Patent Publication Number: US-2023140255-A1

Title: Mid-infrared waveguide sensors for volatile organic compounds

Description:
PRIORITY 
     This application is a continuation of, and claims the benefit of priority to, U.S. patent application Ser. No. 16/589,818, filed Oct. 1, 2019, which claims priority to U.S. Provisional application Ser. No. 16/739,610, filed Oct. 1, 2018, which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to the detection of volatile organic compounds (VOCs), and more specifically to waveguide-based VOC sensors. 
     BACKGROUND 
     VOCs are organic (i.e., carbon-based) chemical compounds that have a high vapor pressure at room temperature, resulting in a low boiling point, which causes large numbers of molecules to evaporate or sublime from the liquid or solid form of the compound and enter into the surrounding air. Although VOCs exist widely throughout our surroundings, they can be damaging to the environment and harmful or even fatal to humans. Exposure to high concentrations of VOCs can, for instance, cause throat irritation, headaches, and internal organ damage, and continuous low-level exposure can have long-term adverse health effects. On the other hand, some VOCs are biomarkers for various diseases and, as such, have been utilized to monitor health conditions. Acetone, for example, is widely used as a biomarker for diabetes since it is found in the exhaled breath of diabetes patients, its concentration being indicative of the patient&#39;s glucose level. Accordingly, the detection of VOCs is relevant to human health monitoring as well as tracing environmental toxins. 
     Currently, the most prevalent method for organic compounds analysis is gas chromatography mass spectrometry. This method uses bulky and expensive instrumentation, and is time-consuming because gas chromatography can take several minutes to separate different gases, which poses a challenge for instantaneous VOC measurements. Another method widely applied in VOC detection relies on electrical resistance changes in metal oxide semiconductor (MOS) sensors in the presence of VOCs due to charge transfer between the sensor surface and the chemisorption oxygen. MOS-based sensors have shown high sensitivity. However, they provide poor selectivity because different VOCs, as well as background gases such as ozone or water, affect the resistance similarly. An alternative, small-scale sensing platform that can achieve real-time VOC detection while retaining high specificity and accuracy is desirable. 
     SUMMARY 
     Described herein are waveguide-based sensor devices and systems, along with associated methods of manufacture and use, for the detection and identification of VOCs based on their characteristic “fingerprint” absorptions in the mid-IR wavelength regime, herein understood as the range between 2.5 μm and 15 μm. The operating principle of the sensor devices is the attenuation of an optical mode travelling in the waveguide due to the absorption of the evanescent field by VOC molecules at the waveguide surface. The magnitude of the attenuation is indicative of the VOC concentration, whereas the wavelength dependence of the attenuation allows an inference of the particular kind of VOC being detected. Accordingly, to discriminate between different VOCs, a sensor device in accordance with various embodiments may utilize a tunable mid-LR lights source (e.g., a tunable laser) to measure attenuation at multiple wavelengths or across a continuous wavelength range. 
     Waveguide-based VOC detection is challenging, in comparison to the waveguide-based sensing of many other chemical or biochemical analytes (which are usually dissolved in liquid samples), due to the typically low concentrations of the VOCs in the gas phase. In accordance with various embodiments, VOC sensing is rendered feasible by a combination of waveguide material, geometric, and surface properties that achieves high sensitivity to VOCs through low-loss light guiding in conjunction with a strong evanescent field. In particular, in preferred embodiments, a chalcogenide, e.g., arsenic triselenide (As 2 Se 3 ), is selected as the waveguide material. Chalcogenides provide high mid-IR transparency across a wide wavelength range, and have relatively high refractive indices resulting in a high index contrast with standard oxide undercladdings, allowing for good optical mode confinement and, thus, efficient light guiding. The waveguide geometry features, in preferred embodiments, an upper waveguide surface (meaning the waveguide surface that is exposed to the environment, as distinguished from the waveguide interface with the undercladding) that is free of edges, which substantially diminishing scattering losses, as well as a large width-to-height aspect ratio (e.g., greater than two, preferably greater than five), which results in a strong evanescent field. For example, the waveguide may have a “flattened semielliptical” cross-sectional profile, rather than the common rectangular cross section. Such a rounded, edge-free surface and high aspect ratio can be achieved in the manufacturing process by depositing, following photolithographic definition of the waveguide, the chalcogenide layer on the substrate using a new sputtering process in which the substrate is tilted or offset relative to the sputtering target and rotates continuously. With a chalcogenide waveguide thus manufactured, VOC concentrations of as little as tens of parts per million (ppm) can be detected in some embodiments. Beneficially for many practical sensing applications, waveguide-based sensor devices as described herein are amenable to chip-scale implementations and capable of real-time VOC detection and concentration monitoring. 
     The foregoing summary introduces various concepts, principles, and aspects of the inventive subject matter, and certain features of various embodiment, but does not exhaustively list all inventive features and aspects, and is not intended to limit the scope of the claimed subject matter in any way. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects and example embodiments are described herein with reference to the accompanying drawings, in which: 
         FIGS.  1 A and  1 B  are schematic perspective and cross-sectional views of an example waveguide structure in accordance with various embodiments; 
         FIG.  2    is a flow chart of a method of manufacturing a waveguide structure as shown in  FIGS.  1 A and  1 B , in accordance with various embodiments; 
         FIGS.  3 A and  3 B  are schematic diagrams illustrating sputtering configurations in accordance with various embodiments; 
         FIGS.  4 A and  4 B  are two-dimensional and one-dimensional optical mode profiles at a mid-infrared wavelength for the waveguide structure of  FIGS.  1 A and  1 B ; 
         FIGS.  5 A and  5 B  are block diagrams of chemical sensing systems incorporating the waveguide structure of  FIGS.  1 A and  1 B , in accordance with various embodiments; 
         FIG.  6    is a cut-away perspective view of a waveguide sensor enclosed in a microfluid chamber, in accordance with various embodiments; 
         FIG.  7    is a flow chart of a method for VOC sensing in accordance with various embodiments; 
         FIG.  8 A  is a graph of the relative absorption by acetone as measured with an example As 2 Se 3  waveguide sensor according to  FIGS.  1 A and  1 B  over a range of mid-infrared wavelengths, along with two-dimensional optical model profiles; 
         FIG.  8 B  is a graph of the relative absorption by ethanol as measured with the example As 2 Se 3  waveguide sensor according to  FIGS.  1 A and  1 B  over a range of mid-infrared wavelengths, along with two-dimensional optical model profiles; 
         FIG.  9 A  is a graph illustrating the transient response of the example As 2 Se 3  waveguide sensor according to  FIGS.  1 A and  1 B  to ethanol during pulsed ethanol exposure; 
         FIG.  9 B  is a graph illustrating the transient response of the example As 2 Se 3  waveguide sensor according to  FIGS.  1 A and  1 B  to stepwise increases in ethanol concentration; and 
         FIG.  9 C  is a graph illustrating the dependence of the relative waveguide mode intensity of the example As 2 Se 3  waveguide sensor according to  FIGS.  1 A and  1 B  on ethanol concentration. 
     
    
    
     DESCRIPTION 
       FIGS.  1 A and  1 B  are schematic perspective and cross-sectional views of a waveguide structure  100  as may be used in VOC sensor devices in accordance with various embodiments. The waveguide structure  100  includes a ridge waveguide  102  formed on a substrate  104  that comprises a thick handle  106  for mechanical stability and a thin top layer serving as an undercladding  108  for the ridge waveguide  102 . The substrate handle  106  may, for instance, be made from a standard silicon wafer. The undercladding  108  may be an oxide layer (e.g., made of silicon dioxide (SiO 2 ), magnesium oxide (MgO), or aluminum oxide (Al 2 O 3 )) or other dielectric layer (e.g., made of calcium fluoride (CaF 2 )), and may be a few micrometers thick. The waveguide  102  is made, in accordance with various embodiments, from a chalcogenide, i.e., a crystalline or amorphous semiconductor compound including one or more chalcogen elements (e.g., sulfur, selenium, tellurium) covalently bonded to an electropositive element. Beneficially, chalcogenides provide wide windows of infrared transparency (e.g., with at least 50% transmission, at least 70% transmission, or at least 90% transmission), ranging from about 2 μm or less to at least about 10 μm, at least about 15 μm, or even up to about 20 μm, depending on the particular material. In addition, chalcogenides have a suitable index of refraction, e.g., in the range from about 2 to about 3 within the mid-infrared regime; such a relatively high index provides good index contrast to that of the undercladding  108 , which is, for SiO 2 , about 1.45. In one embodiment, the waveguide  102  is made of As 2 Se 3  glass, which has a refractive index of about 2.79 and is transparent to light from optical wavelengths up to mid-infrared wavelength of about 15 μm. Other suitable chalcogenides include zinc sulfide, arsenic sulfide, antimony sulfide, antimony selenide, etc. The waveguide  102  is directly exposed to the surrounding without any intervening overcladding; it is, thus, usually air-clad during use. 
     As shown in  FIGS.  1 A and  1 B , the boundaries of the cross-sectional profile of the ridge waveguide  102  are defined by the flat interface  110  with the undercladding  108  and a smooth, rounded top surface  112 , corresponding to the surface that will be exposed to the VOC during use. The exposed top surface  112  is free of the sharp corners (in the two-dimensional profile) or edges (in the three-dimensional structure) that are characteristic of conventional rectangular waveguides, where such edges tend to result in significant scattering losses; the absence of edges in the top surface  112  of the waveguide  102 , accordingly, contributes substantially to low light propagation losses and, as a consequence, high waveguide sensitivity, as needed for VOC detection at low concentrations. In some embodiments, the waveguide  102  has an approximately semi-elliptical or flattened semi-elliptical profile, although, as shown, the flanks of the waveguide may fall off more gradually than the edges of a true semi-elliptical cross section, preventing a sharp edge at the interface with the undercladding  108 . The cross section of waveguide  102  exhibits a high aspect ratio of width to height, e.g., a ratio greater than five in various embodiment, and generally a ration in the range from two to twenty. The waveguide is dimensioned to guide light at wavelengths in the mid-IR regime, and the low waveguide height, compared to the waveguide width, causes a substantial portion of the optical mode (e.g., the fundamental mode) to leak into the surrounding region as an evanescent wave, likewise contributing to high sensitivity. In one example embodiment, the waveguide has a width of about 10 μm (as measured, e.g., between the two points of the profile at which the height is at half the maximum waveguide height) and a (maximum) height of about 1.5 μm. 
       FIG.  2    is a flow chart of a method  200  of manufacturing a waveguide structure  100  as shown in  FIGS.  1 A and  1 B , in accordance with various embodiments. The method  200  starts, at  202 , with a suitable substrate  102 , e.g., a silicon wafer with a SiO 2  cladding layer (e.g., 3 μm thick) serving as undercladding  108  for the waveguide  102 . The waveguide layout is negatively defined on the substrate, in step  204 , by depositing a photoresist layer on the undercladding and photolithographically patterning the photoresist layer, as known to those of ordinary skill in the art. The patterned photoresist layer exposes the undercladding in areas where the undercladding will interface with the waveguide, and covers the remaining surface of the undercladding. In one embodiment, negative tone photoresist NR9-3000PY is used in this process. In step  206 , a chalcogenide (e.g., As 2 Se 3 ) film is deposited over the patterned substrate by sputtering. For example, in one embodiment, a 99.99% As 2 Se 3  target is radio-frequency sputtered with a power of 40 W. Importantly, in order to obtain an approximately (flattened) semi-elliptical waveguide profile with a smooth, edge-free top surface, the sputtering target and substrate may be tilted with respect to each other (as shown in  FIGS.  3 A and  3 B ), rather than being oriented in parallel and directly facing one another as is the case in conventional sputtering. Further, the substrate is continuously rotated during the sputtering process about an axis perpendicular to the substrate. Following deposition of the chalcogenide, the photoresist layer and chalcogenide layer thereabove are removed, in step  208 , by a lift-off process, leaving only the chalcogenide waveguide  102  on the undercladding  108 , which completes manufacture of the waveguide sensor at  210 . 
       FIGS.  3 A and  3 B  are schematic diagrams of tilted and/or offset sputtering configurations in accordance with various embodiments, illustrating the relative position and orientation of the sputtering target  300  (shown on a target holder  302 ) and the substrate  304  (shown on a substrate holder  306 ). The sputtered particles generally emanate from the target  300  anisotropically, with a rotational symmetry about an axis  308  normal to the target and maximum particle density along this axis  308 . The anisotropy is exploited, in accordance with various embodiments, to achieve the desired waveguide profile by rotating the substrate  304  about a rotational axis  310  perpendicular to the substrate  302 , and tilting and/or laterally off-setting the substrate  304  relative to the target  300 . For example, as shown in  FIG.  3 A , the substrate  304  may be tilted relative to the target  300  by an angle θ enclosed between the axis  308  normal to the target  300  and an axis, such as rotational axis  310 , normal to the substrate  304 . This tilt angle θ defines the radius and aspect ratio of the waveguide structure, that is, for a semielliptical structure, the lengths of the minor and major axes. Suitable tilt angles θ are in the range from about 15 to about 75 degrees. Alternatively, as shown in  FIG.  3 B , the substrate  304  may be oriented parallel to the target  300 , but positioned with an off-set between its rotational axis  310  and the symmetry axis  308  of the target. The waveguide profile is determined by the combination of tilt angle and off-set. 
     Rotational sputtering as depicted in  FIG.  3    allows manufacturing waveguides with a smooth, extensive top surface and a large width-to-height ratio, both of which are important for achieving increased waveguide sensitivity (as compared with conventional waveguide geometries) to enable VOC detection. In some embodiments, VOC concentrations on the order of only tens of ppm are detectable with waveguide-based sensors as described herein. 
       FIGS.  4 A  an  4 B illustrate an infrared optical mode in an example As 2 Se 3  waveguide (e.g., as depicted in  FIGS.  1 A and  1 B ) with a semi-elliptical cross section, 10 μm wide and 1.5 μm thick, on an SiO 2  undercladding. The refractive indices of the waveguide and cladding are 2.79 and 1.45, respectively.  FIG.  4 A  displays the two-dimensional optical field, as can be measured or computationally simulated (e.g., with the finite difference method), of the fundamental transverse optical mode excited in the waveguide at a wavelength of 3.4 μm. The mode has a generally ellipsoid (as depicted close to spherical) intensity profile in the waveguide center, and an evanescent field that extends both into the undercladding below (z≤0) and the air above (z≥1.5 μm) the As 2 Se 3  layer. The evanescent field is more easily seen in  FIG.  4 B , which illustrates the one-dimensional intensity profile of the fundamental optical mode along the z axis. As shown, at a distance of 0.5 μm from the exposed waveguide surface, the intensity of the evanescent wave is still above about 10% of the peak intensity of the optical mode in the chalcogenide waveguide. The evanescent field intensity determines the sensitivity of the sensor to VOCs. Due to the high y:z aspect ratio (y being the width dimension of the waveguide), the evanescent field is much stronger in the z direction than in the y direction; therefore, a transverse magnetic (TM) polarization is applied in the simulation. Further, as comparative simulations and measurements at different wavelengths have shown, the evanescent field becomes stronger towards longer wavelengths. 
       FIGS.  5 A and  5 B  are schematic block diagrams of VOC sensing systems, in accordance with various embodiments, that incorporate the waveguide structure  100  of  FIGS.  1 A and  1 B . Shown in  FIG.  5 A  is a system  500  that includes, in addition to the waveguide sensor  502  (as implemented, e.g., by waveguide  102 ), a tunable mid-infrared light source  504  (e.g., a tunable laser), a mid-infrared detector  506 , and a computational processing module  508 . The light source  504  generates the optical input to the waveguide  102  by coupling light, directly or indirectly, into the waveguide  102  to launch an optical mode. The light propagates through the waveguide  102  and exits at the output, where the intensity of the optical output mode is measured by the detector  506 . The computational processing module  508  processes the detector signal to determine the attenuation of the optical mode in the waveguide  102 , e.g., as a function of wavelength to obtain spectral attenuation information or at a selected wavelength. The computational processing module  508  may be implemented in analog or digital circuitry; if the latter, the electronic output of the detector  506  may be converted into a digital signal by an analog-to-digital converter (not shown). In some embodiments, the computational processing module  508  is provided by a programmable processor (e.g., a field-programmable gate array (FPGA) or general-purpose central processing unit (CPU)) executing suitable software. 
     The light source  504  is tunable over an operating wavelength range of the sensing system  500 , facilitating measurements of absorption spectra, across that wavelength range, of samples in contact with the waveguide sensor  502  and detection of analytes (including VOCs) with characteristic absorptions at wavelengths within that range. In some embodiments, the operating wavelength range extends from about 2.5 μm or less to about 15 μm or more. The detector  506  may be, for instance, a photodetector that measures the overall intensity of the light output by the waveguide sensor  502 , or, alternatively, a camera (e.g., an array of photosensors) that allows imaging the optical mode at the waveguide sensor output. Either way, the detector  506  is selected or configured to be sensitive to light within the operating wavelength range. In various embodiments, for instance, an indium antimonide (InSb) infrared camera, which is responsive to light from less than 1 μm up to 5.3 μm, an HgCdTe (MCT) camera, which is sensitive up to at least 7 μm, or a pyroelectric detector, which operates at wavelengths up to at least 15 μm, is used. 
     The computational processing module  508  may be configured to create a spectrum by associating the measured output signal of the sensor  502  at a given time with the respective wavelength input by the light source at that time. The computational processing module  508  may have knowledge of the light-source wavelength by virtue of controlling the tunable wavelength itself, or by receiving a signal indicative of the wavelength from a separate light-source controller (not shown). In addition to computing a spectrum, the computational processing module  508  may also implement processing logic for analyzing the spectrum, e.g., based on data about the absorption characteristics of a various VOCs (e.g., as stored in memory of the computational processing module  508 ), to identify the types of VOCs present within the sample and/or determine their concentration. Alternatively to acquiring a spectrum by varying the wavelength with time, the system  500  can also be operated continuously at a given wavelength, e.g., corresponding to a characteristic absorption line of a certain VOC, to measure a time-resolved absorption signal indicative of a (possibly variable) concentration of the VOC in the sample. 
       FIG.  5 B  shows an alternative sensing system  520 , which includes, instead of a tunable light source  504 , one or more light source  522  providing broadband light (collectively) covering the operating wavelength range. To facilitate the acquisition of a spectrum, the system  500  may further include a dispersive element  524  at the output of the waveguide sensor  502 , preceding the detector(s)  506 , to spatially spread out the light by wavelength. Using a camera as the detector  506 ; the output intensity at different wavelengths can then be measured at different respective locations within the sensor array of the camera. Alternatively to a camera, multiple photodetectors (e.g., a photodiode array detector) may be placed at different locations corresponding to different respective wavelengths, or a single detector (or camera) may be moved to measure the intensity for different wavelengths. In a broadband-light sensing system  520 , the computational processing module  508  generates a spectrum by associating the location of the measured light intensity with wavelength. 
     In both sensing systems  500 ,  520 , the light emitted by the light source  504 ,  522  may be collimated, e.g., with a refractive lens, into an optical fiber, which may then be butt-coupled to the waveguide sensor  502 . Similarly, the light output by the waveguide sensor  502  may be focused by a lens (e.g., a barium fluoride biconvex lens) onto the camera or other detector  406 . Alternatively, the light source  504 ,  522  and/or detector  506  may be implemented as photonic circuit components and monolithically integrated with the waveguide sensor  502  on the same substrate, e.g., in a semiconductor device layer (or multiple such layers) disposed above the undercladding. Lasers and detectors may be formed, e.g., by silicon device structures in conjunction with III-V structures serving as active regions and associated electrodes, which may be patterned and manufactured using standard CMOS processes; suitable photonic-component structures and manners of manufacturing same are well-known to those of ordinary skill in the art. To provide just one example, in some embodiments, a quantum cascade laser, which can emit light in the mid-infrared regime, may be used as the light source. Alternatively to creating the photonic circuit components in a separate semiconductor device layer, they may also in part be created in the chalcogenide layer disposed on the undercladding during manufacture of the waveguide sensor  502 . Integrating light source, sensor, and detector on the same substrate provides a self-contained, chip-scale VOC sensor device. In some embodiments, furthermore, the computational processing module  506  is implemented, in whole or in part, by microelectronic circuitry, which is likewise capable of integration on the same chip in the semiconductor device layer. In some embodiments, such integrated microelectronic circuitry, instead of performing the signal-processing itself, includes a signal transmitter for wireless transmission of the sensor data (e.g., as captured in the photodetector output). Such data transmission enables a compact sensor device to communicate with a separate computing device, such as a general-purpose computer executing software for processing the signals. 
       FIG.  6    is a cut-away perspective view of a waveguide sensor  502  (implemented by waveguide  102 ) enclosed in a microfluidic chamber  600 , in accordance with various embodiments; this configuration may be used during sensor calibration as well as, optionally, for measurements in the field. The microfluidic chamber  600  may be made, e.g., of polydimethylsiloxane (PDMS) or some other organosilicon, using micro-fluidic or opto-fluidic chip manufacturing techniques known to those of ordinary skill in the art. The microfluidic chamber  600  includes an inlet  604  and an outlet  606  through which a gas flow through the chamber  600  and across the waveguide sensor  502  can be established. During sensor calibration, a mixture of VOC vapor and a diluting gas (e.g., nitrogen (N 2 ) gas), is delivered to the microfluidic chamber  600 , where the waveguide sensor  502  is exposed to the analytes. Different VOC concentrations can be achieved by regulating relative flow rates of the VOC and diluting gas. As the waveguide sensor  502  is exposed to the VOCs, the optical output mode intensity is monitored, using system components as described above with respect to  FIGS.  5 A and  5 B . 
       FIG.  7    is a flow chart of a method  700  for VOC sensing in accordance with various embodiments. The method  700  involves measuring the attenuation of an optical mode in a chalcogenide waveguide sensor by coupling infrared light from a light source into the waveguide to launch an optical mode (e.g., the fundamental transverse mode) and measuring the intensity of the optical mode at the waveguide output. Optical mode attenuation is measured as a function of wavelength (over a range within the mid-infrared regime) while the waveguide sensor is exposed to a VOC (act  704 ) to determine spectral attenuation information associated with the VOC (act  706 ). In some embodiments, the optical mode attenuation is also measured, for comparison, in the absence of the VOC (act  702 ), allowing relative attenuation information to be determined (in act  706 ). Such relative attenuation information may be, e.g., the relative absorption, which is defined as the normalized difference between the output mode intensities I 0  and I F , respectively, before and after VOC exposure of the waveguide: A=(I 0 −I F )/I 0 ; or the relative intensity, which is defined as I=1−A. The spectral (relative or absolute) attenuation information generally differs between different compounds, and thus provides a means to identify and discriminate between compounds based on their associated spectral absorption characteristics. Accordingly, in various embodiments, spectral (e.g., relative) attenuation information  710  is measured for multiple VOCs and stored (e.g., in computer memory) for later retrieval and comparison, to facilitate identifying VOCs among multiple VOCs or chemical compounds in general.  FIGS.  8 A and  8 B , described below, provide examples of such compound-specific absorption characteristics. 
     For a given detected VOC, the method  700  may further include, in act  712 , measuring or monitoring optical mode attenuation (e.g., in terms of a relative absorption or intensity) at a selected wavelength (e.g., a wavelength within a strong absorption band of the VOC) to determine the VOC concentration or concentration changes over time ( 714 ). The concentration-dependent attenuation of the optical mode in the waveguide sensor can, for this purpose, be calibrated, e.g., using the set-up shown in  FIG.  6   . Such monitoring can, in some embodiments, be performed in real time or near real time. 
       FIGS.  8 A- 9 C  illustrate various VOC measurements with an example chalcogenide waveguide, characterizing the capabilities of waveguide-based VOCs in accordance with some embodiments. The example sensing waveguide used in these measurements was made of As 2 Se 3  (on an SiO 2  undercladding) and had a flattened semi-elliptical cross-sectional profile about 10 μm wide and 1.5 μm thick. The waveguide featured a smooth exposed waveguide surface without bumps, indentations, or sharp edges; a sharp interface between the As 2 Se 3  and SiO 2  layers; and a homogeneous material composition across the film surface and along the film depth. The high degrees of surface smoothness and material uniformity minimized optical losses due to scattering at surface features and refractive-index variations and, along with a high refractive index contrast between waveguide and undercladding, achieved efficient light wave guiding. Mode intensity measurements at different waveguide lengths revealed optical losses of about 0.16 dB/cm, which is comparable with reported waveguide-based sensors for other chemical compounds. 
       FIGS.  8 A and  8 B  illustrate the sensitivity and specificity of the waveguide sensor to acetone and ethanol, respectively. For the depicted measurements, the wavelength of a tunable light source was sequentially scanned from 3.4 μm to 3.62 μm because this spectral range overlaps with the characteristic absorption bands caused by the C—H functional group.  FIG.  8 A  shows the two-dimensional optical mode profiles at discrete wavelengths within this range, measured both in the absence (first row) and in the presence (second row) of acetone. Also shown is the relative absorption computed from the mode profiles with and without acetone, which is plotted as a function of wavelength and correlated with respective pairs of mode profiles. As can be seen, a bright and sharp optical mode was observed from 3.4 μm to 3.6 μm before any chemical was present. Upon exposure of the waveguide sensor to acetone (as acetone vapor was injected into a PDMS chamber enclosing the waveguide sensor), the mode intensity sharply decreases at 3.40 μm to 3.42 μm. The characteristic absorption associated with acetone was clearly observed at 3.40 μm to 3.44 μm. 
     For comparison,  FIG.  8 B  shows the two-dimensional optical mode profiles measured in the absence (first row) and in the presence (second row) of ethanol, along with a graph of the relative absorption derived from the mode profiles. When ethanol vapor was injected into the PDMS chamber, the mode intensity decayed over a broad spectrum from 3.40 μm to 3.52 μm, reflecting that the absorption band of ethanol is wider than that of acetone. Thus, the chalcogenide sensor was shown to be able to differentiate ethanol and acetone vapor by their distinct mid-infrared C—H absorption. 
       FIGS.  9 A- 9 C  illustrate the ability of the waveguide sensor to monitor ethanol (as an example of a VOC) concentrations in (near) real time. For purposes of characterizing the real-time sensing performance, the diluting gas (99.999% N 2 ) was mixed with ethanol vapor at different gas flow rates. The laser wavelength was tuned to 3.46 μm, aligned with the characteristic ethanol C—H absorption, to continuously trace the ethanol concentration.  FIG.  9 A  is a graph showing the transient response of the waveguide sensor, in terms of the relative intensity of the output mode, to a sequence of ten-second ethanol vapor pulses injected into the PDMS chamber. As can be seen, the output mode intensity dropped abruptly whenever the waveguide sensor was exposed to ethanol. Once the ethanol was purged from the chamber by injecting N 2 , the light intensity recovered to the original level. During ethanol vapor monitoring, a fast response of less than five seconds was observed; this response is attributable to the high sensitivity of mid-infrared detection as well as the small gas chamber volume. 
     To quantitatively correlate the mode intensity variation with ethanol vapor concentration, ethanol vapor with concentrations of 0%, 25%, 50%, 75%, and 100% in N 2  was sequentially injected in to the PDMS chamber.  FIG.  9 B  is a graph illustrating the transient response of the waveguide sensor to these stepwise increases in ethanol concentration. As shown, the light intensity decreased as the ethanol vapor concentration increased. By fitting the intensity and concentration plot, a monotonic dependence of the relative waveguide mode intensity on ethanol concentration was observed; this dependence is plotted in  FIG.  9 C . 
     Although embodiments have been described with reference to specific example embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. The accompanying drawings that form a part hereof show by way of illustration, and not of limitation, specific embodiments in which the subject matter may be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be used and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled.