Abstract:
In accordance with embodiments of the present invention, a nano structure optical wavelength filter is provided. A film made of a negative dielectric constant material such as a metal has embossing structures of subwavelength scale, located thereon in an array in a pattern. The array pattern and the structures are configured such that when light is incident on the array structures, at least one plasmon mode is resonant with the incident light to produce a transmission spectral window with desired spectral profile, bandwidth and beam shape.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    The present application claims benefit of U.S. provisional application 60/877,660, filed Dec. 29, 2006, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Optical filtering is an important concept in optics, and is commonly involved in a variety of optical components and instruments. One example is to use optical filters for optical detectors. Optical detectors are normally sensitive to a broad spectrum of light so that light of broad range of lights all might be detected. Therefore it would be much more useful to have a material or a device that operates exactly in a reverse manner that it selectively transmits light only in a narrow range of frequencies within a broad spectrum. 
         [0003]    Filters made from wire-mesh or metallic grids have been used extensively for filtering light in the far IR (infrared) spectrum. Such filters and devices incorporating the filters are disclosed in U.S. application Ser. Nos. 10/566,946 and 11/345,673 filed on Jul. 22, 2004 and 2/206, respectively, both of which are incorporated herein by reference in their entirety. These filters comprise thin metallic wires (much thinner than the wavelengths to be transmitted) deposited on an optically transparent substrate. The filters are characterized by a transmission spectrum having a peak at approximately 1.2 times the periodicity of the mesh. The peak is very broad, typically greater than half of the periodicity of the mesh. These filters would be much more useful if their transmission spectra could be narrowed to make them more selective. 
       SUMMARY OF THE INVENTION 
       [0004]    A film made of a negative dielectric constant material such as a metal has embossing structures of subwavelength scale, located thereon in an array in a pattern. The array pattern and the structures are configured such that when light is incident on the array structures, at least one plasmon mode is resonant with the incident light to produce a transmission spectral window with desired spectral profile, bandwidth and beam shape. Such embossing structures can be used as various wavelength filtering devices for chip scale spectrometer, color image sensor, hyperspectral image sensor or color flat panel display, and for beam shaping devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a perspective view of an embossing array on a metallic film; 
           [0006]      FIG. 2  is a cross sectional view of an embossing array on a metallic film shown in  FIG. 1 ; 
           [0007]      FIGS. 3A and 3B  are graphical representations of transmission intensity as a function of wavelength for different embossing array geometries; 
           [0008]      FIGS. 4A through 4K  show perspective views of examples of different embossing structures; 
           [0009]      FIGS. 5A ,  5 B,  5 C and  5 D show examples of different plan view layouts of embossing structures; 
           [0010]      FIG. 6A  is schematic representation of a monochromator; 
           [0011]      FIG. 6B  is schematic representation of a chip scale spectrometer; 
           [0012]      FIG. 6C  is schematic representation of a multispectral image sensor. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    Unless otherwise specified, the words “a” or “an” as used herein mean “one or more”. The term “light” includes visible light as well as UV and IR radiation. The invention includes the following embodiments. 
         [0014]    In  FIG. 1 , a thin metallic film  100  containing an array of embossing structures  101 , such as metal structures  101 , in a square pattern, is shown (not to scale). The metal may be any metal and is preferably Ag, Au, Cr or Al or alloys thereof. The gap between embossing structures is G. The width, length and height of the embossing structures are W, L and H respectively. The thickness of metallic film or plate  100  is preferably in the range of approximately 1 to 50 nm which is partially optically transparent. The intensity of the incident light is L incident  and the intensity of the transmitted light after traveling through the gaps in the embossing structures and film is L transmitted . In  FIG. 1  an unsupported thin metal plate is shown, however, a thin metal film deposited on an optically transparent substrate, such as a glass, quartz or polymer, is also contemplated by the present invention. Thus, the metal film  100  is continuous and does not need to contain any openings or holes which extend through the entire film in the gap region. Thus, the film  100  is preferably continuous and contains no through openings in the gap or the feature  101  regions. In contrast, prior art plasmonic resonance structures contain through holes which extend through the entire metal film. 
         [0015]    The embossing structures  101  may be formed by any suitable method. For example, the structures  101  may be formed by embossing grooves into the film to form the gaps G. Alternatively, the structures may be formed by photolithographic etching of the gaps G in the film. Alternatively, the structures  101  may be formed by direct deposition of the structures  101  on the metal film  100  or by forming a metal layer on the film  100  and patterning the layer into the structures  101 . Alternatively, the structures  101  may be formed by electroplating or electroless plating. Alternatively, the structures  101  may be formed by combination of aforementioned methods. 
         [0016]    The arrays of embossing structures shown in  FIG. 1 , exhibit distinct transmission spectra with well defined peaks. The power level of transmitted light is much greater than the expected power level from conventional theory.  FIG. 3  shows FDTD simulations which indicate that the unusual optical properties are probably due to the resonance of the incident light with the surface plasmons of the embossing structure array in metal. It is possible that other phenomena, such as interference due to array geometry, also contributes to the wavelength selective enhanced transmission. 
         [0017]    In  FIG. 3 , the solid line represents transmission of light through a 30 nm thick Ag film, having embossing feature  101  height of 200 nm and a 40 nm gap G between features  101 . For the simulation shown in  FIG. 3A , the embossing width is 200 nm and embossing length is 200 nm. For the simulation shown in  FIG. 3B , the embossing feature width and length are both 400 nm. The peaks occur in relation to the thickness of embossing, gap between embossing and period of the array. The bandwidth of the peaks is also strongly dependent on the gap between embossing and period of array. 
         [0018]    The structures do not have to have rectangular shapes. For example, various other shapes shown in  FIGS. 4A through 4K  show perspective views of examples of different embossing structures which can be used to provide the plasmonic coupling effect between the film  100  and the incident radiation. 
         [0019]      FIGS. 5A ,  5 B,  5 C and  5 D show examples of different plan view layouts of embossing structures. Thus, the structures  101  may be laid out in a square grid as shown in  FIG. 5A  or they may be offset from the square grid along the width or length directions, as shown in  FIGS. 5B ,  5 C and  5 D. Furthermore, the structures  101  may comprise non-rectangular structures, such as hexagonal structures which are arranged in a honeycomb pattern, as shown in  FIG. 5D . 
       Applications 
       [0020]    The novel optical filtering functions that have been revealed and demonstrated with subwavelength scale array of metallic embossing structures proposed here are expected to bring a major impact on various fields that involves optics. 
         [0021]      FIG. 6A  is schematic illustration of wavelength separation using the embossed film as a micron-scale monochromator device  201 . As shown in  FIG. 6A , incident radiation having a range of wavelengths λ 1  to λ n  is provided onto the metal film  100  having the plurality of features  101 . The transmitted radiation is provided through the plurality of gaps between the features such that the transmitted radiation is simultaneously separated into a plurality of passbands having different peak wavelengths λ i , λ j , and λ k . The incident radiation may be provided onto either side of the film  101 . 
         [0022]    The metal film  101  is divided into a desired number of cells or regions  108 , such as at least two cells, where the size of the features  101  and gaps G is substantially the same within each cell. However, the size of the features  101  and/or gaps G and/or a period between the gaps G differs between cells. For example, three cells  108 A,  108 B and  108 C are illustrated in  FIG. 1 . 
         [0023]    The configuration of the features  101  and gaps G in each cell  108  is designed to produce a passband at a certain peak wavelength in the transmission spectrum. Thus, a transmission of the radiation having one peak wavelength is enhanced due to the geometry in the first cell  108 A. A transmission of the radiation having a different peak wavelength is enhanced due to the different geometry in the second cell  108 B. 
         [0024]    Preferably, the device  201  contains at least ten cells, more preferably at least 30 cells, such as 30 to 3,000 cells, for example 30 to 1,000 cells. Preferably, the passband radiation transmitted through each cell  108  has a peak wavelength that differs by at least 1 nm, such as by at least 10 nm, for example by 10 to 100 nm, from peak wavelengths of radiation transmitted through the other cells  108 . 
         [0025]    The wavelength separation device  201  can be used together with a photodetector  302  to form a spectrum analyzer or spectrometer  304 , as shown in  FIG. 6B . Any device which can detect visible, UV and/or IR passband transmitted radiation may be used as the photodetector  302 . The photodetector  302  is adapted to detect radiation transmitted through the wavelength separation device  201 . Preferably, an array of solid state photodetector cells, such as a semiconductor photodetector array is used as a photodetector. Most preferably, charge coupled devices (CCDs), a CMOS active pixel sensor array or a focal plane array are used as the photodetector. The photodetector  302  shown in  FIG. 6B  includes a substrate  313 , such as a semiconductor or other suitable substrate, and a plurality of photosensing pixels or cells  306 . Preferably, each photodetector cell or pixel  306  is configured to detect passband radiation having a given peak wavelength from each respective cell  108  of the wavelength separation device  201 . 
         [0026]      FIG. 6C  is schematic representation of a multispectral imaging system, when the monochromator is extended to a two dimensional array configurations. A multispectral imaging system is a system which can form an image made up of multiple colors. One example of a multispectral imaging system is a digital color camera which can capture moving and/or still color digital images of objects or surroundings. Another example of a multispectral imaging system is an infrared camera, which forms a digital image in visible colors of objects emitting infrared radiation, such as a night vision camera. The camera contains a processor, such as a computer, a special purpose microprocessor or a logic circuit which forms a color image (i.e., as data which can be converted to visually observable image or as an actual visually observable image) based on radiation detected by the photodetector. The multispectral imaging system may store the color image in digital form (i.e., as data on a computer readable medium, such as a computer memory or CD/DVD ROM), in digital display form (i.e., as a still or moving picture on a screen) and/or as a printout on a visually observable tangible medium, such as a color photograph on paper. 
         [0027]      FIG. 6C  shows a multispectral imaging system comprising a three dimensional wavelength separation device (i.e., the metallic embossing array)  110  and a photodetector  302 . The system contains an array of cells or pixels  408  arranged in two dimensions in the wavelength separation device  201 . Preferably, the cells  408  are arranged in a rectangular or square matrix layout. However, any other layout may be used instead. Each cell  408  is adapted to produce a multicolor portion of a multidimensional image. 
         [0028]    Each cell or pixel  408  comprises at least three subcells or subpixels  108  shown in  FIG. 6A , such as nine subpixels. Each subcell  108  in a particular cell  408  is designed to transmit one particular color (or a narrow IR, VIS or UV radiation band). Each cell  418  of the array  110  is preferably identical to the other cells in the array because each cell contains the same arrangement of subcells  108 . 
         [0029]    The array  110  can also be used in a liquid crystal display as a color filter for each pixel of the LCD. In this case, the array  110  is positioned over a back light which emits white light and the array filters particular light colors for each pixel. 
         [0030]    Although the foregoing refers to particular preferred embodiments, it will be understood that the present invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the present invention. 
         [0031]    All of the publications, patent applications and patents cited in this specification are incorporated herein by reference in their entirety.