Patent Publication Number: US-9903805-B2

Title: Cavity enhanced polarimeter and related methods

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This document is a continuation in part of earlier U.S. patent application Ser. No. 14/706,743, entitled “Cavity Enhanced Polarimeter and Related Methods” to T. Peter Rakitzis, which was filed May 7, 2015, now pending, which claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/990,634, entitled “Cavity Enhanced Polarimeter” to T. Peter Rakitzis, which was filed on May 8, 2014, the disclosures of each of which are hereby incorporated by this reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     This disclosure relates to a polarimeter. In particular, the polarimeter may be used to measure the chirality of molecular samples through the measurement of the optical rotation of plane polarized light or circular dichroism. 
     2. Background Art 
     Detecting and measuring chirality has applications in a diverse array of fields such as the pharmaceutical, cosmetics, and food industries as well as the more general fields of medicinal and biological chemistry, chemical synthesis and analysis of microsamples, proteomics, and ecology, surface catalysis, nucleation, and cell membrane reactions, to name a few. 
     Conventional polarimeters, which measure an angle of rotation caused by passing polarized light through an optically active material, work by passing light through a material sample and using typical configurations, the light makes only a single pass through the sample. While useful for the typical purposes of polarimeters, the structure which permits the single pass of light through the sample also limits the potential usefulness of the polarimeter. 
     Gas-phase chirality measurements, using a bowtie cavity and continuous-wave (cw) lasers and measurement, are described in the article by L. Bougas, et al.,  Cavity - Enhanced Parity - Nonconserving Optical Rotation in Metastable Xe and Hg , Physical Review Letters, 25 May 2012, vol. 108, 210801 (2012), the disclosure of which is incorporated herein by reference. The gas-phase measurements, however, do not address how to accomplish liquid-phase and interfacial determination of chirality, or how to measure chirality using a pulsed laser 
     The measurement of chirality of materials is most commonly measured optically using circular dichroism (CD), the preferential absorption of right compared to left circularly polarized light, or optical rotatory dispersion (ORD), the rotation of the plane of linearly polarized light as it is transmitted through a sample. Experimental signals from both CD and ORD are usually very small. For example, the CD asymmetry parameter g is typically of order 10 −4 , so that nearly optically thick chiral samples demonstrate differences in absorption between right and left circularly polarized light of order 10 −4 . The ORD angle is proportional to optical pathlength and the sample concentration. Typically, commercial polarimeters for ORD measurements use cells with pathlengths of 1-10 cm, so that the ORD angle can be large enough to be measured accurately which require a large sample sizes most likely on the order of many milliliters of a substance. The optical principles behind measuring CD and ORD have remained largely unchanged for many decades. 
     The measurement of CD or ORD in an evanescent wave at the total internal reflection (TIR) of a light beam at the surface of a prism can give experimental signals of order g (i.e. CD signal differences of about 10 −4 , or ORD angles of about 10 −4  rad), where g is the chiral asymmetry in the refractive index of right and left circularly polarized light given by n ± =n±g (where n +  and n −  are the refractive indices of a chiral medium for right and left circularly polarized light, respectively). Silverman and Lekner showed that close to the TIR critical angle, the chiral signals are enhanced. This is shown by the following equation (derived following the treatment of Lekner) for the optical rotation angle Θ EW  following TIR, from the placement of a chiral sample in the evanescent wave of the prism: 
     
       
         
           
             
               
                 
                   
                     Θ 
                     EW 
                   
                   ≅ 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         n 
                       
                       n 
                     
                     ⁢ 
                     
                       N 
                       
                         1 
                         - 
                         
                           N 
                           2 
                         
                       
                     
                     ⁢ 
                     
                       
                         cos 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         θ 
                       
                       
                         
                           
                             
                               sin 
                               2 
                             
                             ⁢ 
                             θ 
                           
                           - 
                           
                             N 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where θ is the incidence angle, Δn=(n + −n − ), n=(n + +n − )/2, n +  and n −  are the refractive indices of the chiral sample for left and right circularly polarized light, respectively, N=(n/n p ), and n p  is the refractive index of the prism. The Θ EW  increases sharply near critical angle (sin θ≈N), and also near index matching (N≈1). 
     In the last 15 years, Poirson and Vaccaro pioneered the use of optical cavities to increase the path length to measure ORD of chiral vapors, by introducing two quarter wave plates in and at either ends of a two-mirror cavity. The pair of quarter wave plates carries out two functions: 1) for each pass, the light polarization is reflected through the optical axis of the wave plate so that the optical rotation of the forward and backward light path sums instead of cancelling, and 2) the optical axes of the two waveplates are offset by angle α, so that the light polarization is rotated by 2α for each roundtrip pass. Vaccaro showed that if 2α&gt;&gt;η (where η is the linear birefringence in the cavity, which normally causes great problems in measuring the small optical rotation of the sample), then the negative effects of the linear birefringence become negligible. However, this cavity-based technique has only been applied to vapor samples. The main problem of these linear cavities is that the signal of the chiral sample needs to be compared to the signal of a null sample. As it is difficult to alternate reproducibly between the sample and the null, and significant time is needed to do so (typically many seconds), the background subtraction of the null signal does not typically allow the measurement of small signals (e.g. smaller than about 0.1 mdeg/pass). 
     Very recently, Bougas et al. described a bowtie cavity with counter-propagating laser beams and an intercavity magneto-optical crystal using an applied longitudinal magnetic field. This crystal caused an optical rotation of the polarization with the same advantages as that caused by the quarter wave plates described above. However, in addition, Bougas et al. showed that the sample optical rotation angle can be reversed in sign by inverting the sign of the magnetic field. This gives an experimental control for isolating small experimental signals, without needing to remove the sample and replace with a null sample. This magnetic field reversal can be used as an efficient and rapid subtraction procedure to isolate the small chiral signal from much larger backgrounds. Bougas et al. described an experimental setup that allowed measurements using continuous wave (cw) lasers and measurement methods, on low-loss gas-phase samples. 
     To reduce the length and complexity of the detailed description and to establish a current state of the art, Applicant hereby incorporates by reference in their entirety each reference listed in the numbered paragraphs below:
     M. P. Silverman, J. Badoz, “Large enhancement of chiral asymmetry in light reflection near critical angle” Optics communications 74, 129 (1989).   J. Lekner, “Optical properties of isotropic chiral media” Pure Appl. Opt. 5, 417 (1996).   J. Poirson, M. Vallet, F. Bretenaker, A. L. Floch, and J. Thepot, Anal. Chem. 70, 4636 (1998).   T. Müller, K. B. Wiberg, P. H. Vaccaro, J. Phys. Chem. A, 104, 5959 (2000).   J. L. Hall, J. Ye, and L.-S. Ma, Phys. Rev. A 62, 013815 (2000).   G. Bailly, R. Thon, and C. Robilliard, Rev. Sci. Instrum. 81, 033105 (2010).   V. M. Baev, T. Latz, P. E. Toschek, Appl. Phys. B 69, 171-202 (1999).   W. von Klitzing, R. Long, V. S. Ilchenko, J. Hare, V. Lefevre-Seguin, Optics Letters 26, 166-8 (2001).   

     SUMMARY 
     According to one aspect, a polarimeter for measuring chirality of a material includes an optical ring cavity comprising a plurality of reflective elements configured to promote bi-directional propagation of a laser beam within the cavity, and a laser-emitting device configured to introduce a first input laser beam and a second input laser beam into the ring cavity. The polarimeter further includes a Faraday rotator and a phase compensator, the Faraday rotator and the phase compensator configured to suppress a birefringent background as the first and second laser beams pass through the ring cavity, and an acousto-optic modulator configured to receive a first output laser beam from the optical ring cavity, shift the frequency of the first output laser beam by an offset frequency, and produce a first shifted laser beam. Furthermore, the polarimeter includes an optical recombination device configured to receive the first output laser beam, a second output laser beam from the cavity, and at least the first shifted laser beam to produce a first combined laser beam and a second combined laser beam, and a first detector and a second detector. The first detector is configured to receive the first combined laser beam and the second detector is configured to receive the second combined laser beam. 
     The plurality of reflective elements, Faraday rotator, and phase compensator are configured such that light from the first and second input laser beams passes through a chiral material located within the ring cavity a sufficient number of times for a measurement of optical rotary dispersion (ORD) and circular dichroism (CD) of light transmitted through the chiral material and output as the first and second output laser beams to be obtained at the first and second detectors. 
     Particular embodiments may comprise one or more of the following features. The first combined laser beam may comprise the first shifted beam and the second output beam, and the second combined laser beam may comprise the first shifted beam and the first output beam. The acousto-optic modulator may be further configured to receive the second output laser beam from the ring cavity, shift the frequency of the second output laser beam by the offset frequency, and/or produce a second shifted laser beam. The optical recombination device may be further configured to receive the second shifted laser beam. The first combined laser beam may comprise the first shifted beam and the second output beam, and the second combined laser beam may comprise the second shifted beam and the first output beam. The polarimeter may further comprise a prism having at least a light input and light output surface comprising an anti-reflective coating such that light from the first and second laser beams is totally internally reflected for at least one surface of the prism at which the chiral material is located within an evanescent wave at the at least one surface of the prism. The polarimeter may further comprise a flow cell comprising one or more anti-reflective coated windows within which the chiral material is located. The light from the first and second laser beams may pass through the chiral material in a range of 100 to 10,000 times. A magnetic field of the Faraday rotator may be reversed such that the ORD signal is reversed, so that an absolute measurement of ORD of light passing through the chiral material is obtainable without requiring a null sample. The polarimeter may further comprise a whispering gallery mode microresonator (WGMM) configured to pass an electric current therethrough to produce a Faraday effect, such that a measurement of ORD and CD is obtained when the chiral material is located at a surface of the WGMM. The laser-emitting device may be configured to emit a continuous wave laser beam. The plurality of reflective elements may comprise four reflective elements and the optical ring cavity may comprise a bowtie ring cavity. 
     Implementations of a method of measuring chirality of a material includes promoting bi-directional propagation of a laser beam within an optical ring cavity of a polarimeter comprised of a plurality of reflective elements, introducing a first input laser beam and a second input laser beam into the ring cavity using a laser emitting device, suppressing a birefringent background as the first and second laser beams pass through the ring cavity using a Faraday rotator and a phase compensator, 
     The method further includes shifting the frequency of the first output laser beam by an offset frequency using the acousto-optic modulator to produce a first shifted laser beam, combining the first output laser beam, a second output laser beam produced by the ring cavity, and at least the first shifted laser beam into a first combined laser beam and a second combined laser beam using an optical recombination device, and measuring the first combined beam using a first detector and the second combined beam using a second detector. The plurality of reflective elements, Faraday rotator, and phase compensator are configured such that light from the first and second input laser beams passes through a chiral material located within the ring cavity a sufficient number of times for a measurement of optical rotary dispersion (ORD) and circular dichroism (CD) of light transmitted through the chiral material and output as the first and second output laser beams to be obtained at the first and second detectors. 
     Particular aspects may comprise one or more of the following features. The method may further comprise receiving the second output laser beam at the acousto-optic modulator, and shifting the frequency of the second output laser beam by the offset frequency using the acousto-optic modulator to produce a second shifted laser beam. The second shifted beam may be combined with the first and second output beams and the first shifted beam using the optical recombination device to produce the first and second combined beams. The first combined laser beam may comprise the first shifted beam and the second output beam, and the second combined laser beam may comprise the second shifted beam and the first output beam. The method may further comprise totally internally reflecting light from the first and second laser beams for at least one surface of a prism having at least a light input and light output surface comprising an anti-reflective coating, the chiral material being located within an evanescent wave at the at least one surface of the prism for which the light is totally internally reflected. The method may further comprise obtaining an absolute measurement of ORD of light passing through the chiral material without requiring a null sample by reversing a magnetic field of the Faraday rotator. The method may also further comprise increasing a number of times the light from the first and second input laser beams passes through the chiral material using an intracavity gain medium within the cavity. The intracavity gain medium may amplify the light independent of a polarization state of the light. The laser-emitting device may emit a continuous wave laser beam. 
     Aspects and applications of the disclosure presented here are described below in the drawings and detailed description. Unless specifically noted, it is intended that the words and phrases in the specification and the claims be given their plain, ordinary, and accustomed meaning to those of ordinary skill in the applicable arts. The inventor is fully aware that he can be his own lexicographer if desired. The inventor expressly elects, as his own lexicographer, to use only the plain and ordinary meaning of terms in the specification and claims unless he clearly states otherwise and then further, expressly sets forth the “special” definition of that term and explains how it differs from the plain and ordinary meaning. Absent such clear statements of intent to apply a “special” definition, it is the inventor&#39;s intent and desire that the simple, plain and ordinary meaning to the terms be applied to the interpretation of the specification and claims. 
     The inventor is also aware of the normal precepts of English grammar. Thus, if a noun, term, or phrase is intended to be further characterized, specified, or narrowed in some way, then such noun, term, or phrase will expressly include additional adjectives, descriptive terms, or other modifiers in accordance with the normal precepts of English grammar. Absent the use of such adjectives, descriptive terms, or modifiers, it is the intent that such nouns, terms, or phrases be given their plain, and ordinary English meaning to those skilled in the applicable arts as set forth above. 
     Further, the inventor is fully informed of the standards and application of the special provisions of post-AIA 35 U.S.C. § 112(f). Thus, the use of the words “function,” “means” or “step” in the Description, Drawings, or Claims is not intended to somehow indicate a desire to invoke the special provisions of post-AIA 35 U.S.C. § 112(f), to define the invention. To the contrary, if the provisions of post-AIA 35 U.S.C. § 112(f) are sought to be invoked to define the claimed disclosure, the claims will specifically and expressly state the exact phrases “means for” or “step for”, and will also recite the word “function” (i.e., will state “means for performing the function of [insert function]”), without also reciting in such phrases any structure, material or act in support of the function. Thus, even when the claims recite a “means for performing the function of . . . ” or “step for performing the function of . . . ,” if the claims also recite any structure, material or acts in support of that means or step, or that perform the recited function, then it is the clear intention of the inventors not to invoke the provisions of post-AIA 35 U.S.C. § 112(f). Moreover, even if the provisions of post-AIA 35 U.S.C. § 112(f) are invoked to define the claimed disclosure, it is intended that the disclosure not be limited only to the specific structure, material or acts that are described in the preferred embodiments, but in addition, include any and all structures, materials or acts that perform the claimed function as described in alternative embodiments or forms of the invention, or that are well known present or later-developed, equivalent structures, material or acts for performing the claimed function. 
     The foregoing and other aspects, features, and advantages will be apparent to those artisans of ordinary skill in the art from the DESCRIPTION and DRAWINGS, and from the CLAIMS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations will hereinafter be described in conjunction with the appended drawings, where like designations denote like elements, and: 
         FIG. 1  is a block diagram of an exemplary configuration of a polarimeter comprising three reflective elements which introduces a chiral sample in an evanescent wave of a prism. 
         FIG. 2  is a block diagram of an exemplary configuration of a polarimeter comprising three reflective elements which introduces a chiral sample using a flow cell. 
         FIG. 3  is a block diagram of an exemplary configuration of a polarimeter comprising four reflective elements in a bowtie configuration which introduces a chiral sample in an evanescent wave of a prism. 
         FIG. 4  is a block diagram of an exemplary configuration of a polarimeter comprising four reflective elements in a bowtie configuration which introduces a chiral sample using a flow cell. 
         FIG. 5  is a block diagram of an exemplary configuration of a polarimeter comprising polarization beam splitters (PBS) and separate detection devices. 
         FIG. 6  is a block diagram of an exemplary configuration of part of a polarimeter comprising a continuous beam laser. 
         FIG. 7  is an exemplary mode structure of the ring cavity output of  FIG. 6 . 
         FIGS. 8A-8B  provide two embodiments of a top view of a whispering gallery mode microresonator (WGMM). 
         FIG. 9  provides a side view of a whispering gallery mode microresonator (WGMM) of either of the  FIG. 8A or 8B . 
         FIG. 10  is a block diagram of an exemplary configuration of a polarimeter comprising an intracavity gain medium. 
         FIGS. 11A-11B  provide graphical examples of normalized polarization-oscillation signals without and with the use of an intracavity gain medium, respectively. 
         FIG. 12  provides an exemplary diagram of detection of CD as known in the prior the prior art. 
         FIG. 13  provides an exemplary diagram of detection of ORD as known in the prior art. 
     
    
    
     DESCRIPTION 
     This disclosure, its aspects and implementations, are not limited to the specific polarimeters shown and described, or other system component examples, or methods disclosed herein. Many additional components, manufacturing and assembly procedures known in the art consistent with polarimeters are contemplated for use with particular implementations from this disclosure. Accordingly, for example, although particular implementations are disclosed, such implementations and implementing components may comprise any components, models, types, materials, versions, quantities, and/or the like as is known in the art for such systems and implementing components, consistent with the intended operation. 
     The word “exemplary,” “example” or various forms thereof are used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” or as an “example” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Furthermore, examples are provided solely for purposes of clarity and understanding and are not meant to limit or restrict the disclosed subject matter or relevant portions of this disclosure in any manner. It is to be appreciated that a myriad of additional or alternate examples of varying scope could have been presented, but have been omitted for purposes of brevity. 
     While this disclosure includes embodiments in many different forms, there is shown in the drawings and will herein be described in detail particular embodiments with the understanding that the present disclosure is to be considered as an exemplification of the principles of the disclosed methods and systems, and is not intended to limit the broad aspect of the disclosed concepts to the embodiments illustrated. 
     The following disclosure is directed to systems and methods relating to a polarimeter that utilizes cavity-enhancement techniques for the measurement of ORD and CD of chiral liquid and solid samples, both in bulk form and in the evanescent wave of prisms. These implementations allow the measurement of chirality of micro-volume and monolayer samples at the total internal reflection (TIR) surfaces of prisms. Such techniques may also be applied for measuring ORD and CD in chiral samples in a solution. 
     A ring optical cavity comprising at least three reflective elements allows bi-directional propagation of laser beams, which may be referred to as having clockwise (CW) and counter-clockwise (CCW) polarity. These two laser beams may be configured to enter and exit the cavity in different directions so that they can be controlled and probed separately. A magnetic field B may be applied to an intracavity magneto-optic window, such that a Faraday rotation θ F  is induced, with a sign determined by the sign of the B field. A chiral sample causes a rotation φ C . Subsequently, the cavity modes are split by frequencies 2ω CW  and 2ω CCW  (or, alternatively, linearly polarized light that is introduced into the cavity will rotate with frequency of ω CW  for the CW beam, and ω CCW  for the CCW beam). These frequencies given by:
 
ω CW (± B )=(±θ F +φ C ) c/L   (2)
 
ω CCW (± B )=(±θ F −φ C ) c/L   (3)
 
     where L is the round-trip length of the cavity, c is the speed of light, and the dependence on the (lab-frame) angles θ F  and φ C  and B field are shown explicitly. The frequencies ω CW  and ω CCW  can be measured by frequency metrology of cavity modes, polarimetry, or by direct measurement of the polarization rotation in the cavity ring down signals described here, and demonstrated recently by Sofikitis et al. “Evanescent-wave and ambient chiral sensing by signal-reversing cavity-ringdown polarimetry”  Nature  514, 76 (2014), the disclosure of which is incorporated herein by reference. 
     When a pulsed laser is used or a continuous (cw) laser is used to excite a cavity mode and is turned off quickly, the time-dependent experimental signals I CW  and I CCW  resemble an exponential decay with polarization oscillation, described by the following equations:
 
 I   CW   =I   0   [e   −t/τ     CW    cos 2 (ω CW   t +φ)+β]  (4)
 
 I   CCW   =I   0   [e   −t/τ     CCW    cos 2 (ω CCW   t +φ)+β]  (5)
 
     In equations (4-5), τ CW , τ CCW  and ω CW , ω CCW  are the decay times and the polarization beating frequencies, respectively, for the CW and CCW propagation directions, β is a parameter to account for experimental imperfections in the light polarization, φ is a parameter that accounts for error in the determination of the phase of ω (and is affected by the relative angle between the axes of the input and output polarizers), and I 0  is a normalization constant for the signals. 
     After the signals are analyzed to yield ω CW  and ω CCW  (as a function of the magnetic field polarity), the optical rotation angle φ C  can then be determined using Eqs (2-3), by performing the following two subtractions:
 
Δω(± B )=|ω CW (± B )|−|ω CCW (± B )|=±2φ C ( c/L )  (6)
 
Δω(+ B )−Δω(− B )=4φ C ( c/L )  (7)
 
     As shown in Eqs. (6-7), the optical rotation angle φ C  can be determined by subtracting ω CW  and ω CCW , for both positive and negative B fields. These two subtractions allow the measurement of the absolute value of φ C  without needing to remove the chiral material sample. 
     For the detection of CD, each output passes through a quarter-wave plate (QWP), followed by a polarizing beam splitter (PBS). The angle between the axis of the QWP and PBS is 45°. Subsequently, as shown in  FIG. 12 , detector one detects right circularly polarized light, with ring-down time τ R , and detector two detects right circularly polarized light, with ring-down time τ L . These ring-down times are given by: 
     
       
         
           
             
               
                 τ 
                 R 
               
               = 
               
                 
                   
                     
                       L 
                       / 
                       c 
                     
                     
                       1 
                       - 
                       
                         R 
                         n 
                       
                       + 
                       
                         
                           α 
                           R 
                         
                         ⁢ 
                         
                           l 
                           0 
                         
                       
                     
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   and 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     τ 
                     L 
                   
                 
                 = 
                 
                   
                     L 
                     / 
                     c 
                   
                   
                     1 
                     - 
                     
                       R 
                       n 
                     
                     + 
                     
                       
                         α 
                         L 
                       
                       ⁢ 
                       
                         l 
                         0 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
     where L is the cavity roundtrip length, R is the reflectivity of each of the n mirrors, α R  and α L  are absorption coefficients of the chiral sample per unit length for right and left circularly polarized light, respectively, and I 0  is the length of the chiral sample in one arm of the cavity. The CD signal Δα=α R −α L , is then given by: 
     
       
         
           
             Δα 
             = 
             
               
                 L 
                 ⁡ 
                 
                   ( 
                   
                     
                       τ 
                       R 
                     
                     - 
                     
                       τ 
                       L 
                     
                   
                   ) 
                 
               
               
                 c 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   
                     l 
                     0 
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         τ 
                         R 
                       
                       ⁢ 
                       
                         τ 
                         L 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
     The CD signal can be measured for CW, CCW, +B and −B: Δα(CW,+B), Δα(CW,−B), Δα(CCW,+B), and Δα(CCW,−B). Finally, the desired CD signal for the chiral sample Δα c  is given by the average of pairs of these signals, or of all four of these signals: 
     Δα c =[Δα(CW,+B)+Δα(CW,−B)]/2, or Δα c =[Δα(CCW,+B)+Δα(CCW,−B)]/2, or Δα c =[Δα(CW,+B)+Δα(CW,−B)+Δα(CCW,+B)+Δα(CCW,−B)]/4. These averages remove background CD signals that depend on the light propagation direction (CW or CCW), or the magnetic field polarity (+B or −B) such as magnetic circular dichroism. 
       FIG. 13  similarly provides a corresponding diagram showing a conventional ORD detection method. 
     While the optical cavity may comprise any appropriate number of reflective elements, such reflective elements comprising, for example, mirrors, reflectors, prisms, or any other components capable of reflecting light, in some implementations disclosed herein, particular advantages may be provided by the use of three, four, five, or any appropriate number of reflective elements within the optical cavity. 
       FIGS. 1-2  provide exemplary configurations of implementations of the disclosed polarimeter comprising three reflective elements and highlight two ways in which a sample of a chiral material may be introduced into the optical cavity.  FIG. 1  depicts an implementation of a polarimeter comprising three reflective elements. One or more laser-emitting devices  101  introduce first  102  and second  103  laser beams into the optical cavity. While any appropriate type of laser emitting device may be used, in some implementations, it is advantageous to utilize a pulse of continuous wave (cw) laser. As shown, this implementation introduces a sample of a chiral material  105  into the cavity by placing the sample  105  in an evanescent wave  106  of a side of a prism  120  at which total internal reflection occurs. The prism is configured such that at least the surfaces of the prism which receive an input of light reflected by one or more reflective elements  104  and provide an output of light from the prism are coated with an anti-reflective coating material that is associated with the specific index of refraction of the chiral material sample  105  which results in total internal reflection of light from the laser beams  102 ,  103  for at least one side of the prism, at which the chiral material sample  105  is placed in the resultant evanescent wave  106 . While  FIG. 1  depicts only two reflective elements  104 , prism  120  may be considered to comprise the third reflective element due to the manner in which it receives an input and provides an output of light. It should be understood that throughout the remainder of this disclosure, the term “reflective element” may be used to describe a mirror, prism, or any other type of reflective component, however, for clarity in describing some of the embodiments of this disclosure, prisms may be referred to and labeled separately in the figures when it is practical to point out that a specific reflective element in fact, comprises a prism. 
       FIG. 2  provides an example of an alternative configuration of a cavity enhanced polarimeter comprising three reflective elements  104 . As shown, rather than placing the chiral material sample in an evanescent wave of a total internally reflected prism, the chiral material sample may be introduced into the optical cavity using a flow cell  111 . While the flow cell  111  may comprise any appropriate shape and dimensions, the flow cell may be substantially cylindrical in shape and comprise an anti-reflective coating  112  on the sides of the flow cell which receive light input and provide light output of the laser light passing through the optical cavity. A sample of the chiral material  105  is located within the flow cell  111  between the anti-reflectively coated input and output sides of the flow cell  111 . 
       FIGS. 3-4  provide exemplary configurations of implementations of a cavity-enhanced polarimeter comprising four reflective elements  104 , with  FIG. 3  comprising additional an additional reflective element in the form of prism  120 .  FIG. 2  depicts a chiral sample introduced into the optical cavity in the evanescent wave of a totally internally reflected side of a prism in the manner of  FIG. 1  and  FIG. 4  utilizes the flow cell for introduction of the chiral material sample in the manner of  FIG. 2 . Each of these implementations is depicted in a bowtie format for the optical cavity, however, it should be noted that a rectangular ring configuration may also be used instead of a bowtie configuration. Regardless of the number of reflective elements present in the optical cavity, as long as three or more reflective elements are present in the disclosed configurations, light from the first and second laser beams makes many passes through the sample of chiral material, typically in the range of 100 to 10,000, but in some implementations, this range may be as broad as from 100 to 1,000,000 passes without utilizing an intracavity gain medium and as high as 100,000,000 passes when an intracavity gain medium is introduced, which enhances the differences in the polarization oscillation frequencies, ω CW  and ω CCW , detected in output signals  109 ,  110 . 
     The measurement of ORD and CD requires that linear birefringence in the cavity be minimized as otherwise, it will also be amplified by the cavity, and will suppress the ORD and CD signals. One solution is to make sure that the linear birefringence is naturally small, which can be achieved by ensuring that there is negligible linear birefringence at each optic. For example, if the angle of reflection at each mirror is very small (less than a few degrees), then the linear birefringence can be kept below 10 −4  rad per reflection (and as low as 10 −6  rad). A four-mirror bowtie ring cavity, as shown in  FIG. 4 , is consistent with these constraints. However, if the linear birefringence is large (e.g. from large incidence angles of a three-mirror cavity, or from birefringence of intracavity optics such as a prism) then a low-loss compensator  108  can be introduced into the cavity, which can be used to tune the linear birefringence η to be small, so that the condition 2θ F &gt;&gt;η can be satisfied. The use of an intra-cavity magneto-optic window such as a Faraday rotator  107  as discussed above also serves to reduce undesirable linear birefringence. 
       FIG. 5  provides an exemplary configuration of a cavity-enhanced polarimeter that utilizes a single pulsed laser beam (with pulse width shorter than L/c), or a continuous beam that is rapidly turned off or detuned from the cavity resonance (in time shorter than L/c), emitted from the laser emitting device  101  in conjunction with a polarizing beam splitter (PBS)  113  to split the incoming laser beam into CW and CCW beams. The resulting CW and CCW output ring-down signals from the cavity then pass through separate polarizers and reach separate detectors  114  for CD and ORD measurement and analysis. 
       FIG. 6  provides an exemplary configuration of part of a cavity-enhanced polarimeter that utilizes a continuous beam laser that is not pulsed. Specifically,  FIG. 6  provides an exemplary configuration for performing CD and ORD measurement and analysis on the two ring cavity output signals (e.g. first output laser beam  109 , second output laser beam  110 ) produced by any of the cavities contemplated herein utilizing a laser emitting device  101  that is continuous, such that the output lacks a ring-down signal. As shown, measurement and analysis is performed by recombining the CW (e.g. first output laser beam  109 ) and CCW (e.g. second output laser beam  110 ) output beams (with polarization oscillation frequencies ω CW   0  and ω CCW   0 , respectively). 
     First, the CW and CCW beams pass through an acousto-optic modulator  122  (AOM), operating at frequency ω AOM  (e.g. offset frequency  138  of  FIG. 7 ), producing beams cw 1  (e.g. first shifted beam  124 ) having a frequency  140  of ω CW   1 =ω CW   0 +ω AOM , and ccw 1  (second shifted beam  126 ) having a frequency  142  of ω CCW   1 =ω CCW   0 +ω AOM . The mode structure of the cavity output after passing through the AOM  122  is shown in  FIG. 7 , including the frequencies of the unshifted and shifted beams cw 0  (frequency  134 ), ccw 0  (frequency  136 ), cw 1  (frequency  140 ), and ccw 1  (frequency  142 ). 
     Subsequently, the beams cw 1  (first shifted beam  124 ) and ccw 0  (second output beam  110 ) are recombined within an optical recombination device  128  forming a first combined beam  130  which is observed at first detector  114   a , and produces a beating signal proportional to Sin(ω 1 t+φ 1 ). Similarly, beams cw 0  (first output beam  109 ) and ccw 1  (second shifted beam  126 ) are recombined into a second combined beam  132  which is observed at second detector  114   b , and produces a beating signal proportional to Sin(ω 2 t+φ 2 ). Frequencies ω 1  and ω 2  can each be measured with frequency counters, each measuring the signals from detector  1  and detector  2 , respectively. Also, the phase difference (φ 2 −φ 1 ) between signals Sin(ω 1 t+φ 1 ) and Sin(ω 2 t+φ 2 ) can be measured with a frequency mixer. For a laser linewidth much larger than the cavity linewidth (Δω laser &gt;&gt;Δω cavity ), the chiral cavity splitting Δω chiral  is given by Δω chiral =(ω 2 −ω 1 )/4. For a laser linewidth much smaller than the cavity linewidth (Δω laser &lt;&lt;Δω cavity ) the chiral, cavity splitting Δω chiral  is given by: Δω chiral =(φ 2 −φ 1 )(c/NL), where N is the average number of cavity passes which the light undergoes. 
     If only one output beam (e.g. first output beam  109 ) is shifted by the AOM  122  (for experimental convenience), then parts of beams cw 1  (first shifted beam  124 ) and ccw 0  (second output beam  110 ) are recombined and observed at detector  114   a , producing a beating signal proportional to Sin(ω 1 t+φ 1 ); similarly parts of beams cw 0  (first output beam  109 ) and cw 1  (first shifted beam  124 ) are recombined and observed at detector  114   b , producing a beating signal proportional to Sin(ω 2 t+φ 2 ). As a result, the chiral signals (ω 2 −ω 1 ) and (φ 2 −φ 1 ) are half the size, as: Δω chiral =(ω 2 −ω 1 )/2, and Δω chiral =2(φ 2 −φ 1 )(c/NL). 
     In any of the implementations described herein, measurements of absolute values of CD and ORD may be obtained without the need for removing the chiral material sample and obtaining a null sample as is required according to conventional methodology which relies on background subtraction. This may be accomplished by achieving a reversed chiral signal by inverting the magnetic field in the Faraday rotator, or by inverting the helicity of the input light (for the continuous wave case). This is advantageous because such an absolute measurement of the chiral signal allows measurements in situations in which a null measurement is impossible, such as for thin films on surfaces or open-air measurements, and also allows the cancelling of slow background drifts through rapid signal-reversing background subtractions. 
     In some implementations of cavity enhanced polarimeters, one or more whispering gallery mode microresonators (WGMMs) may be used as the Faraday material. WGMMs may be microspheres (typically of radius of 50-500 micrometers), toroidal, or bottle resonators, in which ring cavity propagation is supported, for both CW and CCW directions. WGMMs may be comprised of fused silica, polymethyl methacrylate (PMMA), or materials with high Verdet constants, such as terbium gallium garnet (TGG) or any other appropriate materials. As shown in  FIGS. 8A, 8B and 9 , a magnetic field along the CW and CCW directions can be applied through a wire  116  that is perpendicular to the cavity plane having an electric current that passes through the center of the microresonator microsphere  115  that produces a magnetic field that is parallel or anti-parallel to the direction of the light propagation. The linear birefringence of the microsphere can be compensated to zero using stress-induced birefringence (as described in W. von Klitzing, R. Long, V. S. Ilchenko, J. Hare, V. Lefevre-Seguin, Optics Letters 26, 166-8 (2001)). CW and CCW cavity modes can be excited through CW and CCW fibers  117 ,  118  that are brought close to the microsphere  115 .  FIG. 8A  illustrates a side view of a first direction of light input, and  FIG. 8B  illustrates a side view of a second direction of light input.  FIG. 9  illustrates a top view of either  FIG. 8A or 8B . While it may appear that fibers  117 ,  118  are in contact with the microsphere  115 , fibers  117 ,  118  are not in direct contact with microsphere  115 , but rather, an evanescent wave couples light between fibers  117 ,  118  and the microsphere  115 . Finally, measurement of ORD and CD at the output of the fibers  117 ,  118  allow measurement of chirality of samples near the microsphere surface. Chirality is measured as differences in ORD(CW) and ORD(CCW). As WGMMs have already been used to measure the presence of single molecules attached to the WGMMs, using the ORD and CD measurements proposed here may reach the ultimate sensitivity limit of measuring the chirality of single molecules. 
     As discussed above, the sensitivity enhancement of these cavity-enhanced polarimetry system and methods is given by the average number of cavity passes as limited by the single pass cavity losses. As shown in  FIG. 10 , to improve this sensitivity even further, an intracavity gain medium  119  may be introduced that amplifies the light independent of the polarization state of the light which may result in few or even no net cavity losses. While the intracavity gain medium  119  is depicted in the particular systematic arrangement of  FIG. 10 , it is intended that such an intacavity gain medium  119  is applicable in any and all embodiments of cavity enhanced polarimeters described in this disclosure. Using such methods, effective absorption path lengths of 70,000 km may be achieved. Therefore, introducing intracavity gain media  119  to implementations of the cavity-enhanced polarimeters disclosed herein to amplify the chiral signal in a polarization-independent manner may increase the average number of cavity passes to more than 10 7 . Examples of possible intracavity gain media may include, but are not limited to, laser dyes, laser crystals (e.g. titanium sapphire, Nd:YAG), diode lasers, and gaseous gain media (e.g. He—Ne). A graphical depiction of the normalized chiral signal amplitude without the use of intracavity gain medium and with such an intracavity gain medium are provided in  FIGS. 11A-11B , respectively. 
     Through use of a polarimeter configured in accordance with any of the implementations described above, additional surface measurements may be performed that could not previously be made. Such implementation allow for the surface chirality of microscopic samples to be measured, which was not previously possible. 
     Where the above examples, embodiments and implementations reference examples, it should be understood by those of ordinary skill in the art that other protective gear and manufacturing devices and examples could be intermixed with or substituted for those provided. In places where the description above refers to particular embodiments of protective gear and customization methods, it should be readily apparent that a number of modifications may be made without departing from the spirit thereof and that these embodiments and implementations may be applied to other to protective gear customization technologies as well. Accordingly, the disclosed subject matter is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the disclosure and the knowledge of one of ordinary skill in the art. 
     In places where the description above refers to particular implementations of telecommunication systems and techniques for transmitting data across a telecommunication channel, it should be readily apparent that a number of modifications may be made without departing from the spirit thereof and that these implementations may be applied to other to telecommunication systems and techniques for transmitting data across a telecommunication channel.