Patent Publication Number: US-2023152211-A1

Title: System for Optically Analyzing a Test Sample and Method Therefor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This case claims priority of U.S. Provisional Pat. Application Serial No. 63/278,162, filed Nov. 11, 2021 (Attorney Docket: CIT-8737-P), which is incorporated herein by reference. If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to chemical analysis and, more particularly, to spectroscopy methods, systems, and sample holders suitable for use in optical analysis systems. 
     BACKGROUND OF THE INVENTION 
     Optical analysis methods, such as spectroscopy, and the like, are important analytical tools for identifying analytes contained within a test sample. In spectroscopy, a characteristic pattern of light-absorption peaks is detected, where this pattern is unique to the bonding structure of any chemical. The pattern of absorption peaks for a chemical, therefore, can function as a spectral “fingerprint” for that chemical. 
     The mid-infrared (MIR) spectral range (defined herein as the wavelengths within the range of approximately 2 microns to approximately 15 microns) represents a particularly information-rich spectral region because of the wealth of absorption peaks that exist within it for most chemicals. The MIR spectral range, therefore, is an attractive operating range for infrared spectroscopy. 
     Typically, during any optical analysis approach, the test sample is held in a sample holder. Ideally, such a sample holder is “optically efficient” in that it does not significantly affect the optical characteristics imparted by the sample, such as by absorbing wavelengths within the spectral range of the light signal used to interrogate the sample (i.e., the interrogation signal). Unfortunately, sample holders available in the prior art are quite expensive and not particularly optically efficient - particularly those suitable for use in the MIR spectral range. As a result, a lack of suitable sample holders has limited the use of spectroscopy and other optical analysis methods in many applications. 
     An optical analysis system having a low-cost, optically efficient sample holder would be a significant advance in the state of the art. 
     SUMMARY OF THE INVENTION 
     An advance in the art is made according to aspects of the present disclosure, which describes systems and apparatus for performing optical analysis of a test sample collected on an open sample-collection surface of a sample holder. The sample-collection surface is an exposed structure that is configured to increase the surface area of a planar surface while simultaneously enabling direct optical access to the test sample, thereby facilitating high-performance measurement of its characteristics. Embodiments of the present invention are particularly well suited for use in optical analysis systems, such as spectroscopy systems, calorimetry systems, differential scanning calorimetry Fourier-transform infrared spectroscopy systems, and the like. 
     An illustrative embodiment in accordance with the present disclosure is an optical analysis system operative for performing mid-infrared spectroscopy on a test sample. The system includes a source of an interrogation signal, which is directed toward a sample holder having an outer front surface that is configured as a sample-collection surface. The sample-collection surface includes a planar surface and a plurality of features that project from the planar surface, thereby increasing its surface area while enabling the interrogation signal to reach the test sample in an unimpeded manner. Each of the plurality of features has a base, sidewall, and tip, where the planar surface, the plurality of sidewalls, and the plurality of tips collectively define the sample-collection surface. 
     The illustrative sample-holder is reflective such that energy of the interrogation signal passes through the sample twice. In some such embodiments, the sample-collection surface is reflective, while in other such embodiments, the back surface of the sample holder is reflective. In some embodiments, the sample-collection surface is disposed on a reflective coating. 
     In some embodiments, the features are shaped and/or arranged on the planar surface to collectively define a geometric anti-reflection layer that mitigates reflection of the interrogation signal at the sample-collection surface. 
     In some embodiments, the sample holder is transmissive such that the interrogation signal passes through the sample once and transits the sample holder, exiting as an output signal that includes information about the test sample. In such embodiments, the back surface of the sample holder is configured to mitigate reflection of the output signal. 
     In some embodiments, the features are macro-features that hold the test sample and/or improve its interaction with the interrogation signal. 
     An embodiment in accordance with the present disclosure is an apparatus that includes: a sample holder ( 106 ) comprising: a body ( 302 ) having a first surface ( 310 ) and a plurality of features ( 308 ) that project from the first surface, each feature having a sidewall ( 318 ) and tip ( 320 ); wherein the first surface, the plurality of sidewalls, and the plurality of tips collectively define a sample-collection (SC) surface ( 116 ) for locating a test sample ( 114 ). 
     Another embodiment in accordance with the present disclosure is a method comprising: providing a sample holder ( 106 ) comprising a body ( 302 ) having a first surface ( 310 ) and a plurality of features ( 308 ) that project from the first surface, each feature having a sidewall ( 318 ) and tip ( 320 ), wherein the first surface, the plurality of sidewalls, and the plurality of tips collectively define a sample-collection (SC) surface ( 116 ) for locating a test sample ( 114 ); and collecting the test sample on the sample collection surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    depicts a block diagram of an illustrative embodiment of a chemical analysis system in accordance with the present disclosure. 
         FIG.  2    depicts operations of a method for performing analysis of a test sample in accordance with the illustrative embodiment. 
         FIGS.  3 A-B  depict schematic drawings of perspective and sectional views of an exemplary reflective sample holder in accordance with the present disclosure. 
         FIG.  4 A  depicts a sectional view of a second example of a reflective sample holder in accordance with the present disclosure. 
         FIG.  4 B  depicts a sectional view of a third example of a reflective sample holder in accordance with the present disclosure. 
         FIG.  4 C  depicts a sectional view of a fourth example of a reflective sample holder in accordance with the present disclosure. 
         FIGS.  5 A-B  depict schematic drawings of side and top views of a portion of yet another example of a sample holder in accordance with the present disclosure. 
         FIG.  6    depicts a block diagram of another exemplary embodiment of an optical analysis system in accordance with the present disclosure. 
         FIG.  7    depicts a block diagram of yet another embodiment of a chemical analysis system in accordance with the present disclosure. 
         FIGS.  8 A-B  depict schematic drawings of perspective and sectional views of an exemplary transmissive sample holder in accordance with the present disclosure. 
         FIG.  9    depicts measured transmission for a transmissive sample holder in accordance with the present disclosure at different angles of incidence. 
         FIG.  10    depicts measured transmission through a test sample disposed on the sample-collection surface of a vertically oriented transmissive sample holder in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    depicts a block diagram of an illustrative embodiment of a chemical analysis system in accordance with the present disclosure. System  100  is a spectrometer that is operative for analyzing a test sample by determining the absorption characteristics of the test sample over the wavelength range from approximately 2.0 microns to approximately 15 microns (i.e., the mid-infrared spectral range). System  100  includes light source  102 , filter  104 , sample holder  106 , detection system  108 , processor  110  and associated optics, such as collimating and focusing lenses, etc. 
     In the depicted example, light source  102 , filter  104 , and detection system  108  (and their associated optics) collectively define optical engine OE1. 
     Optical engine OE1 is optionally contained within enclosure  122 , which is a conventional housing that is configured to protect the optical engine, provide a stable environment, protect it from dust and humidity, and the like. 
     Enclosure  122  includes window  124 , which enables optical engine OE1 to provide interrogation signal  122  to a test sample located outside the enclosure and receive output signal  118  from the test sample. In the depicted example, window  124  comprises germanium; however, it will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use a window  124  that includes a different suitable material. Examples of alternative window materials suitable for use in accordance with the present disclosure include, without limitation, silicon (Si), silicon compounds, zinc selenide (ZnSe), and the like. 
     In some embodiments, enclosure  122  and optical engine OE1 collectively define a portable spectrometer system. Typically, such a portable system also includes a power source, such as a battery, etc. 
     Although the illustrative embodiment is a spectrometer, the teachings here can be applied to a wide range of optical-analysis instruments, such as other spectrometers, calorimetry systems, and the like. 
       FIG.  2    depicts operations of a method for performing analysis of a test sample in accordance with the illustrative embodiment. Method  200  is described with continuing reference to  FIG.  1    as well as reference to  FIGS.  3 A-B . 
     Method  200  begins with operation  201 , wherein test sample  114  is provided such that it is disposed on sample-collection (SC) surface  116  of sample holder  106 . 
     Sample holder  106  is a sample-collection device that is configured to locate test sample  114  on SC surface  116 . In the depicted example, test sample  114  is blood; however, the teachings of the present disclosure can be directed to a wide range of test samples in fluid or fluid-like form (e.g., liquids, gasses, gels, creams, suspensions, mixtures, solutions, biological fluids and sera, etc.). Embodiments in accordance with the present disclosure are particularly well suited for use in the analysis of biological fluids and sera, chemical solutions and compounds, organic solutions, petroleum products, cosmetics, gasses, and the like. 
       FIGS.  3 A-B  depict schematic drawings of perspective and sectional views of an exemplary reflective sample holder in accordance with the present disclosure. Sample holder  106  includes body  302  and reflector  304 . 
     Body  302  is a monolithic, homogeneous structure comprising material M1 and includes substrate  306  and features  308 , which project from planar surface  310  of the substrate. In the depicted example, body  302  is formed by etching into a surface of substrate  306  to define features  308  and planar surface  310 , as described in U.S. Pat. Applications 16/212,499, filed Dec. 6, 2018 (Attorney Docket: 3105-004US1) and 16/212,347, filed Dec. 6, 2018 (Attorney Docket: 3105-005US1), each of which is incorporated herein by reference. 
     Although sample holder  106  includes a body that is formed by etching into a monolithic substrate, sample holders in accordance with the present disclosure can be formed, at least in part, using any of a wide range of known fabrication techniques such as conventional, precision molding techniques suitable for forming optical elements, MEMS fabrication methods (e.g., deep reactive-ion etching (DRIE), etc.), hybrid fabrication and assembly, and the like. In some embodiments, features are formed on a planar surface of a sample-holder body by depositing material on the surface via, for example, selective-area growth, self-assembly, the Langmuir-Blodgett method, and the like. 
     Material M1 can be any material suitable that is substantially non-reactive with test sample  114  and does not significantly absorb light at wavelengths within interrogation signal  112 . In the depicted example, material M1 is silicon; however, other materials suitable for use in accordance with the present disclosure include, without limitation, germanium, Ultem®, polycarbonate, thermoplastics, optical plastics, glasses, silicones, acrylics, and the like. 
     Sample holder  106  is configured to reflect the energy of interrogation signal  112  such that it passes through test sample  114  twice. In some embodiments, little or no energy of an interrogation signal passes through sample holder  106  and a wider range of materials can be used in body  302 , since the primary considerations are mechanical strength and non-reactivity with test sample  114 . In such embodiments, therefore, suitable materials include, without limitation, silicon, polysilicon, silicon compounds (e.g., silicon carbide, silicon germanium, etc.), compound semiconductors, silicon nitrides, oxynitrides, fluorides, ceramics, polymers, composite materials, and the like. 
     Furthermore, although in the depicted example body  302  is a monolithic, homogeneous structure, in some embodiments, body  302  includes layers and/or regions of different materials. 
     Each of features  308  includes a base  316 , sidewall  318 , and tip  320 . Each of bases  316  has diameter, d1, and is located at planar surface  310 , which defines plane P1, while each of tips  320  has diameter, d2, and is located in plane P2 at a height of h1 above planar surface  310 . Features  308  are distributed on planar surface  310  such planar surface  310 , sidewalls  318 , and tips  320  collectively define SC surface  116 . 
     Furthermore, features  308  are arranged in a two-dimensional arrangement in which they have inter-feature spacing, s1. For the purposes of this Specification, including the appended claims, the term “inter-feature spacing” is defined as the minimum center-to-center spacing between adjacent features of a sample-collection surface. 
     It is an aspect of the present disclosure that the shape of features  308  and their inter-feature spacing give rise to an ability to hold a liquid test sample, in an open structure, at sample-holder orientations from horizontal through vertical, and even when the sample holder is upside down. 
     In some embodiments, features  308  are arranged in a periodic fashion in at least one dimension. In some embodiments, features  308  are arranged periodically in two dimensions. In some embodiments, the features are arranged in a non-periodic arrangement. Feature arrangements suitable for use in accordance with the present disclosure include, without limitation, periodic in one or two dimensions, rectilinearly periodic, periodic in two dimensions with different inter-feature spacings, multiple regions having different periodicities, hexagonally close-packed, deterministically aperiodic, randomly periodic, or any other suitable arrangement. 
     In the depicted example, features  308  are configured to realize a layer having a refractive index that slowly increases in substantially adiabatic fashion from a relatively lower effective refractive index, n e , at plane P2 to the refractive index of the material of planar surface  310 . As a result, features  308  collectively define geometric anti-reflection (GAR) layer  312 , which functions as a graded-index layer that mitigates reflection of light at the wavelengths within interrogation signal  112 . 
     To facilitate anti-reflection characteristics of GAR layer  312 , inter-feature spacing s1 (also referred to as “spacing s1”) is preferably smaller than the shortest wavelength in interrogation signal  112 . It should be noted, however, that s1 can have any practical dimension without departing from the scope of the present disclosure. In some embodiments, the features are not on a regular spacing. In some embodiments, not all of tips  316  are coplanar in plane P2. Furthermore, in some embodiments, features  308  have different inter-feature spacings in different regions of sample-collection to enable sample holder  106  to concentrate particles, cells, etc., having different dimensions in different regions. 
     In the depicted example, each of features  308  is a frustum of a cone, h1 is approximately 8 microns, d1 is approximately 1.8 microns, and d2 is approximately 460 nm, and s1 is approximately 2 microns. It should be noted that a wide range of dimensions and spacings for features  308  can be used without departing from the scope of the present disclosure. It should be further noted that a frustum of a cone is merely one example of a suitable shape for features  308  within the scope of the present disclosure, as discussed in U.S. Pat. Application 16/212,499. 
     Reflector  304  is a conventional reflective element disposed on back surface  314  of substrate  306 . In some embodiments, reflector  304  is a conventional broadband reflective element that includes one or more layers of reflective material, such as metals, dielectric layers, metals and dielectric layers, and the like. In the depicted example, reflector  304  comprises a broadband reflector comprising a titanium adhesion layer and a thick, reflective layer of gold. In some embodiments, reflector  304  includes a spectral element, such as a band-rejection filter that reflects radiation in the wavelength range of interest while not reflecting wavelengths outside that range. In some embodiments, reflector  304  includes an edge filter. 
     As will be understood by one skilled in the art, after reading this Specification, a reflective sample holder advantageously mitigates back-reflections toward light source  102  since it receives interrogation signal  112  at a substantial angle (i.e., incidence angle θ). Furthermore, since light passes through the test sample twice, the absorption of characteristic wavelengths by the chemicals in test sample  114  is enhanced. In fact, as compared to a transmissive sample holder oriented such the interrogation signal is normal on SC surface  116 , the path of light through the test sample is more than doubled by a reflective sample holder since it is equal to twice the inverse of the cosine of angle θ S  inside the test sample, where angle θ s  is related to the incidence angle θ by Snell’s Law (not shown for clarity). 
     Returning now to method  200 , at operation  202 , interrogation signal  112  interacts with test sample  114  to produce output signal  118 . 
     In the depicted example, interrogation signal  112  is provided by light source  102  and filter  104 . 
     Light source  102  is a source, such as a thermal source, operative for providing broadband radiation that includes wavelengths that span the spectral range from approximately 2 microns to approximately 15 microns. In other words, light source  102  is a source of mid-infrared (MIR) radiation. Typically, the light from light source  102  is collimated (via a lens and/or reflector) en route to filter  104 . 
     It should be noted that, for convenience, the term “light” is used throughout this Specification to characterize any radiation on which the described systems operate. However, systems in accordance with this Specification can be configured for use at virtually any wavelengths throughout the electromagnetic spectrum. For example, in some embodiments, system are designed for operation at wavelengths within the ultraviolet spectral range, the visible spectral range, near-infrared spectral range, far-infrared spectral range, as well as at longer wavelengths, such as terahertz, millimeter, microwave, radiowave radiation, and the like. As a result, for the purposes of this Specification, including the appended claims, the term “light” is defined as any radiation within the electromagnetic spectrum. 
     Filter  104  is spectral filter that receives the broadband radiation from light source  102  and spatially disperses the wavelengths within it along at least one dimension. The spatially dispersed wavelengths are provided by filter  104  as interrogation signal  112 . In the depicted example, filter  104  is a hyperspectral filter, such as is described in U.S. Pat. Application 16/782,674, which is incorporated herein by reference. In some embodiments, filter  104  is a different spectral filter. Some non-limiting examples of spectral filters suitable for use in embodiments in accordance with the present disclosure are described in U.S. Pat. Applications 15/065,792 and 15/990,114, each of which is incorporated herein by reference. 
     Filter  104  is imaged onto detection system  108  by a focusing lens such that interrogation signal  112  passes through test sample  114  twice en route to the detection system. 
     It should be noted that test sample  114  and sample holder  106  can be located elsewhere in the optical path between light source  102  and detection system  108  without departing from the scope of the present disclosure. For example, in some embodiments, test sample  112  and sample holder  106  can be located:
     i. between the focusing lens and detection system  108 ; or   ii. between light source  102  and the collimator; or   iii. between the collimator and filter  104 ; or   iv. at other locations in system  100 .   

     At operation  203 , the absorption characteristics of test sample  114  are imprinted on interrogation signal  114  to produce output signal  118 . Specifically, during each pass of interrogation signal  112  through test sample  114 , each constituent chemical in the test sample absorbs some of the energy of the interrogation signal at wavelengths characteristic of that chemical. The resulting light signal, modified by the absorption characteristics of test sample  114 , defines output signal  118 . 
     At operation  204 , detection system  108  receives output signal  118  and generates spectral signal  120 . 
     In the depicted example, detection system  108  includes a conventional detector array that is configured with filter  104  such that each detector element in the detector array detects a different sub-range of wavelengths (i.e., wavelength signal) within output signal  118 . Some non-limiting examples of spectral filters and detector arrays suitable for use in accordance with the present disclosure are described in detail in U.S. Pat. No. 10,488,256, which is incorporated herein in its entirety. 
     At operation  205 , processor  110  receives spectral signal  120  from detection system  108 . Spectral signal  120  includes spectral content, such as absorption peaks, that is based on the interaction of interrogation signal  112  with the chemical constituents of test sample  114 . 
     At operation  206 , one or more potential chemical constituents in test sample  114  are identified based on the spectral signal  120 . Typically, these potential chemical constituents are identified by observing absorption peaks in spectral signal  120  and correlating these absorption peaks with standard absorption spectra for a library of known chemicals stored in a look-up table included in processor  110 . In some embodiments, processor  110  includes communications capability that enables it to access a remotely located database that includes such absorption spectra. 
     In some embodiments, to identify one or more potential chemical constituents in test sample  114 , processor  110  performs one or more sub-operations that include:
     i. modifying the raw data of spectral signal  120  to correct variations in the response of detection system  108 ; or   ii. subtracting one or more calibration spectral signals (e.g. spectra taken with no test sample  114 , taken with a different filter  104 , etc.) from spectral signal  120 ; or   iii. modifying spectral signal  120  to, for example, decrease noise, increase contrast, decrease contrast, and the like; or   iv. applying thresholding to spectral signal  120  to enhance detectability; or   v. applying artificial intelligence techniques to improve analysis of spectral signal  120 ; or   vi. any combination of i, ii, iii, iv, and v.   

     It should be noted that sample holder  106  is merely one example of a reflective sample holder suitable for use in optical analysis systems in accordance with the present disclosure. 
       FIG.  4 A  depicts a sectional view of a second example of a reflective sample holder in accordance with the present disclosure. Sample holder  400  is analogous to sample holder  106 ; however, in sample holder  400 , reflector  304  is immediately beneath features  308  and functions as surface  310 . The sectional view shown in  FIG.  4 A  is analogous to the sectional view of sample holder  106  taken along line a-a, as shown in  FIG.  3 B . 
     In the depicted example, body  402  includes features  308 , reflector  304 , and handle substrate  404 . 
     Reflector  304  is disposed on the top surface of handle substrate  404 . As will be apparent to one skilled in the art, after reading this Specification, handle substrate  404  can be any substrate having suitable mechanical strength and optical flatness. Preferably, reflector  304 , substrate  406  and features  308  are all non-reactive with test sample  114 . 
     In the depicted example, features  308  are formed by etching them from substrate  406 , which is provided such that it is disposed directly on reflector  304 . Suitable methods for providing substrate  406  such that it is disposed on reflector  304  include, without limitation, wafer bonding, hybrid assembly, mechanical application, and the like. In some embodiments, substrate  406  is a layer of material that is formed on reflector  304  by a conventional process such as evaporation, spin coating, sputter deposition, etc. 
     As will be appreciated by one skilled in the art, after reading this Specification, the energy of interrogation signal  112  must pass through the material of substrate  406  twice in sample holder  400  but does not pass through handle substrate  404 . Therefore, the material of the handle substrate does not need to be transmissive, nor does its back surface need to be polished to mitigate scatter. Additionally, since interrogation signal  112  is highly reflected from reflector  304 , features  308  may or may not define a GAR layer, or may have intermediate reflectivity, however a GAR layer is likely to substantially maximize the interaction of interrogation signal  112  with test sample  114 . 
     In addition to the methods described briefly above, myriad alternative fabrication methods can be used to produce sample holder  400 , such as:
     i. forming reflector  304  on handle substrate  404 , producing features  308  in substrate  406 , and subsequently joining substrate  406  and the reflector; or   ii. forming reflector  304  on handle substrate  404 , forming a material layer onto reflector  304 , and forming features  308  in or on the material layer; or   iii. forming features  308  in reflector  304 ; or   iv. any other suitable fabrication process.   

       FIG.  4 B  depicts a sectional view of a third example of a reflective sample holder in accordance with the present disclosure. Sample holder  408  is analogous to sample holders  106  and  400 ; however, sample holder  408  includes reflector  410 , which is disposed on features  308 . The sectional view shown in  FIG.  4 B  is analogous to the sectional view of sample holder  106  taken along line a-a, as shown in  FIG.  3 B . 
     Reflector  410  is analogous to reflector  304  and includes one or more metal layers and/or one or more dielectric layers. 
     Since transmission through features  308  is not necessary in the depicted example (as well as some other embodiments in accordance with the present disclosure), the spacing and dimensions of features  308  can have any suitable value that affords the ability for the features to hold test sample  114 . As a result, in some embodiments, features  308  are configured to alter the reflected light for one or more desired characteristics, such as reflectivity control, beam control, beam steering, beam focusing, spectral dispersion, polarization control, enhancing beam coupling into a waveguide or other structure, and the like. 
       FIG.  4 C  depicts a sectional view of a fourth example of a reflective sample holder in accordance with the present disclosure. Sample holder  412  is analogous to sample holder  400 ; however, in sample holder  412 , body  414  includes features  308 , a portion of substrate  406 , and handle substrate  404 , where features  308  are formed such that they do not extend through the entire thickness of substrate  406 . As a result, reflector  304  is in close proximity to features  308  beneath the unetched portion of substrate  406 . Preferably, the unetched portion of substrate  406  is very thin (e.g., much thinner than substrate  306 ). The sectional view shown in  FIG.  4 C  is analogous to the sectional view of sample holder  106  taken along line a-a, as shown in  FIG.  3 B . 
     For some embodiments in which a sample holder is configured for reflective-mode operation, the dimensions and arrangement of features  308  are based on one or more characteristics of test sample  114 , such as its viscosity, surface tension with respect to material M1, the desired thickness of the test sample when disposed on the sample holder, and the like. In such embodiments, features  308  are configured primarily based on their ability to hold the test sample in place and, in some cases, to optimize the reflected-light characteristics. 
       FIGS.  5 A-B  depict schematic drawings of side and top views of a portion of yet another example of a sample holder in accordance with the present disclosure. Sample holder  500  is analogous to sample holder  106 ; however, in sample holder  500 , features  502  are optimized to effectively hold a test sample  114 , and do not need anti-reflection properties. Features  502  may be asymmetric (e.g. approximately rectangular with a long dimension  512  and short dimension  514 , etc.) Furthermore, although the depicted example includes rectangular features, features  502  can have virtually any shape (e.g., square, circular, triangular, rhomboidal, elliptical, irregular, etc.) without departing from the scope of the present disclosure. 
     Like features  308 , each of features  502  projects from planar surface  310  and comprises sidewall  504  and tip  506  such that the planar surface, sidewalls, and tips collectively define SC surface  508 . However, features  502  are much larger than typical features  308 , having lateral dimension on the order of microns or millimeters. It should be noted, however, that features  502  can have any practical lateral dimensions without departing from the scope of the present disclosure. 
     Features  502  are dimensioned and arranged to both effectively hold test sample  114  and substantially optimize the reflective characteristics of SC surface  508 . As such, features  502  have an inter-feature spacing, s2, which is selected such that tips  506  occupy only a small fraction of the total area of SC surface  508 . As a result, most of the light of interrogation signal  112  reflects from reflector  410  on planar surface  310  without striking sidewalls  504  while completing a double pass through test sample  114 . In some embodiments, reflector  410  is also disposed on sidewalls  504  and/or tips  506 . 
     It should be noted that  FIG.  5 B  shows optional features  516 , which are included in some embodiments in at least some of the regions between features  502  to provide additional optical and other functionality, such as anti-reflection capability, spectral filtering, test sample retention, and the like. 
     Preferably, planar surface  310  is “optically flat” so that the beam quality of interrogation signal  112  is preserved as it reflects from the planar surface. 
     Furthermore, it should be noted that features  502  can be included in the SC surface of any sample holder in accordance with the present disclosure without departing from its scope. In some embodiments, features  502  are located only at the perimeter of a SC surface, thereby enclosing the SC surface within a border. 
     It should be further noted that sample holder  500  need not include any elements that give rise to a significantly wavelength-sensitive response. Most simple optical-surface components, such as thin-film coatings or features  308  can be configured such that they are effective over a wavelength range spanning a factor of about 2-3 (e.g. 7-to-15 microns, etc.). For example, in embodiments in which a layer of gold is used for reflector  304 , the reflector will be highly reflective (90% or more) over a wavelength range that spans from 2 to 20 microns or more. 
       FIG.  6    depicts a block diagram of another exemplary embodiment of an optical analysis system in accordance with the present disclosure. System  600  is analogous to system  100 ; however, system  600  is a Fourier-Transform Spectrometer (FTS) that includes light source  602 , mirrors  604 , sample holder  500 , detector  606 , and processor  110 , as well as associated optics, such as a collimator, focusing lens, and the like (not shown for clarity). In some embodiments, system  600  is an optical analysis system other than an FTS. 
     Light source  602  is analogous to light source  102  and filter  104 ; however, light source  602  is configured such that it can be translated to change the length of the optical path between itself and detector  606 . 
     Mirrors  604  are conventional reflectors suitable for redirecting interrogation signal  112  toward test sample  114  and output signal  606  toward detector  606  after the output signal is reflected by the test sample. In some embodiments, at least one of mirrors  604  includes spectral filtering capability. 
     Detector  606  is a conventional detector configured to receive energy of interrogation signal  112  from test sample as output signal  608  and provide spectral signal  610  to processor  110 . 
     It should be noted that, although the depicted example includes sample holder  500 , any reflective sample holder in accordance with the present disclosure can be used in system  600 . 
     Although the optical analysis systems discussed thus far are configured to operate in reflection mode, the teachings of the present disclosure are also suitable for use in optical analysis systems that operate in transmission mode (i.e., in which an interrogation beam passes completely through a test sample and its sample holder). 
       FIG.  7    depicts a block diagram of yet another embodiment of a chemical analysis system in accordance with the present disclosure. System  700  is a spectrometer that is operative for analyzing a test sample by determining the absorption characteristics of the test sample over the wavelength range from approximately 2.0 microns to approximately 15 microns (i.e., the mid-infrared spectral range). System  700  includes light source  102 , filter  104 , sample holder  702 , detection system  108 , processor  110  and associated optics, such as collimating and focusing lenses, etc. 
     Sample holder  702  is analogous to sample holder  106 ; however, sample holder  702  is configured to enable the energy of interrogation signal  112  to pass through it and emerge as output signal  704 . Output signal  704  is analogous to output signal  118 . 
       FIGS.  8 A-B  depict schematic drawings of perspective and sectional views of an exemplary transmissive sample holder in accordance with the present disclosure. Sample holder  702  includes body  302  and AR structure  802 . The sectional view shown in  FIG.  8 B  is taken through line c-c as shown in  FIG.  8 A . 
     AR structure  802  is formed on or in surface  314  and is configured to mitigate reflection of the wavelengths included in interrogation signal  112  at that surface. 
     In the depicted example, AR structure  802  is a second GAR layer  312 . In some embodiments, AR structure  802  includes a different anti-reflection structure, such as a GAR layer having a different feature configuration, a conventional anti-reflection coating comprising one or more material layers, such as dielectric materials (e.g., germanium, zinc selenide, etc.), and the like. It will be clear to one skilled in the art, after reading this Specification, how to specify, make, and use AR structure  802 . 
     In some embodiments, AR structure  802  has functionality beyond just simple reflection suppression. For example, in some embodiments, AR structure  802  functions as a spectral filter, such as:
     i. an edge filter that passes only wavelengths longer than a particular wavelength within interrogation signal  112 ; or   ii. an edge filter that passes only wavelengths shorter than a particular wavelength within interrogation signal  112 ; or   iii. a bandpass filter that transmits only a wavelength range and rejects (reflects or absorbs) wavelengths outside the range; or   iv. any combination of i, ii, and iii.   

     In the depicted example, interrogation signal is incident on the test sample and sample holder at incidence angle, θ1, relative to axis A1, where axis A1 is normal to planar surface  310 . In the depicted example, A1 is 0° such that interrogation signal  112  is aligned with axis A1. However, it is an aspect of the present disclosure that transmissive and reflective sample holders having a sample-collection surface as taught herein has less angular sensitivity than sample holders of the prior art. As a result, a sample-holder in accordance with the present disclosure can be aligned at angles of up to at least 40° between its normal axis, A1, and the propagation direction of interrogation signal  112 , thereby increasing the optical path length of the interrogation signal through analyte and improving the sensitivity of an optical measurement system. 
     It should be noted that, in such embodiments, AR structure  802  preferably retains its low-reflection characteristics. 
     In some embodiments, AR structure  802  has additional optical functionality. For example, in some embodiments, AR structure  802  includes a metasurface configured to function as field lens, focusing lens, beam deflector and the like. 
       FIG.  9    depicts measured transmission for a transmissive sample holder in accordance with the present disclosure at different angles of incidence. Plot  900  includes traces of the transmissivity of sample holder  106  for an interrogation signal received at incidence angles, θ, of 0, 10, 20, 30, and 40 degrees (relative to normal axis A1), respectively. 
     As evinced by plot  900 , for angles of incidence up to 20 degrees, sample holder  106  has approximately 80% transmission for the spectral range that spans from approximately 5 microns to approximately 15 microns, which encompasses most of the entire MIR spectrum . It should be noted that, as a point of comparison, a plain, unstructured silicon substrate exhibits approximately 30% reflection from each of its surfaces; therefore, would have a transmission of approximately 50%. 
     Furthermore, plot  900  indicates that transmission is not severely degraded even for angles of incidence between 20 degrees and 40 degrees, with only a slight loss of transmission for wavelengths below about 7 microns. 
     As the wavelength of light approaches the value of s1, some degradation in transmission becomes evident. For example, for wavelengths less than approximately 5.5 microns (i.e., approximately 2.75 times s1), transmission begins to decline and more dependence on angle of incidence is evident. It should be noted, however, that reflection is also reduced at wavelengths less than 2.75 times s1; therefore, it is preferably that features  308  have an inter-feature spacing, s1, that is less than about 1/2.75 of the wavelength of interest. In many cases, but not all, the value of s1 is based on the shortest wavelength, λ m , included in interrogation signal  112 . 
     It is an aspect of the present disclosure, therefore, that the inter-feature spacing, s1, is preferably less than approximately one-half the shortest wavelength in interrogation signal  112 . 
     Still further, at longer wavelengths, the transmission increases and has less angular dependence than at the shorter wavelengths. For a 2-micron wavelength, GAR layer  312  can be effective with s1 being less than 2 microns; however, it would be more effective with s1 being less than 2/2.75 (0.72) microns. In other cases, such as for a 100-micron wavelength (i.e., 3 Terahertz frequency), GAR layer  312  can be effective with s1 being less than 200 microns; however, it would be more effective with an s1 of less than 36 microns. In similar fashion, for radiation having a wavelength of 10,000 microns (i.e., 10 millimeters), GAR layer  312  can be effective with s1 being less than 20 millimeters; however, it would be more effective with s1 being less than 3.6 millimeters. It should be noted the wavelengths discussed here are merely exemplary and that scaling of the inter-feature spacing, s1, scales with substantially any wavelength of radiation. 
       FIG.  10    depicts measured transmission through a test sample disposed on the sample-collection surface of a vertically oriented transmissive sample holder in accordance with the present disclosure. Plot  1000  includes traces of the transmission of interrogation signal  112  through a test sample composed of a 1:1 mixture of ethanol and methanol at different times after the test sample is dispensed on SC surface  116 . 
     It is an aspect of the present disclosure that a sample holder configured as taught herein can be oriented at a wide range of angles relative to the propagation direction of interrogation signal  112 , as well as to the direction of gravity. The fact that sample holders disclosed herein enable direct access to a test sample, while still retaining even a liquid sample on SC surface  116  (as noted above) at extreme orientations with respect to gravity, affords embodiments in accordance with the present disclosure significant advantages over optical analysis systems of the prior art. 
     Inset A shows the orientation of test sample  114  and sample holder  702  during the measurements made to generate plot  1000 . As indicated, the normal axis A1 of sample holder  106  is held such that it is aligned with the direction of propagation of interrogation signal  112  and output signal  704  and perpendicular to the direction of gravity. As a result, test sample  114  is open to direct access by interrogation signal  112 ; however, there is nothing to keep the test sample in place on SC surface  116  other than its adhesion to features  308  arising from surface tension. 
     It is an aspect of the present disclosure that, by configuring sample holder  106  such that test sample  114  is “open to the environment” while loaded in the sample holder, it is easy to rapidly prepare the test sample. Furthermore, such a configuration facilitates high-fidelity measurement of the light transmission/absorption by the test sample (and/or other properties of the test sample). 
     Plot  1000  shows that the absorption characteristics of test sample  114  change over time as the volatile ethanol and methanol evaporate at different rates due to the fact that test sample  114  is open to the environment. 
     It is to be understood that the disclosure teaches just some embodiments in accordance with the present disclosure and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.