Abstract:
A microoptoelectromechanical integrated spectrometer with a photonic element assembly having metal foil removably disposed on a first transparent substrate surface, the substrate having no foil on any other surface. A means is provided for directing source photons that are reflected from or transmitted through a sample, over a range of angles of incidence, into the transparent substrate and onto the metal foil such that source photons are incident at the Brewsters angle. A means is also provided for detecting an induced exponential field in the metal foil. A means is also provided for relating the induced exponential field to a known exponential field for the sample and determining the identity of the sample. The spectrometer performs ultraviolet-to-visible-to-infrared spectroscopy using photon tunneling and surface plasmon excitation.

Description:
[0001] The United States Government has rights in this invention pursuant to contract no. DE-AC05-000R22725 between the United States Department of Energy and UT-Battelle, LLC. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a spectrometer or filter device. In particular, the present invention relates to a spectrometer device in which the wavelengths of a beam of photons are controllably detected and measured in relative intensity after being distributed to a set of photon receptor elements via a thin metal foil. Further, the present invention relates to spectral information in the source beam of photons that is subject to differentiation by wavelength and distance within the exponential field within one wavelength of the foil. The separation of wavelengths into multiple parts occurs by the stimulation of surface plasmons in the metal foil at the Brewster angle for each incident wavelength or by detection of the decay length of an evanescent field. An embodiment of the invention employs surface plasmons and provides filtering of all wavelengths not incident at the Brewster angle due to the high reflectivity of the metal foil at angles other than Brewster&#39;s angle.  
         BACKGROUND OF THE INVENTION  
         [0003]    The use of surface plasmons to filter and hence to separate wavelengths of photons by using a supplementary dielectric second layer has been described in U.S. Pat. No. 5,986,808 by Wang and in U.S. Pat. No. 6,122,091 by Russell, both herein incorporated by reference. Each of these requires a second layer of dielectric beyond a metal foil that is essential to the phenomena considered and the filtering is not therefore solely dependent upon the surface plasmons. Nor is the case considered in these references one in which the surface plasmons are detected within one wavelength of the metal foil.  
           [0004]    Similarly the use of surface plasmons in controlling photons by using a second dielectric layer has been described in U.S. Pat. No. 6,034,809 by Anemogiannis and is herein incorporated by reference. Anemogiannis teaches optical plasmon-wave attenuation and modulation structures for controlling the amount of coupling between a guided optical signal and a surface plasmon wave, but the second-layer required must be controlled by another device so as to cause a change in optical index of the second layer. Anemogiannis employs no condition in which the surface plasmons are associated with spectrometry or filtering without the presence of the controlling dielectric layer.  
           [0005]    In summary, prior art has the necessity of a special second layer and the lack of use of surface plasmons in the exponentially decaying electric field provided by the surface plasmons within one wavelength of the surface. Therefore, these comprise prior art utilizing different physical phenomena to actually act upon the photons than is described in the present invention. Further they do not provide a means of differentiating the signal so as to determine distance to the surface.  
           [0006]    The following background publications are herein incorporated by reference:  
           [0007]    1. E. Kretschmann, E., Rather, H., Z.  Naturforsch,  216, 398-410, (1968).  
           [0008]    2. Otto, A., Z.  Physik,  216, 398-410, (1968).  
           [0009]    3. Welford, K. R., et al., “Coupled Surface plasmons in a Symmetric System,”  Journal of Modern Optics , Vol. 35, No. 9, Pp.1467-1483, 1988.  
           [0010]    4. Hoyt, Clifford C., “Towards Higher Res, Lower Cost Quality Color and Multispectral Imaging,” Advanced Imaging, pp. 53-55, April 1995.  
           [0011]    5. Kajenski; “Tunable Optical Fiber Using Long-Range Surface Plasmons”;  Society of Photo - Optical Instrumentation Engineers ; Vol. 36, No.19 Pp. 1537-1541, May 1997.  
           [0012]    6. Wang, Yu, “Voltage-induced Color Selective Absorption with Surface Plasmons,”  Appl. Phys. Lett ., Vol. 67, No.19, Pp. 2759-2761, Nov. 6, 1995.  
           [0013]    7. Caldwell et al.; “Surface-Plasmon Spatial Light Modulators Based on Liquid Crystal”;  Applied Optics ; Vol. 31, No. 20; Pp. 3880-3891, Jul. 10, 1992;  
           [0014]    8. Jung et al.; “Integrated Optics Waveguide Modulator Based on Surface Plasmon Resonance”;  Journal of Lightwave Technology ; Vol. 12, No. 10, October 1994; Pp. 1802-1806.  
           [0015]    9. Lozovik Y E; Merkulova S P; Nazarov M M; Shkurinov A P., “From two-beam surface plasmon interaction to femtosecond surface optics and spectroscopy”  Physics Letters A , Vol 276, Iss  1 - 4 , Pp. 127-132, Oct. 30, 2000.  
           [0016]    10. Challener W. A.; Edwards J. D., McGowan R. W., Skorjanec J., Yang Z., “A multilayer grating-based evanescent wave sensing technique”,  Sensors and Actuators B - Chemica , Vol 71, Iss1-2, Pp. 42-46, Nov. 15, 2000.  
           [0017]    11. Tessier G; Beauvillain P., “Non linear optics and magneto-optics in ultrathin metallic films”,  Applied Surface Science , Vol 164, Pp 175-185 Sep. 1, 2000.  
           [0018]    12. Tominaga et al,  Appl. Phys. Let , Vol. 78, No. 17, Pp. 2417-2419 Apr. 23, 2001.  
           [0019]    13. Ferrell et al., U.S. Pat. No. 5,018,865; issued May 28, 1991.  
         BRIEF SUMMARY OF THE INVENTION  
         [0020]    The tunneling of photons from a region of total-internal reflection is engendered by a probe or detector situated within one photon wavelength of the reflecting surface. The tunneling signal received is converted immediately thereafter to an electronic signal after being transmitted by an optical fiber or waveguide to a detector. This principle has long been used in phenomena of frustrated total internal reflection in the optical region of the spectrum in the case without a metal foil. The probe or detector is positioned with modern piezoelectric crystals to an accuracy of 0.0005 nm by using data on the ratio of the derivative of the signal with respect to wavelength to the derivative with respect to distance. If an unknown wavelength is introduced to the system, the ratio of the aforementioned derivatives can be thence remeasured to produce the value of the wavelength. If multiple wavelengths are introduced in combination, then a second photonic element is required in order to delineate the wavelengths and permit the above-mentioned measurements to obtain the intensity of each wavelength relative to the calibration wavelength. This additional element is a thin metal foil (typically 10-100 nm thick) placed on the reflecting surface. Each wavelength will induce surface plasmon quanta in the foil at an angle unique to that wavelength (Brewster&#39;s angle) with a tolerance of milliradians. The surface plasmon field is of the same functionality (exponential in the tunneling gap) as the evanescent field in the original case, but tunneling now occurs only for a single wavelength with a degree of broadening due to damping of surface plasmons of the combined waves. The angle of incidence is thereafter successively changed by minute amounts in order to permit successive wavelengths and provide a tunneling signal. In this way a complete spectrum of the incident photons is obtained by repeated measurement of the derivative ratio or by simultaneously doing so with multiple probes or an array of detectors. A probe is produced by etching silicon dioxide on a silicon wafer or in the core of an optical fiber. Simultaneous measurement at many wavelengths can alternatively be used if the incident photons are dispersed in wavelength to any given degree if an array of charged-coupled devices or other solid-state devices are formed into a compact two-dimensional array and placed in the near zone of the foil. The resulting probe or detector is driven to resonance using similar electronics to that used for other micromechanical systems (e.g., micro-cantilevers, membranes, etc.). By tracking the bending and resonance behavior of the probe using the tunneling signal, the probe simultaneously functions as a sensor in a large variety of sensing applications. Therefore, the spectrum of a targeted sample is simultaneously obtained while sensing other properties of the sample. The sample may be an element or compound in fluid form, a biological material, or it may simply be desired to measure a physical property of the environment during the process of acquisition of a spectrum. Spectroscopies enabled on silicon by this device include all of the ultraviolet, visible, and infrared spectroscopies, Raman spectroscopy, and photometry. A special advantage is the high stray-light rejection factor provided by the metal foil for wavelengths not incident at the Brewster angle.  
           [0021]    One embodiment of the invention is a microoptoelectromechanical integrated spectrometer with a photonic element assembly having metal foil removably disposed on a first transparent substrate surface, the substrate having no foil on any other surface. A means is provided for directing source photons that are reflected from or transmitted through a sample, over a range of angles of incidence, into the transparent substrate and onto the metal foil such that source photons are incident at the Brewsters angle. A means is also provided for detecting an induced exponential field in the metal foil. A means is also provided for relating the induced exponential field to a known exponential field for the sample and determining the identity of the sample. The spectrometer performs ultraviolet-to-visible-to-infrared spectroscopy using photon tunneling and surface plasmon excitation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a schematic of the major components of the invention.  
         [0023]    [0023]FIG. 2 is an exploded view of the photonic element assembly.  
         [0024]    [0024]FIG. 3 shows the optics of the embodiment of the invention in which the entire spectrum is displayed on an array detector.  
         [0025]    [0025]FIG. 4 is the base component of the invention.  
         [0026]    [0026]FIG. 5 is a photon probe/detector head.  
         [0027]    [0027]FIG. 6 is a graph showing the surface plasmon excitation reflectance dips, or transmission peaks, versus wavelength. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    In the context of this specification, distribution means the temporal or spatial separation of a beam of photons so that the separated parts are each of a given range of wavelengths, this being performed by a device that collects, waveguides, detects, amplifies, or otherwise processes the photons. Surface plasmons are the quanta associated with collective electron motion comprising a longitudinal wave in a material medium. A signal can be spectral information consisting of the intensity of photons within a given range of wavelengths.  
         [0029]    Spectrometers are very widely used in science and medicine and industry. Any such spectrometer can be replaced by the instant invention so long as; 1) the sample to be analyzed spectroscopically can be obtained in size to fit the scale of the instant invention without untoward effects on the sample, 2) stray light might otherwise obscure the signal, and 3) provided high-resolution of the wavelengths can be sacrificed for stray-light rejection qualities. Additionally, the application must be one in which the sample can be physically obtained rather than as in remote sensing spectroscopy, except in cases in which sufficient return or transmitted photons can be collected and directed into the subject invention. It may be undesirable in remote sensing to gather a dispersed return signal, but the use of external photon sources, including natural sources, that are located so as to provide photon transmission through a sample and thence into the invention, is desirable for long-range spectroscopic analysis.  
         [0030]    This invention finds applications in certain cases wherein it is necessary to spectroscopically analyze, filter, or distribute photon signals. In particular it applies to photons transmitted through or reflected from a sample that is to be spectroscopically analyzed in a manner to minimize the presence of wavelengths that may obscure the signal or saturate the detector, or it is necessary to rapidly spatially separate wavelength content in a controllably varying manner within a small volume. The minimum time quality requires any processing or distribution elements to provide response times of the order of milliseconds.  
         [0031]    A variety of compounds may be coated onto the probe in order to provide a surface that is optically selective for targeted compounds reflecting or transmitting photons as a matter of sensing.  
         [0032]    A configuration for exciting surface plasmons with photons has been provided by Kretschmann (E. Kretschmann, E., Rather, H., Z.  Naturforsch,  216, 398-410, (1968)) and by Otto (Otto, A., Z. Physik, 216, 398-410, (1968)). In a preferred embodiment of the present invention the Kretschmann configuration is used. The configuration is extended so that a probe or detector is placed within the exponential field of the surface plasmons. In the Kretschmann configuration a beam of photons enters a transparent medium bounded by a flat, thin, metal foil. The polarization component of the beam that lies in the plane of incidence is the only effective component. The beam is directed at an angle of incidence  1  relative to the foil normal such that the energy and momentum of the photons matches that of the surface plasmons (Brewster&#39;s angle for media of complex index of refraction). The surface plasmon momentum is proportional to the surface plasmon wave vector K by Planck&#39;s constant divided by 2π. Here K is related to the vacuum wave vector k of the incident photons by K=nk sin β, where n is the index of refraction of the medium supporting the metal foil. This relation provides conservation of the lateral component of momentum. The frequency and thus the energy of the surface plasmon must equal that of the engendering photon in order to satisfy the conservation of energy. The degree of excitation is then dependent upon the complex index of refraction of the metal foil and is determined by the application of Maxwell&#39;s equations and the Cartesian boundary conditions.  
         [0033]    A diagram of a preferred embodiment of the present invention is shown in FIG. 1. In this embodiment surface plasmons are engendered by the source beam  18  of photons provided by reflection from or transmission through a sample  19 . The source beam  18  is rotated about the center of the surface of the metal foil  30  changing the incident angle of the emitted photons that are reflected from or transmitted through sample  19 . Photons enter the cylindrical lens  20  at the optional entrance element  24  and pass through cylindrical lens  20  to impact the bottom surface of metal foil  30  that is coated onto the microscope slide  28 . Photons reflected from the foil  30  exit the cylindrical lens  20  through the optional exit element  26  and impact a detector  22 . A second source of photons (not shown) that is for reference purposes and emits photons of a known intensity and wavelength may also be incident upon the metal foil  30  within the supporting medium.  
         [0034]    A detection device that is spatially arranged at the point of maximum intensity near the metal foil collects each separated energy peak at a given angle of incidence upon the foil. The detection device can be a photosensitive detector or an optical system or waveguide that collects the photons and directs the collected photons to a photosensitive detector. Further, the detection device may be a system that converts the photons to a different energy for purposes of improved propagation in waveguides or for purposes of enhancing the spectral resolution. The detection device in FIG. 1 is positioned in an evanescent near-field generated by the photons being totally reflected at the foil  30 . The sample  19  modulates the evanescent near-field and will manifest itself as spatial variations in the near-field intensity at a given height above the foil surface. A fiber optic probe  12  is introduced into the evanescent near-field such that photons will tunnel between the foil  30  and the probe  12 . The probe  12  is mounted to one end of a piezoelectric translator  32  so that the probe  12  may be scanned across the foil  30 . The translator  32  moves the probe  12  over the foil  30  in a standard raster scan. An end opposite to the probe end of the translator  32  is connected to a photomultiplier tube  34  which detects the photons received by the probe  12 . The tube  34  produces an output signal that is proportional to the number of photons received by the probe  12  and provides an electrical current signal proportional to the detected light intensity. This signal drives an electronic feedback circuit  16  which regulates the distance between the probe  12  and the foil  30 . The motion of the probe  12  is monitored and controlled by a computer  14 , which also serves to collect and process the information generated by the scan of the probe  12  over the foil  30 .  
         [0035]    [0035]FIG. 2 is an exploded view of a photonic element assembly showing a transparent plano-convex cylindrical lens  20 , an optional movable entrance element  24  to prevent focusing of entering photons, an optional movable exit element  26  to prevent defocusing of exiting photons, a slide  28  (such as a microscope slide) mated to the surface of the lens  20  using a photonic gel that matches the index of refraction of the slide  28  and lens  20 , a thin metal (typically aluminum, silver, or gold) foil  30  evaporated at nanoscale thickness by vacuum evaporation onto the surface of slide  28 , and a detector array  12 ′ or fiber optic probe  12  used to detect photons near the surface of foil  30 .  
         [0036]    [0036]FIG. 3 shows the optics of one embodiment of the invention in which the entire spectrum is displayed on an array detector  40  (schematically represented here as a screen) such as a charge coupled device. The plano-convex cylindrical lens in this case is replaced by a prism  43  having foil  30  on a surface. Photons enter the prism  41  and pass through a two dimensional cylindrical lens  42  to focus the photons incident on the foil  30  surface. The absence of light along the curve on the array detector  40  placed in the exponential field of the surface plasmons (distance exaggerated for clarity) is the surface plasmon dispersion relation and shows up clearly as a curve of varying darkness. A photon of given wavelength will excite the surface plasmon at only a single angle within the tolerance range of milliradians. This results in a dip in the foil reflectivity. The surface plasmon field thus engendered has an exponential decay with distance from the foil and is available on the opposite side of the foil from the incident light. Measurement of the intensity variation along the curve and derivatives thereof provide a measure of the intensity at each wavelength. Spectrometry can be carried out with only slight changes in incident angle.  
         [0037]    [0037]FIG. 4 shows the support base of the invention. The incident photons enter from the left into a hollow windlass  50  containing adjustable mirrors  52  at the windlass joints. The photons are directed to fall upon a plano-convex cylindrical lens  20  visible partially in the upper center of the interior. A transparent slide  28  having a nanoscale thickness foil  30  coating is removably disposed on the lens at the top center. A micrometer  54  changes the foil  30  position relative to the lens by moving the transparent slide  28  on the lens  20 . The slide  28  is made movable by having an optical gel layer between the slide  28  and the lens  20 . In this way the direction of incidence of the photons is altered only by the windlass rotation. The windlass rotation is motorized with an angular positioner  56  with a precision of one tenth of a degree. Other micrometers  58  adjust the overall position of the lens  20  in the orthogonal directions. A detector  22  detects any reflected photons.  
         [0038]    [0038]FIG. 5 shows a preferred embodiment of a photon probe/detector head  60  that is mounted during operation on the top of the base of FIG. 4 and which contains a piezoelectric translator for the probe/detector.  
         [0039]    [0039]FIG. 6 is a graph of the surface plasmon excitation reflectance dips (transmission peaks). These show that, if photons at each wavelength are set at the surface plasmon excitation angle in the Kretschmann configuration, then the transmitted photons (those not reflected) at each wavelength have a very small bandwidth permitting spectroscopic resolution on the nanometer scale. Thus, the intensity transmitted at each incident wavelength and corresponding angle of surface plasmon excitation is measured. The collection of measurements in a range of angles comprises the spectrum of the sample without the noise associated with stray light, said stray light being reflected to a very high degree by the metal foil. The spectral resolution may be further improved by using a spectrally sensitive detector with a calibrated wavelength response.  
         [0040]    Although a preferred embodiment is described above, it is understood that the invention is capable of numerous rearrangements, modifications and substitutions of parts without departing from the scope of the invention as defined by the appended claims.