by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under FA9550-10-1-0264 and FA9550-09-1-0673 awarded by the Air Force. The government has certain rights in the invention. 
    
    
     FIELD 
     The present disclosure relates to plasmonic color filtering. More particularly, it relates to a system and method for color imaging with integrated plasmonic color filters. 
     BACKGROUND 
     In contemporary Si-based image sensor technologies such as charge-couple devices (CCDs) and complementary metal-oxide semiconductor (CMOS) image sensors, color sensitivity is added to photo detective pixels by equipping them with on-chip color filters, comprised of organic dye-based absorption filters. However, organic dye filters are not durable at high temperatures or under long exposure to ultraviolet (UV) radiation, and can be difficult to fabricate much thinner than a few hundred nanometers due to low absorption coefficient of the dye material. Furthermore, on-chip color filter implementation using organic dye filters rely on carefully aligned lithography steps for each type of color filter over the entire photodiode array, thus making such fabrication costly and highly impractical for multi-color and hyperspectral imaging devices having more than three primary or complementary colors. 
     SUMMARY 
     Embodiments of the invention are directed to a device for filtering selected colors of light. According to the embodiments, a plurality of holes can be formed in a thin metallic layer to filter the selected colors of light due to interference of surface plasmons between the plurality of holes. Thus, the color filter formed by plasmonic hole arrays can be combined together with individual pixels and sub-pixels or an imaging sensor to provide color imaging characteristics. 
     According to an aspect of the embodiments of the invention, a color imaging device is described, including: an optical sensor having a plurality of pixels, each pixel of the plurality of pixels including a plurality of sub-pixels, wherein each sub-pixel of the plurality of sub-pixels are adapted to sense light; and a metal film on the optical sensor. The metal film includes a group of nano-holes arranged over at least one sub-pixel according to a periodic lattice formation and is configured to pass light having, a preselected first range of wavelengths, wherein a group of nano-holes arranged over an adjoining sub-pixel is configured to pass light having ranges of wavelengths different from the first range of wavelengths. 
     According to another aspect, a diameter of each nano-hole of one of the groups of nano-holes is selected to pass light having a preselected range of wavelengths. 
     According to another aspect, spacing between adjoining nano-holes of the group of nano-holes is selected to pass light having a preselected range of wavelengths. 
     According to another aspect, the optical sensor is a CMOS image sensor or a CCD image sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the present invention, and many of the attendant features and aspects thereof, will become more readily apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate like components. 
         FIG. 1  is a perspective view of a plasmonic hole array integrated with a CMOS image sensor according to an embodiment of the present invention. 
         FIG. 2  is a schematic diagram depicting a film having an array of holes arranged in a triangular lattice. 
         FIG. 3A  is a schematic diagram of a film having a triangular lattice hole array showing a scattering-absorption model of surface plasmons. 
         FIG. 3B  depicts transmission efficiency curves based on the schematic of  FIG. 3A . 
         FIGS. 4A-4K  is a series of schematic diagrams illustrating a method of fabricating a thin metallic film layer having a plasmonic hole array, and integrating the metallic film with a CMOS image sensor according to an embodiment of the invention. 
         FIGS. 5A-5D  show various plots of absolute and relative transmission efficiencies of plasmonic hole arrays at various wavelengths. 
         FIGS. 6A-6C  show exemplary steady state field intensity distributions of three integrated color filters for each of the three colors, read, green and blue. 
         FIG. 7  is a flow chart for processing raw image data taken with a CMOS image sensor integrated with a plasmonic color filter according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described more fully with reference to the accompanying drawings, in which example embodiments thereof are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough and complete, and will fully convey the concept of the present invention to those skilled in the art. 
     While the described embodiments of the invention may be modified in various ways, the described embodiments are presented as examples in the drawings and in the detailed description below. The intention, however, is not to limit the invention to the particular embodiments described. To the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. Moreover, detailed descriptions related to well-known functions or configurations have been omitted in order not to unnecessarily obscure the subject matter of the present invention. 
     It will be understood that although the terms “first,” “second,” etc. may be used herein to describe various components, these components should not be limited by these terms. These descriptors are only used to distinguish one component from another. The terminology in this application is used to more clearly describe the presented embodiments and is not intended to limit the scope of the present invention. 
     As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” as used herein, specify the presence of the stated features or components, but do not preclude the presence or addition of one or more other features or components. “/”, as used in herein may be interpreted as “and,” or may be interpreted as “or” depending on the situation. 
     The sizes of the layers and regions in the drawings may be exaggerated for convenience of explanation. Like reference numerals refer to like elements throughout. It will be understood that when a layer, region, or component is referred to as being “on,” “formed on,” “over,” or “formed over,” another layer, region, or component, it can be directly or indirectly on or formed on the other layer, region, or component. That is, for example, intervening layers, regions, or components may be present. 
     Plasmonic hole arrays formed in thin metal films (e.g., 150 nm thick) can behave as optical band-pass filters due to interference of surface plasmons (SPs) between adjacent holes. Thus, a plasmonic hole array can be utilized as a plasmonic color filter to allow a certain wavelength (or a range or spectrum of wavelengths) of light to pass through the plasmonic hole array, and prevent other wavelengths of light from passing. For example, the plasmonic hole array can be configured by selecting the size of the holes (i.e., diameter of the holes), the period or periodicity of the lattice (i.e., distance between the holes, lattice constant), and even the shape(s) of the holes, such that a predetermined color of light (e.g., red) will pass through the plasmonic hole array while preventing other colors of light (e.g., blue, green, etc.) from passing through the plasmonic hole array. According to an embodiment of the invention, the thickness of the metal film is in the range of 50-200 nm. However, the metal film may be as thin as 30 nm, or as thick as 1000 nm (i.e., 30-1000 nm). In the context of the present disclosure, the term “plasmonic hole array” is used interchangeably with the terms “plasmonic color filter” and “color filter,” and the term “color” means a wavelength or a distribution of wavelengths of electromagnetic radiation incident on a plasmonic hole array. 
     According to an embodiment of the invention, a plasmonic hole array can be disposed over a black and white image sensor (i.e., an image sensor that is not capable by itself of recognizing color), and behave as a color filter for the image sensor. As such, the integrated black and white image sensor in combination with the color filter is able to recognize a color image. By way of example and not of limitation, the image sensor can be a CMOS image sensor  102  as shown in  FIG. 1 , with the plasmonic hole array  100  positioned over the CMOS image sensor  102  to allow a selected color of light to pass through the plasmonic hole array  100  such that the selected color of light is detected by the CMOS sensor. More specifically, according to the exemplary embodiment of  FIG. 1 , the plasmonic hole array  100  is divided into four substantially square-shaped arrays  104 ,  106 ,  108 , such that the hole arrangement of array  104  is configured to pass green light, the hole arrangement of array  106  is configured to pass blue light, and the hole arrangement of array  108  is configured to pass red light. 
     In the illustrated embodiment, the four arrays are aligned and positioned over a single pixel of the CMOS image sensor, with each array of the four arrays positioned precisely over one sub-pixel of the CMOS image sensor. Thus, one pixel of the CMOS image sensor is configured to sense each of three colors, red, green and blue. A person having ordinary skill in the art will recognize that red, green and blue light can be arranged, for example, in a Bayer filter mosaic arrangement. However, the number and/or configuration of sub-pixels in a pixel, or the colors of light, are not necessarily limited to those mentioned herein. 
     In contrast to on-chip organic color filters, the plasmonic color filters are highly tunable across the visible (and non-visible) electromagnetic spectrum by varying characteristics (size, spacing, periodicity, shape, etc.) of the holes of the array. Plasmonic color filters may also utilize a single perforated metallic layer to fabricate filters for many colors of light. That is, a filter for any desired color can be fabricated on a single metallic film at once. Furthermore, spatial color-crosstalk is reduced among the adjacent hole arrays. 
     In accordance with the invention, it has been discovered that the principal filtering function of a plasmonic hole array results from “near-field effects,” i.e., from plasmonic interactions between each hole and its nearest and/or next-nearest neighbor holes (e.g., second-nearest neighbor holes). Thus, it has been found that filtering efficiency does not require an infinite or near-infinite array of holes (i.e., an array relying on “far-field effects”), but rather, a relatively compact array may suffice. In specific embodiments, a filtering array may be as small as a 2×2 lattice unit cell (i.e., 4 holes), a 3×3 lattice unit cell (i.e., 9 holes), or a 4×4 lattice unit cell (i.e., 16 holes). Furthermore, it has been determined that arrays of these sizes can be placed directly adjacent arrays having different wavelength characteristics (i.e., in front of different pixels or sub-pixels) in a sensor array without exhibiting significant “crosstalk” between pixels or sub-pixels. 
       FIG. 2  shows an array of holes arranged in a triangular lattice. Each of the holes is substantially circular, and has a diameter  200  and a period  202  that varies in size and distance depending on the color that the plasmonic hole array is selected to filter. That is, an optimal hole size and period is selected based on the desired wavelength of light that the plasmonic hole array is designed to filter/pass. According to an embodiment of the present invention, the holes for filtering the green light (˜550 nm) are about 180 nm in diameter with a period of about 340 nm; the holes for filtering the red light (˜650 nm) are about 240 nm in diameter with a period of about 420 nm; and the holes for filtering the blue light (˜450 nm) are about 140 nm in diameter with a period of about 260 nm. Thus, a substantially square shaped array of holes in a triangular pattern can be made on the metal film layer, such as aluminum or any other suitable conductor or insulator material. With regard to conductors, a variety of metals, such as, gold, silver, copper, platinum, titanium nitride, indium tin oxide, or doped zinc oxides, for example, may be used to create the plasmonic color filter. In the context of the present disclosure, the term “nano-holes” is used interchangeably with the term “holes.” However, the diameters, periodicity, and shapes of the holes are not limited to those specifically described here. For example, the sizes of the holes can be, but not necessarily limited to, a diameter of approximately 50-1000 nm. In some embodiments, the size of the holes can be unequal. In such a case, the transmittance of light distribution passing through the array of holes is reduced, but the bandwidth profile becomes sharper. In some embodiments, the periodicity of the holes can also be unequal. 
       FIG. 3A  shows a schematic of a square-shaped triangular lattice hole array, showing a scattering-absorption model of the surface plasmons. According to an embodiment of the invention, transmission spectra of a relatively large array of holes (e.g., an infinite size hole array) can be produced by accounting for hole interactions of only nearest neighbor and second nearest-neighbors within the array. The scattering-absorption efficiencies can be determined by fitting finite-difference time-domain (FDTD) transmission data to a scattering-absorption model consisting of a truncated Fabry-Perot resonance between hole pairs. Although subwavelength holes are known to scatter both in circularly expanding surface plasmons  306  and in linear surface plasmons, the scattering contribution of circular surface plasmons  306  dominate the transmission spectra of the array. Thus, hole positions that are within “line-of-sight” of the hole of interest can be observed to approximate the transmission of the array to analyze the second nearest-neighbor interactions, since scattering from holes at larger distances can be thought of as being screened by the first and second nearest neighbors. 
     For example, considering the hole of interest  300  shown at the center of the array of  FIG. 3A , the holes  302  are the first nearest-neighbor holes relative to the hole of interest  300 , and the holes  304  are the second nearest-neighbor holes relative to the hole of interest  300 . The transmission spectra has a maximum amplitude at a surface plasmon wavelength that is approximately twice the diameter of the hole. That is, d˜λ SP /2. According to an embodiment of the present invention, the array of holes can comprise as few as just four holes. Accordingly, there is negligible spatial crosstalk between the different color hole arrays (e.g., adjacent hole arrays), and therefore there is negligible scattering of light into adjacent sub-pixels from the hole array after the light is coupled through the plasmonic filter. 
       FIG. 3B  shows transmission efficiency curves based on the schematic of  FIG. 3A , where the transmission spectra obtained by FDTD (solid curves) for a large-size hole array are in agreement with the second nearest-neighbor scattering model (dashed curves). 
     Although the square-shaped array can be of various sizes, the array described in the present disclosure is about ˜3×3 μm 2  to correspond to the size of the pixels generally found on a CMOS image sensor. By systematically increasing the size of the hole array by adding rows and columns to the ˜3×3 μm 2  array, and normalizing to an infinite array for reference, an asymptotic behavior of the spectra can be analyzed. Based on this analysis, the peak transmission value for a ˜4×4 μm 2  array (consisting of 10×12=120 holes) is ˜80% of the peak efficiency with respect to an infinite array for size. The peak transmission value for a ˜6×6 μm 2  array increases to ˜90% of the peak efficiency with respect to an infinite array, with only a minimal increase for array sizes beyond ˜10×10 μm 2 . Thus, the ˜6×6 μm 2  hole array shows near-infinite array transmission properties, with any additional increase in size providing marginal increases in transmission. 
       FIGS. 4A-4K  show a method of making a color filter by fabricating a plasmonic hole array with a metallic film layer, and integrating the plasmonic hole array with the CMOS image sensor. As discussed above, for purposes of this description, a Bayer filter mosaic layout is utilized, which consists of a pixel having a 2×2 color sub-pixel with two green filters, a blue filter and red filter. In other embodiments, the layout of filters may be in any other suitable mosaic pattern depending on physical parameters and constraints of use. 
     According to an embodiment of the invention, the fabrication process starts with a thin chromium layer  400  over a quartz substrate  402  as shown in  FIG. 4A . A layer of polymethyl methacrylate (PMMA)  404  is coated on the chromium layer  400  and a series of electron beam lithography steps are performed to develop a pattern  406  that represents the holes, as shown in  FIGS. 4B-4D . Next, a layer of aluminum  408  is coated and the remaining chemicals on the quartz substrate are removed ( FIG. 4D ), thus obtaining a thin metallic film layer  408  with a plurality of holes  410  arranged in an array. Finally, a layer of glass  412  is spin coated on the metal film as shown in  FIG. 4G .  FIG. 4H  shows conceptually, the resulting plasmonic color filter having a red filter region R, a green filter region G, and a blue filter region B. 
     According to an embodiment, a 180×160 pixel color filter (e.g., Bayer mosaic) is created using this method, onto a quartz substrate having a total of 360×320 sub-pixels (e.g., dimensions 2016×1792 μm 2 ). Each of the pixels is separated by 11.2 μm wide grid lines separating the color filter into 40×40 filter blocks (corresponding to 224×224 μm 2 ), which were created during the fabrication process to both prevent overexposure of the peripheral region of each block during electron beam lithography and to serve as guide lines for alignment with the CMOS image sensor pixel array. 
       FIGS. 4I-4K  show a method of integrating of the plasmonic hole array color filter with a front-side-illumination black and white CMOS image sensor  414  composed of 1920×1200 2.8×2.8 μm 2  size pixels, corresponding to a 5376×3360 μm 2  effective pixel area. In some embodiments, the CMOS image sensor  414  is coated with a layer such as, for example PMMA  416  to create a flat surface over the microlens  418  of the CMOS image sensor as shown in  FIG. 4J . The plasmonic hole array color filter  420  can be disposed on the PMMA  416  to directly couple with the CMOS image sensor  414 . In some embodiments, a microlens  418  of the CMOS image sensor  414  is first, flattened/smoothed by, for example, shaving or grinding the microlens  418  portion. The plasmonic hole array can be placed directly on the flattened microlens  418  of the CMOS image sensor  414 , or the PMMA  416  can be layered between the CMOS image sensor  414  and the plasmonic hole array  420 . The specific type and size of CMOS image sensor described above is an example for purposes of this disclosure, and instead any other suitable image sensor known by those skilled in the art can be utilized. 
     In some embodiments, the plasmonic color filter with the integrated CMOS image sensor is combined together with an optical lens, such as, for example, a C-mount camera lens to focus light (containing an image) onto the plasmonic color filter. 
       FIG. 5B  shows an example of computed spectral transmission data of an unmounted plasmonic color filter embedded in the quartz substrate, and illuminated from a normal angle of incidence.  FIG. 5A  shows an example of measured spectral transmission data of a plasmonic color filter, where each of the three RGB filters have peak efficiencies in the 50-60% range with full-width at half maximum (FWHM) in the 150-200 nm range, which is consistent with transmission data extracted from full field electromagnetic simulations using FDTD calculations for the unmounted plasmonic color filters of  FIG. 5B . 
       FIG. 5C  shows an example of measured spectral transmission data of a plasmonic color filter that is integrated with the CMOS image sensor. The measured spectral image for red, green and blue color bands with a FWHM of 40 nm and center wavelengths ranging from 400-750 nm in 50 nm steps is shown plotted in  FIG. 5C . The focal length of the C-mount lens utilized to obtain the spectral transmission data is 12.5 mm and an f-number of 5.6. A comparison of the transmission efficiencies between the unmounted ( FIG. 5A ) and CMOS integrated ( FIG. 5C ) plasmonic color filters show a quantitative measurement of how well the plasmonic color filters are integrated onto the CMOS image sensor. In comparing these two data sets, the integrated peak transmission efficiencies are in the 50% range, which is slightly lower than those of the unmounted color filters ( FIG. 5A ). Thus, integrating the color filter in direct contact with the CMOS image sensor negligibly degrades the efficiency. 
       FIG. 5D  shows the relative efficiency of the CMOS image sensor integrated with the plasmonic hole array color filters. The horizontal error bars correspond to the spectral width of the band-pass filter used for the measurements and the vertical error bars correspond to averaged data. 
       FIGS. 6A-6C  show examples of steady state intensity field distributions of three integrated color filters for each of the three colors, read, green and blue. According to an embodiment, a significant factor in the high light coupling efficiency comes from the nitride waveguide which directs the light into the active region of the Si image sensor pixel. Moreover, blue light absorption occurs in the Si pixel near-surface region, while green light absorption occurs at a depth of approximately 1 μm, and red light absorption occurs at a depth of approximately 3 μm below the Si surface. 
       FIG. 7  is a flow chart illustrating the processing of 10-bit raw image data ( 701 ) taken by a CMOS image sensor integrated with a plasmonic color filter. Since the pixels on the CMOS image sensor are filtered by a Bayer color filter array, each sub-pixel of the pixel has intensity information of only one of three colors, red, green, or blue. To reconstruct the full-color image, a bilinear demosaicing algorithm ( 702 ) is used to interpolate the set RGB values for each pixel from its neighbors. For example, the red value of a non-red pixel may be computed as the average of the two or four adjacent red pixels, and similarly for blue and green. 
     After demosaicing, the image is white balanced ( 703 ) by equalizing the RGB signal levels of the gray patches. A linear matrix correction ( 704 ) is applied to correct for spectral color crosstalk from the color filter. A purer green spectra with a narrower FWHM and smaller linear matrix cross-terms can be obtained by slightly reducing, for example, the hole diameters and/or changing the period to compensate for the effective surface plasmon path length between holes. 
     After applying the linear matrix pixel transformation, a gamma correction ( 705 ) is applied to convert the linear response brightness of the CMOS image sensor to a logarithmic response of a human eye. As such, the demosaicing ( 702 ), white balance adjustment ( 703 ), linear matrix ( 704 ), and gamma correction ( 705 ) can be applied by varying the amplification values corresponding to each pixel and/or sub-pixel of the CMOS image sensor. 
     For proper filter performance, the filters should function as intended among all aperture angles. The definition of f-number is a ratio of focal length (f) to aperture diameter (D). The half-aperture angle for a given f-number is equal to θ 1/2 =arctan(½ F). As such, for an f-number of 16, the maximum half-aperture collection angle is 1.8 degrees, which means that the image sensor collects a nearly collimated incident light. On the other hand, for and f-number of 1.8, the half aperture collection angle is 15.5 degrees. The plasmonic hole array filters according to the embodiment of the invention retain their designed filter functionality for larger aperture angles as well as for smaller aperture angles. 
     It will be recognized by those skilled in the art that various modifications may be made to the illustrated and other embodiments of the invention described above, without departing from the broad inventive step thereof. It will be understood therefore that the invention is not limited to the particular embodiments or arrangements disclosed, but is rather intended to cover any changes, adaptations or modifications which are within the scope and spirit of the invention as defined by the appended claims and their equivalents.