Abstract:
A spectrometer system includes an array of micro-zone plates (MZP) each having coaxially-aligned ring gratings, a sample plate for supporting and illuminating a sample, and an array of photon detectors for measuring a spectral characteristic of the predetermined wavelength. The sample plate emits an evanescent wave in response to incident light, which excites molecules of the sample to thereby cause an emission of secondary photons. A method of detecting the intensity of a selected wavelength of incident light includes directing the incident light onto an array of MZP, diffracting a selected wavelength of the incident light onto a target focal point using the array of MZP, and detecting the intensity of the selected portion using an array of photon detectors. An electro-optic layer positioned adjacent to the array of MZP may be excited via an applied voltage to select the wavelength of the incident light.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Application 61/089,226 filed on Aug. 15, 2008, which is hereby incorporated by reference in its entirety. 
    
    
     ORIGIN OF TITLE INVENTION 
     The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without payment of any royalties thereon or therefor. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to spectrometers, and in particular to spectrometers having an array of thin-film micro-zone plates (MZP) suitable for acquiring photons of a specific wavelength from one or more point light sources. 
     BACKGROUND OF THE INVENTION 
     Spectroscopic analysis or spectroscopy pertains to the study of the dispersion of light into its component wavelengths. By analyzing the absorption and dispersion of incident light and other radiation by matter, scientists are able to study various properties of the matter such as temperature, mass, luminosity, composition, etc. Optical instruments known as spectrometers are used to measure and study such light dispersion. Spectrometers therefore play an essential role in the study and design of various scientific monitoring devices, for example multi-spectral imaging (MSI) systems, hyper-spectral imaging (HSI) systems, and the like. 
     In a conventional spectrometer, incident light passes through a first linear opening or slit in a mirror or an optical lens. A beam of incident light passing through the first slit illuminates a prism or a linear grating device. The grating device may have a series of vertically-aligned gratings which diffract the incident light into its component colors, with each color corresponding to a particular band of wavelengths of the electromagnetic spectrum. 
     Spectrometers may include multiple aperture slits, with the first slit positioned in front of the linear grating device to initially select light in a relatively narrow band of wavelengths. The linear grating device spreads this portion of the light beam at different wavelength-dependent angles. A second slit in another mirror or optical lens may be positioned to allow selective passage of a narrower band of the light beam from the linear grating device. The second slit may be used to direct selected wavelengths to a measurement device to determine a desired spectral characteristic. In this manner, a specific wavelength or set of wavelengths may be selected for detailed spectral analysis. 
     Conventional spectrometers as described above are often used to acquire and analyze light from a single point source, but may require additional lenses or mirrors to capture a point light source. Because a single spectrometer cannot make a two-dimensional (2D) image without scanning, the total speed of acquiring 2D spectral data is also relatively slow. For example, if it takes approximately 10 seconds to resolve a first spot, 1024×768 images will require 7,864,320 seconds or 91 days to complete. A multi-channel position-sensitive device or Charged Coupled Detector (CCD) array may eliminate the need for a linear aperture slit, but nevertheless may require the additional dimension in order to function properly. A red-green-blue (RGB) CCD array may be used to capture 2D images. However, the color filters of such CCD arrays may have less than optimal spectral resolution. 
     SUMMARY OF THE INVENTION 
     Accordingly, a miniature arrayed spectrometer system is provided herein that can be used to rapidly build a high-resolution two-dimensional (2D) image of desired spectral information or data while potentially providing significant size advantages relative to the designs of the prior art. Within the scope of the present invention, the spectrometer system uses an array of photon detectors in conjunction with an array of thin-film micro-ring gratings in the form of micro-zone plates (MZP). The MZP may be configured with an electro-optic layer that can be energized to optimize data acquisition times. The arrayed spectrometer system does not require the use of a focusing lens for micro-objects due to its multiple built-in optical focal points. 
     In particular, the arrayed spectrometer system includes a plurality of MZP, hereinafter referred to as an MZP array, with each MZP in the MZP array having a plurality of coaxially-aligned annular or ring-shaped gratings. Each MZP diffracts a selected wavelength of incident light from sample matter onto a target focal point, e.g., incident light from a micro/nano object. The spectrometer system also includes a sample layer configured for supporting and illuminating the sample via passage of light through a plurality of apertures, and an array of photon detectors for measuring a desired spectral characteristic of the selected wavelength, e.g., light intensity. 
     According to one embodiment, each MZP in the MZP array is configured as one of a positive MZP or a negative MZP as those terms are described herein. That is, each MZP in the MZP array is identically configured. An electro-optic layer may be energized to select a wavelength. A beam separator may be positioned adjacent to the MZP array to separate unwanted wavelengths of light from the desired or selected wavelengths. 
     The sample layer may be configured to emit an exponentially-decaying electrical field or an evanescent wave in response to the incident light, with the evanescent wave exciting molecules of the sample to thereby cause an emission of secondary photons from the sample. These photons are detectable using the array of photon detectors or detector array. A data recorder or other suitable recording device may be placed in communication with the detector array and used to record the intensity of the selected wavelengths or another desired spectral characteristic. 
     A method of detecting the intensity of selected wavelengths of incident light includes directing the incident light onto an MZP array, with each MZP having a plurality of coaxially-aligned annular or ring-shaped gratings. The gratings are configured for diffracting a predetermined wavelength of light from a sample onto a target focal point. The method includes directing source light onto one side of a sample plate and through a plurality of apertures defined thereby in order to illuminate a sample on the other side of the sample plate. Illumination of the sample generates the incident light, which passes to the MZP array. The MZP array diffracts the selected wavelength(s) onto the target focal point. The method includes detecting the intensity of the selected wavelength(s) using an array of photon detectors. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic illustration of a positive micro-zone plate (MZP) usable within an arrayed spectrometer system in accordance with the present invention; 
         FIG. 1B  is a schematic illustration of an alternate negative MZP; 
         FIG. 2  is schematic illustration of a micro-ring spectrometer usable within the arrayed spectrometer system of the present invention; 
         FIG. 3A  is a schematic perspective view illustration of a sample layer of the arrayed spectrometer system; 
         FIG. 3B  is another schematic perspective view illustration of the sample layer of  FIG. 3A ; 
         FIG. 4  is a schematic perspective view illustration of an array of MZP and the sample layer of  FIG. 3 ; and 
         FIG. 5  is a schematic perspective view illustration of a micro-ring thin-film spectrometer system in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings wherein like reference numbers represent like components throughout the several figures, and beginning with  FIG. 1A , a positive electro-optic micro-zone plate (MZP)  10  is provided that can be arrayed with a plurality of similar MZP  10  within the arrayed spectrometer system  50  of the present invention, as shown in  FIG. 5  and discussed in further detail hereinbelow. Within the scope of the present invention, the term “MZP” refers to alternating transparent and opaque concentric zones of a predetermined or calibrated width. The MZP  10  may be constructed on a thin film or layer of suitable material, as understood in the art. The MZP  10  can acquire photons of a particular wavelength from multiple point light sources or micro/nano objects, as will described below, and can thereafter build a two-dimensional (2D) image of the detected spectral data. 
     The positive MAP  10  includes a transparent center disk  12  that is circumscribed by a series of progressively larger transparent rings  14 . The transparent rings  14  are separated by an interposed series of progressively larger opaque rings  16 , with the center disk  12  and each of the rings being coaxially-aligned and centered on a common optical axis  11 . For simplicity, the number of rings is kept at a minimum in  FIGS. 1A and 1B , with the actual number of rings depending on the particular design and intended use of the MZP  10 . 
     The transparent center disk  12  and the various rings  14 ,  16  may be configured as optical gratings on thin film. As will be understood by those of ordinary skill in the art, the term “optical gratings” refers to an optical element configured for diffracting incident light and directing it to a predetermined optical focal point. Gratings have a regular pattern which split and diffract incident light into several beams travelling in directions that depend on the spacing between gratings and the wavelength of the incident light. 
     Source light (arrows  13 ) is directed toward the MZP  10  from a source, e.g., a micro/nano object, organism, matter, or other substance serving as the subject of the spectral analysis at hand. The source light is then diffracted by the various rings  14 ,  16  of the MZP  10  into different wavelengths, with each wavelength directed toward a particular focal point P 1 , P 2 , P 3 , P 4 , or P 5 . That is, the particular focal point corresponds to particular wavelengths or frequencies of the source light (arrows  13 ). The transparent center disk  12  allows a constructive interference point at the farthest focal point, i.e., focal point P 1 . Additional constructive interference points are provided at focal points P 3  and P 5 . 
     As is well understood in the art, the transmission of light in the form of waves gives rise to the principals of constructive and destructive wave interference. During any wave interference the shape of the medium is determined by the sum of the separate amplitudes of each wave. The waves interfere when one wave passes through another. When the crest of one wave is superpositioned upon the crest of another, the waves constructively interfere. Constructive interference also occurs when the trough of one wave is superpositioned upon the trough of another. Conversely, destructive interference occurs when the crest of one wave is superpositioned upon the trough of another. During destructive interference, the positive amplitudes from one crest are added to the negative amplitudes from the other trough, with the result being a reduced amplitude or destructive wave interference. Such principles give rise to the different constructive/destructive focal points discussed above. 
     Referring to  FIG. 1B , another type of MZP is the negative MZP  10 A, which has at its optical center an opaque center disk  12 A that is circumscribed by progressively larger opaque rings  16 A. The rings  16 A are separated by a corresponding series of progressively larger transparent rings  14 A, with the center disk  12 A and each of the rings being coaxially-aligned and centered on axis  11 . The MZP  10 A allows a constructive interference point at P 1 ′ but the focused photons at P 1 ′ have 180 degree out of phase relation with respect to the focused photons at P 1 . The source light (arrows  13 ) is diffracted by the MZP  10 A and directed toward a focal point P 1 ′, P 2 ′, P 3 ′, P 4 ′, and P 5 ′, with the particular focal point corresponding to a band of wavelengths or frequencies of the source Light (arrows  13 ). The opaque center disk  12 A allows a constructive interference point at the farthest focal point, i.e., focal point P 1 ′ with photons of 180 degree out of phase. Additional constructive interference points are provided at focal points P 3 ′ and P 5 ′. 
     The MZP  10 ,  10 A may be used as micro-ring gratings that focus parallel photons of source light (arrows  13 ) into the different radial points according to their wavelengths. A photon detector (D)  18  may be placed at any of the focal points P 1 -P 5 , and may relay or transmit detected information (arrow i) to a data recorder (R)  20  to provide a historical record for facilitation of spectral analysis. 0 th  order direct photons from the source light (arrows  13 ) through the transparent center disk  12  of  FIG. 1A  may cause bright irregular background noise at the center of circular aperture at a focal point. It takes an infinite number of micro-ring gratings to completely compensate 0 th  order constant photons through the transparent center disk  12  of the positive MZP  10  of  FIG. 1A . 
     This result is similar to the Fourier transform in which y=c (a constant) is approximated by the sum of infinite sine and cosine waves. However, a negative MAP such as the MAP  10 A of  FIG. 1B  does not have a direct line-of-sight between the focal point and the light source, and therefore all converging light is from higher order photons without a 0 th  order photon. Therefore, the MZP  10 A of  FIG. 1B  may be particularly well suited for use as a micro-ring grating due to the opacity of its center disk  12 A. 
     Referring to  FIG. 2 , the MZP  10  of  FIG. 1A  is used hereinafter to illustrate the core concepts behind the construction of the arrayed spectrometer system  50  of  FIG. 5 , however the MZP  10 A of  FIG. 1B  may also be used without departing from the intended scope of the invention. In some embodiments, the MZP  10  may be placed adjacent to an electro-optic or E/O layer  22 , which in turn may be placed adjacent to a transparent electrode  26 . The E/O layer  22  may be constructed of a material have a variable refractive index in response to an applied electric field, e.g., liquid crystal, non-linear crystal, an electro-optic polymer, etc. The electrode  26  in turn provides a bias voltage for the E/O layer  22 , and may be constructed of Indium Tin Oxide (ITO) or other suitable materials. The MZP  10  may be fixed, and the designated wavelength (arrows  15 ) selected by moving a sample  26 , e.g. a point light source or micro/nano object, around a focal point as indicated by double-sided arrow A. Non-selected wavelengths (arrows  17 ) are directed away from the photon detector  18 . 
     For a positive MZP  10  (see  FIG. 1A ), a good dispersion point is near focal point P 1  shown in that Figure. As an MZP already has a built-in focal point, it does not require an additional converging lenses or mirrors of the type used with linear grating systems of the prior art. The MZP  10  may also be included within a programmable spectrometer. In such a configuration, the designated wavelength (arrows  15 ) may be selected by applying a voltage from a power supply  28  to the E/O layer  22 . The sample  26  remains fixed at the focal point in this particular configuration. 
     Referring to  FIG. 3A , based on the above principles a sample plate  30  may be illuminated using backside light (arrows  19 ). The sample plate  30  may be constructed as an opaque plate or disk sufficient for supporting the sample  26  on a surface  32 . The sample plate  30  defines or includes a series of apertures  34 , which in one embodiment are equally-spaced. The apertures  34  may have a diameter of a few nanometers according to one embodiment, and admit some of the backside light (arrows  19 ) to help define a pixel in the 2D image. 
     Referring to  FIG. 3B , the illuminated sample  26  can emit secondary photons (arrows  21 ) with different wavelengths by a photoluminescence process. Also, different materials and geometric topographies may change the intensity of the penetrating photons (arrows  23 ) through the apertures  34 . The secondary photons (arrows  21 ) and the penetrating photons (arrows  23 ) may be detected and measured for spectral analysis of the sample  26 . 
     When backside light (arrow  19 ) is incident on the sample plate  30  at a glancing angle, total internal reflection occurs so that no photon is able to travel above surface  32 . Exit light (arrow  25 ) is reflected away from the sample plate  30 . However, the tangential component of the electric field ({right arrow over (E)}) is still continuous above the surface  32 , and this tangential component decays exponentially with time (t) above the surface  32  as shown in the superimposed graph  36  plotting Z vs. {right arrow over (E)} 1 . 
     This vertically-decaying time-varying electric field is known as the evanescent wave. Because the evanescent wave is decaying quickly along the Z-axis, it cannot reach the photon detector  18  (see  FIGS. 1A and 1B ) if the detector is positioned at a distance of greater than a few microns from the sample plate  30 . However, the evanescent wave can excite the molecules of the sample  26  on the surface  32  to emit the secondary photons (arrows  21 ), which can travel over a considerable distance. Also, if there are geometric variations due to the sample  26 , some of the light, i.e., the penetrating photons of arrows  21 , can penetrate and travel above the surface  32  absent total-internal-reflection. 
     Referring to  FIG. 4 , in order to construct the arrayed spectrometer system  50  of  FIG. 5 , an MZP array  40  is first positioned above the sample plane  30 . The MZP array  40  includes a plurality of individual MZP, whether a positive MZP as shown in  FIG. 1A  or a negative MZP as shown in  FIG. 1B . The MZP array  40  is placed at a focal distance Z above the sample plate  30  supporting the sample  26  that is being spectrally analyzed. 
     Referring to  FIG. 5 , the arrayed spectrometer system  50  of the present invention includes a beam separator  38  that is placed adjacent to the MZP array  40  to separate unwanted wavelengths of light. A photon detector array  118  constructed of a plurality of photon detectors  18  (see  FIGS. 1A ,  1 B, and  2 ) is positioned adjacent the beam separator  38  and opposite the MZP array  40 , i.e., the beam separator is sandwiched between the detector array  122  and the MAP array  40 . The detector array  122  may be an array of PiN diodes, i.e., diodes having a lightly doped near intrinsic semiconductor region between a p-type and an n-type semiconductor region. Alternately, the array  122  may include M-MOS or P-MOS logic devices, i.e., respective M-type or P-type field effect transistors, a charged-coupled device (CCD) or analog shift register, as those terms are understood in the art. However, other photon detecting devices may also be used without departing from the intended scope of the invention. 
     The detector array  122  receives light of a specific wavelength from each MZP  10  in the MZP array  40 . For a fixed-ring grating MZP, the distance or focal length Z between the plane of the sample plate  30  and the MZP array  40  may be changed to select a different wavelength of light for spectral analysis. As shown in  FIG. 2 , for an electro-optical ring grating MAP a voltage may be applied to the E/O layer  22  to bring a specific wavelength light to each photon detector  18  in the detector array  118 . 
     While acquisition of point-source light is noted above, those of ordinary skill in the art will readily appreciate that the arrayed spectrometer system  50  can also acquire light from greater distances. That is, each aperture  34  can confine parallel light transmitted over a distance and effectively convert the light to an equivalent point-source. In this manner the system  50  can be used to capture 2D spectral data from far objects, thus enabling a host of potential applications including but not limited to multi-spectral imaging (MSI), hyper-spectral imaging (HSI), and other remote sensing and/or spectral analysis applications. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.