Abstract:
A circular dichroism spectrometer which comprises a metasurface. The metasurface has a plurality of anisotropic antennas configured to simultaneously spatially separate LCP and RCP spectral components from an incoming light beam. An optical detector array is included which detects the LCP and RCP spectral components. A transparent medium is situated between the metasurface and the optical detector array.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present patent application is related to and claims the priority benefit of U.S. Provisional Patent Application Ser. No. 62/188727, filed Jul. 5, 2015, the contents of which is hereby incorporated by reference in its entirety into the present disclosure. 
     
    
     STATEMENT REGARDING GOVERNMENT FUNDING 
       [0002]    This invention was made with government support under W911NF-13-1-0226 awarded by the Army Research Office, FA9550-14-1-0389 awarded by the Air Force Office of Scientific Research; and 3002095871 awarded by the National Science Foundation. The government has certain rights in the invention 
     
    
     TECHNICAL FIELD 
       [0003]    The present disclosure relates to nanophotonic devices, and more specifically, production of a circular dichroism spectrometer using nanophotonic techniques. 
       BACKGROUND 
       [0004]    Circular Dichroism (CD) spectrometry is a very important tool in sensing chiral molecular structures which don&#39;t superimpose onto their mirror image. Chiral structures are very recurrent in biological media and organic compounds. Therefore, CD spectrometers find many applications in areas including, but not limited to, biological sensing, stereochemistry, crystallography and DNA structural analysis. 
         [0005]    CD spectrometers measure the spectrum of differential absorption between left circularly polarized (LCP) light and right circularly polarized light (RCP). Conventional prior art CD spectrometers measure LCP and RCP spectra sequentially. The laser sources are tuned to generate LCP across the wavelength range and measure the absorption, and then, the source is switched to RCP and the process is repeated. The process is time consuming, and involves much complicated hardware to switch the polarization of the laser, thereby increasing the dimensions of the device. 
         [0006]    Polarization gratings have been proposed to split LCP and RCP spatially. The gratings can be used to obtain real-time concurrent measurement of LCP and RCP spectra. This can eliminate the need to use complicated switchable sources. However, these polarization gratings require large thickness to accumulate optical phase delays between the major and minor axes of polarization gratings. Therefore, improvements are needed in the field. 
       SUMMARY 
       [0007]    The present disclosure applies optical metasurface technology to perform spatial separation of LCP and RCP spectra using a single deeply subwavelength metasurface layer. The disclosed system can generate strong phase accumulation within a layer about 100 nm thick which can be used to reflect and\or transmit LCP and RCP efficiently in different sides at a wavelength dependent angle. As a result, the entire CD spectrometer device may be sub-millimeter in dimensions, including spatial separation of LCP and RCP spectra and collecting their measurements at a charge-couple device (CCD) array. 
         [0008]    According to one embodiment, a miniature real-time CD spectrometer is disclosed which separates LCP and RCP spectra in space. A metasurface layer is used to efficiently split LCP and RCP spectra, and then, a CCD array is used to collect these spectra. 
         [0009]    Separation of LCP and RCP spectra is performed either in transmission or reflection mode. For transmission mode, a dielectric metasurface is used, while for reflection mode, a plasmonic metasurface is used. 
         [0010]    The metasurface and CCD layers are separated with a polymer or any other transparent medium. The whole spectrometer is sub-millimeter in 3D dimensions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein: 
           [0012]      FIG. 1  shows a conceptual illustration of a CD spectrometer implemented using a reflective metasurface according to one embodiment. 
           [0013]      FIG. 2  shows a conceptual illustration of a CD spectrometer implemented using a transmission metasurface according to one embodiment. 
           [0014]      FIGS. 3(A) and 3(B)  show the structure of the reflective metasurface of  FIG. 1  from a cross-sectional view and top view, respectively, according to one embodiment. 
           [0015]      FIG. 4  shows a 3D schematic of the reflecting metasurface structure showing the direction of the light beam and the reflected LCP and RCP spectral light components according to one embodiment. 
           [0016]      FIGS. 5(A) and 5(B)  show the structure of the transmission metasurface of  FIG. 2  from a cross-sectional view and a top view, respectively, according to one embodiment. 
           [0017]      FIG. 6  shows an image of a fabricated reflecting metasurface according to one embodiment. 
           [0018]      FIG. 7(A)  shows a schematic of a system  700  used to test the fabricated sample in  FIG. 5(A) and 5(B) . 
           [0019]      FIG. 7(B)  shows experimental results of reflected power for LCP and RCP incident beams at different wavelengths as a function of reflected angle showing discrimination of LCP and RCP spectra according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The presently disclosed CD spectrometer is configured to be submillimeter in dimensions.  FIGS. 1 and 2  show conceptual schematics of the whole CD spectrometer, where spectral components are being separated using reflection and transmission metasurfaces respectively. A key component of the spectrometer is the beam splitting metasurface. The metasurface spatially separates LCP and RCP spectra of the incident light. The incident light beam is of typical beam diameter ˜50-500 μm. After the beam falls onto the reflection (or transmission) metasurface, LCP and RCP components are reflected (or transmitted) into opposite directions. In addition, each wavelength is reflected (or transmitted) at a different angle, thereby resolving the individual spectral components of the beam. As a result, the LCP and RCP components are completely separated in space. 
         [0021]    The LCP and RCP spectral components are then projected onto an array of charged coupled detectors (CCD). A distance ˜0.5-1 mm is required between the metasurface and CCD array layers for adequate separation of spectral components of light. For mechanical stability, this distance is filled with a transparent dielectric polymer or any other transparent material. 
         [0022]      FIG. 1  shows a conceptual illustration of a CD spectrometer  100  implemented using a reflective metasurface  104  according to one embodiment. Each side of the spectrometer  100  is ˜0.5-1 mm in length. A broadband light beam  110  enters the spectrometer through a hole  112  having a width of ˜50-500 μm. The light goes through a transparent bulk medium  114 , hitting the reflective metasurface  104 . The metasurface  104  separates the spectral components of LCP and RCP as shown, and a CCD array  120  is used to detect the data from these separated components. 
         [0023]      FIG. 2  shows a conceptual illustration of a CD spectrometer  200  implemented using a transmission metasurface  204 . Each side of the spectrometer  200  is ˜0.5-1 mm in length. A broadband light beam  210  enters the spectrometer  200  through a hole  212  having a width of ˜50-400 μm and hits the transmission metasurface  204 . The metasurface  204  separates the transmitted spectral components of LCP and RCP which go through a transparent bulk medium  214 , and a CCD array  220  is used to detect the data from these separated components. 
         [0024]    The most important part of the CD spectrometer is the beam splitting metasurface. 
         [0025]    The detailed structure of the reflecting metasurface is shown in  FIGS. 3 and 4  and that of the transmission metasurface in  FIG. 5 . 
         [0026]    The reflecting metasurface  104  as shown in  FIGS. 3 and 4  comprises three layers: (1) a backward reflecting metal layer  308 ; (2) an intermediate dielectric spacer layer  310 ; (3) an antenna array layer  311  comprising an array of metallic nano-antennas  312 . The reflecting metal layer  308  may be implemented using any metal, including but not limited to gold, silver, copper, aluminum, titanium nitride, and zirconium nitride. The thickness of the layer  308  can be in the range of 10 nanometers up to a 500 nanometers. The layer  308  can be grown using any chemical vapor deposition (CVD) or physical vapor deposition (PVD) technologies known in the art. The spacer layer  310  may be composed of a dielectric material, including but not limited to silica, alumina, and PMMA, and is in the range of 10-100 nanometers in thickness. Spacer layer  310  can also be fabricated using any chemical vapor deposition (CVD) or physical vapor deposition (PVD) technologies known in the art. In certain embodiments, the antenna array layer  311  may comprise any metal, including but not limited to gold, silver, copper, aluminum, titanium nitride, zirconium nitride. In other embodiments, the antenna array layer  311  may comprise a plasmonic ceramic or transparent conducting oxide, including but not limited to aluminum doped zinc oxide (AZO) and gallium doped zinc oxide (GZO). The antenna array layer may be in the range of 10 to 100 nanometers in thickness. The antennas  312  may be fabricated, for example, using electron beam lithography or photo-lithography. The antennas  312  are preferably plasmonic. Each antenna  312  is preferably anisotropic in shape (e.g., rectangular), and it is preferable that the reflection coefficient across the major and minor axis be out of phase. This is achieved through adjusting the lateral dimensions of the antennas  312 . The dimension selection depends on the wavelength of interest and the materials used. For near infra-red applications, dimensions in the order of 100-300 nm are suitable. Going towards shorter wavelengths, it is suitable to use silver nano-antennas of lateral dimensions in the order of tens of nanometers (e.g., 10-100 nanometers), and if aluminum is used, it is possible to go to ultraviolet wavelengths. Higher wavelengths (mid and far infra-red) can also be achieved by increasing the dimensions of the nano-antenna to few micrometers. 
         [0027]      FIG. 3(A)  shows a cross-sectional view of the metasurface  104  with a conceptual illustration of beam splitting of LCP and RCP components in different directions at a wavelength dependent angle λ.  FIG. 3(B)  shows a top view of the metallic nano-antenna array  311  portion of the metasurface  104 .  FIG. 4  shows a 3D schematic of the reflecting metasurface  104 . The array  311  comprises a periodic structure of nano-antennas  312  (a period P of 4 antennas is shown in the illustrated embodiment). For each period P, the major axes of the antennas are oriented at different angles with respect to each other to span 180 degree orientation across the period. For the case of a 4 antenna period, the orientation angles of the axes are 0, 45°, 90° and 135°. Across each period, the nano-antennas  312  form a phase distribution from 0 to 2π (−2π) for reflected LCP (RCP) beam. This causes the reflected beam not to be normally reflected, but instead, reflected at an angle θ r  defined sin θ r =λ/P for LCP and sinθ r =−λ/P for RCP, where λ is the wavelength and P is the period. This explains the spatial separation of LCP and RCP and wavelength dependence of the reflection angle. The period P must be designed to be larger than any wavelength λ, in our operating band because the equation sin θ r =λ/P implies that λ/P&lt;1. 
         [0028]      FIG. 5(A)  shows the schematics of the transmission metasurface  204 . It comprises an array of high-index nano-antennas  412  embedded inside a low-index medium  410 . The antennas  412  are preferably plasmonic. The metasurface  204  may be fabricated layer by layer, where a layer of low-index medium (e.g., silica, alumina, or PMMA) is deposited using any of the chemical vapor deposition (CVD) or physical vapor deposition (PVD) technologies. This can be in the range of 10-1000 nanometers in thickness. Then the array of high-index dielectric (e.g., silicon, germanium) is patterned using electron beam lithography or photo-lithography. The top view of the pattern is shown in  FIG. 5(B) . Then low-index dielectric is deposited again to fill the space between the nano-antennas  412  and to build some thickness on top of the array (e.g. in the range of 10-1000 nanometers). The thickness of the nano-antennas  412  is in the range of 100-1000 nanometers, and so are the lateral dimensions. The nano-antennas  412  are oriented the same way as the reflecting metasurface  104  metallic array. Similarly, the high-index dielectric nano-antennas  412  are anisotropic and their dimensions are adjusted such that the transmissions along the major and minor axes are out of phase. By similar analysis to that of the reflecting metasurface  104 , the transmission angle θ t  follows the formula sin θ t =λ/P for LCP and sin θ t =−λ/P for RCP, where λ is the wavelength and P is the period. This explains the spatial separation of LCP and RCP and wavelength dependence of the transmission angle. 
         [0029]    The incident light beam applied to the reflection metasurface  104  or transmission metasurface  204  need not be generated from a laser source. Any non-coherent source such as a lamp, a light emitting diode (LED), or a Xenon lamp maybe be used which has equal components of LCP and RCP which may be spatially separated by the metasurface  104  or  204 . 
         [0030]    As a proof of feasibility, a reflecting metasurface was fabricated as shown in  FIG. 6 , with a bottom 50-nm gold layer, on top of which a 50-nm alumina layer, and the outermost layer of a rectangular antenna array of gold of 30-nm thickness and lateral dimensions of 230 nm×280 nm. The separation between antennas is 450 nm. The bottom gold and alumina layers are grown using electron beam deposition (one type of PVD), and then the nano-antenna arrays are fabricated using electron beam lithography.  FIG. 6  shows a top FE SEM image of the fabricated metasurface. 
         [0031]      FIG. 7(A)  shows a schematic of a system  700  used to test the fabricated sample in 
         [0032]      FIG. 5(A) and 5(B) . The system  700  comprises a tunable monochromatic source  702 , a polarizer  704 , and a retarder  706  which are utilized to obtain circularly polarized incident beams  710  for different wavelengths. Measurements are taken using a rotating arm device (e.g., with an optional analyzer  713 ) which allows rotation of a detector  712  to collect the reflected beam  714  as a function of reflection angle θ r .  FIG. 7(B)  shows experimental results of reflected power for LCP and RCP incident beams at different wavelengths as a function of reflected angle showing discrimination of LCP and RCP spectra. LCP is reflected at the right side, and RCP is reflected at the left side. Wavelengths varying in the range 1.2-1.7 μm are reflected at different angles (40°-70°) with RCP reflection angles being mirror images of LCP reflection angles. Power efficiency of up to 40% is demonstrated. Efficient discrimination of LCP and RCP spectra is verified. 
         [0033]    Various aspects described herein may be embodied as systems or methods. Accordingly, various aspects herein may take the form of an entirely hardware aspect, an entirely software aspect (including firmware, resident software, micro-code, etc.), or an aspect combining software and hardware aspects These aspects can all generally be referred to herein as a “service,” “circuit,” “circuitry,” “module,” or “system.” 
         [0034]    Furthermore, various aspects herein may be embodied as computer program products including computer readable program code stored on a tangible non-transitory computer readable medium. Such a medium can be manufactured as is conventional for such articles, e.g., by pressing a CD-ROM. The program code includes computer program instructions that can be loaded into the processor (and possibly also other processors), to cause functions, acts, or operational steps of various aspects herein to be performed by the processor. Computer program code for carrying out operations for various aspects described herein may be written in any combination of one or more programming language(s). 
         [0035]    The invention is inclusive of combinations of the aspects described herein. 
         [0036]    References to “a particular aspect” or “embodiment” and the like refer to features that are present in at least one aspect of the invention. Separate references to “an aspect” (or “embodiment”) or “particular aspects” or the like do not necessarily refer to the same aspect or aspects; however, such aspects are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to “method” or “methods” and the like is not limiting. The word “or” is used in this disclosure in a non-exclusive sense, unless otherwise explicitly noted. 
         [0037]    The invention has been described in detail with particular reference to certain preferred aspects thereof, but it will be understood that variations, combinations, and modifications can be effected by a person of ordinary skill in the art within the spirit and scope of the invention.