This PDF file contains the front matter associated with SPIE Proceedings Volume 10657, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Approaches are demonstrated that enable mobile devices, such as smartphones, to function as spectrophotometers with equivalent performance to laboratory instruments for measuring any diagnostic test that generates a colored liquid, fluorescent liquid, or colored solid surface. We envision mobile health diagnostic applications in which smartphone integrated measurement of point-of-care assays enables smart service systems for efficiently connecting patients with health care providers and other health services. A key to this capability is to offer valid tests that are equivalent to those performed in the laboratory by utilizing the same reagents, experimental controls, and calibration standards as conventional assays.

There are many applications requiring an instrument to be brought to the sample for chemical analysis onsite (rather than bringing a sample to a lab for analysis, as is done traditionally). Ideally, for such applications, a portable chemical analysis instrument must be capable of acquiring data using a smartphone, have wireless capability and it must be able to become part of the Internet-of-Things (IoT). But do smartphones have the required processing power to execute computationally-intensive algorithms, such as a Fast Fourier Transform (FFT)? Among others, FFTs are used for filtering (e.g., de-noising) of periodic signals, thus improving Signal-to-Noise Ratio (SNR). Using non-periodic signals and Fourier-domain interpolation for resolution enhancement, it will be shown that smartphones do have the necessary power.

Over the past 15, Raman systems have evolved from benchtop analyzers interfaced with a laptop computer to a smart phone based sensor. The journey from the first handheld system to newly developed smartphone-based Raman sensors is presented. Combining a Raman sensor with a smartphone enables advanced processors and cloud computing, greatly expanding Raman capabilities. We will discuss a range of applications, from process to security.

There are many applications requiring measurements on-site for example when accidental spills occur either outdoors or on the floor of IoT-enabled smart-factories (e.g., Industry 4.0). In other cases, a portable, fiber-optic spectrometer may be required for “bringing part of the lab to the sample” types of chemical analysis applications. Conceptually, there two approaches that can be used to put an optical spectrometer on IoT. One, is to design it from the ground up. The other, is to purchase a portable, battery-operated fiberoptic spectrometer; to use the manufacturer’s software (without any modification), and to employ a wireless connection, so that user interface and data-display will take place on the screen of a smartphone. In this paper, a dual-processor approach was taken to accomplish these tasks, as will be described in some detail.

Portable spectrometers designed for users with little technical training must be more robust than spectrometers designed for professionals. Common measurement errors (and in fact any imaginable non-random error) must be accounted for, trapped, and the user gently directed to manipulate the system to yield a valid, useful measurement. Such measurements are likely cost-sensitive, as consumers typically want infinite performance at zero cost. These constraints require either simplicity, automation, or software-driven operation using inexpensive components whose limitations are compensated algorithmically. We describe progress towards an instrument of the latter description, providing absorption, diffuse reflectance, and luminescence measurements to be carried out in a hand-held grating spectrometer. The gratings are plastic films, stacked and mutually rotated to generate hundreds of orders, which in turn are detected by a consumergrade camera chosen to mimic those in typical low-end smartphones. Low camera dynamic range is compensated by observing orders with a wide range of throughputs, such that overall dynamic range in a single exposure may be 12 bits from an 8 bit image. This requires significant effort for wavelength calibration and inter-order intensity normalization. The paper discusses progress in automatic calibration algorithms, software modularization, and method development for quantifying nitrate and phosphate in agricultural runoff. While the overall approach has been established for some years, significant details requiring careful raytracing, numerical analysis, algorithmic modularization and error handling, materials choices, and mechanical engineering for manufacturability are of current importance.

High resolution gas spectroscopy in the mid-infrared in a transportable device is a big challenge allowing to address numerous applications: air quality or industrial process monitoring, defense and security, medical diagnostics... Together with high tunability in the mid-infrared, spectral purity, narrow bandwidth, compactness and robustness are needed. Nested Cavity doubly resonant OPO (NesCOPO) fulfill all those requirements. This architecture is already commercialized (in the X-FLR8 portable gas analyzer from Blue Industry and Science) and allows to reach low threshold compatible with the use of compact micro-chip nanosecond YAG laser.

A wide spectral range can be obtain (2 - 10 μm). In the most mature version NesCOPO takes benefit of down-conversion of a laser radiation at 1.064 μm in a PPLN bulk crystal and give rise to two secondary radiation around 1.5 μm and between 3.2 and 4.25 μm. This last radiation is used to probe rovibrational absorption lines of species of interest using absorption or transmission spectroscopy.

Speed in the selection of the emitted wavelength can be an important requirement especially when security is involved. We use engineering of the crystal using fan-out configuration. Evolution of the bandwidth and phase shift between the three waves after reflection onto the end cavity mirror has to be managed to maintain high conversion efficiency. Experiment show more flexible behavior than expected with theory. This lead to fine wavelength control on the overall emission spectrum (over 1 μm) without using crystal temperature tuning that slow down tuning speed.

Optical Parametric Oscillator (OPO) is a well-known solution when wide tunability in the mid-infrared is needed. A specific design called NesCOPO (Nested Cavity doubly resonant OPO) is currently integrated in the X-FLR8 portable gas analyzer from Blue Industry and Science. Thanks to its low threshold this OPO can be pumped by a micro-chip nanosecond YAG (4 kHz repetition rate and a 30 GHz bandwidth). To achieve very high resolution spectra (10 pm of resolution or better), the emitted wavelength has to be finely controlled. Commercial Wavemeter do not meet price and compactness required in the context of an affordable and portable gas analyzer. To overcome this issue, Blue first integrated an active wavelength controller using multiple tunable Fabry-Perot (FP) interferometers. The required resolution was achieved at a 10 Hz measurement rate. We now present an enhanced Wavemeter architecture, based on fixed FP etalons, that is 100 times faster and 2 times smaller. We avoid having FP ‘blind zones’ thanks to one source characteristic: the knowledge of the FSR (Free Spectral Range) of the OPO source and thus, the fact that only discrete wavelengths can be emitted. First results are displayed showing faster measurement for spectroscopic application, and potential future improvement of the device are discussed.

Cost effective multi-wavelength light sources are key enablers for spectroscopic applications at Mid-IR wavelength range. Utilizing a novel Mid-IR Si-based photonic integrated circuit filter and wide-band Mid-IR SLEDs, we show the concept of a light source that covers 2.7…3.5 μm wavelength range with a resolution <1nm. The spectral bands are switchable and tunable and they can be modulated. The source allows for the fabrication of an affordable multi-band gas sensor with good selectivity and sensitivity. The unit price can be lowered in high volumes by utilizing tailored molded IR lens technology and automated packaging and assembling technologies.

The status of the development of the key components of the light source are reported. The Mid-IR PIC is based on the use of thick-SOI technology, SLED is based on AlGaInAsSb materials and the lenses are tailored single crystal, nonoxide glass and heavy metal oxide glasses fabricated by the use of hot-embossing. The packaging concept utilizing automated assembly tools are depicted.

In safety and security applications, the Mid-IR wavelength range covered by the source allows for the detection of several harmful gas components with a single sensor. At the moment, affordable sources are not available. The market impact is expected to be disruptive, since the devices currently in the market are either complicated, expensive and heavy instruments, or the applied measurement principles are inadequate in terms of stability and selectivity.

Recent advances in the deposition of patterned thin film spectral filters have enabled a new class of radically miniaturized spectral sensors. This new technology enables numerically large arrays of spectral bandpass filters with unprecedented manufacturing economy. For example, a 64-channel array occupying two square millimeters and spanning 400-900 nm can be deposited with as few as eight coating steps. Mating this filter array to a photodiode array yields a tiny multispectral sensor with diverse applications.

The bandpass filters are single-cavity Fabry-Perot designs with common top and bottom mirrors. The dielectric spacer layer between them determines the passband wavelength and is patterned to differing thicknesses using a binary scheme, i.e., each successive “sub-spacer” layer is half the thickness of the previous one. The technical challenge is uniformly patterning and depositing thinner and thinner sub-spacers, which can be only a few nanometers thick. We have demonstrated 64-channel arrays covering the spectral range of 400-900 nm and 775-1075 nm.

These arrays have been mated to high-responsivity 2D silicon detectors, in much the same way that linear variable filters are mated to linear detector arrays. The resulting sensor is less than 3 x 3 x 1 mm in size and ideal for integration into mobile devices, wearable electronics, autonomous aerial vehicles, and countless industrial applications. Sensor performance is currently being evaluated for food quality and freshness measurement, drug identification, fuel quality measurement, explosives detection, colorimetry and illumination measurement, solar flux monitoring, remote sensing, and myriad other applications.

This paper discusses the two-step fabrication of a novel in-plane Si-air linear variable optical filter (LVOF). LVOF has alternating quarter-wave stack layers of high refractive and low refractive index materials sandwiching a tapered cavity. Different passbands can be observed at various positions along the length of the filter. Challenges of LVOF fabrication include depositing consistent thickness of quarter-wave stacks and precise control of the taper angle to be in the range of milli-degrees. In many instances, due to the limitations of thin film deposition systems, surface roughness and deposition thickness vary across entire wafer surface. Such deviations could result in different LVOFs possessing varying response to input signal.

Electron-beam lithography (EBL) was utilized for accurate patterning of Si pillars and taper angle which are difficult to achieve using traditional fabrication methods. In the absence of hardmask, SU-8 was used for pattern transfer with Si:SU-8 etch selectivity as high as 60:1. By optimizing SF₆ and C₄F₈ gas flow and time parameters, aspect ratio of 10:1 and almost- 90° pillars were deep etched into Si with scallop depth <30 nm. High Bragg contrast mirrors were obtained with [HLH]-wedge-[HLH] configuration.

This LVOF operates in free space with continuous tuning from 3.1-3.8 μm. FWHM of 95 nm is observed at 3.3 μm. Simulation and other characterization results are discussed. Finally, the proposed LVOF can be wafer-level packaged with normal incidence detector array, suitable light source and other essential optical elements.

Wedge or staircase micro-optics have become important components for building miniature optical spectrometers. These devices create spectral discrimination through interference between beams resulting from reflections at the surfaces of the optic. The literature has examples of low reflectance wedge spectrometer system where the Fourier transform is used to recover the spectrum (with no inherent bandwidth limit), and high-reflectance, band-limited simplex spectrometers where no data processing is required. Instruments in the first category tend to be for the thermal infrared range, and instruments in the second category are more often encountered in the visible band. This second category includes linear variable filters and discrete etalon staircases. Though in practice, the signal treatment for these two types of spectrometers is radically different, the underlying interference mechanism is identical. It follows, that a single signal processing algorithm must exist which correctly treats the two types of signals. We present a mathematical description of the signal model for such spectrometers. We show that in the case of spectrally uniform reflectance, the signal has a specific relationship to the spectrum’s Fourier transform. We cast the spectral recovery problem as a matrix inversion, and derive formulas for calculating the solution matrix. The solution matrix is shown to yield the exact spectrum when applied to modeled wedge spectrometer signals in both low and high reflectance cases.

Due to the growth of miniature unmanned aerial vehicles (UAVs) and small spacecraft (SmallSats) in recent years, there has been a push for the development of miniaturized spectral imagers to be incorporated with them. An efficient, compact hyperspectral imager integrated with these vehicles provides a cost-effective platform for environmental sensing applications that include the monitoring of agriculture, vegetation, geology, and pollutants. We present here the development and integration of a hyperspectral imaging system called the SNAP-IMS, originally used for biomedical detection, with an Octocopter UAV. The entire collected hyperspectral data cube is 350x400x55 (x,y,λ) spatial/spectral samples. The final system enclosure (288 mm x 150 mm x 160 mm) weighs 3.6 kg (7.9 lbs), offering minimal size and weight. The payload’s power consumption is marginal as there are no mechanical scanning components; the existing power requirements are dedicated exclusively to CCD frame acquisition. Experimental testing included several flights on board the Octocopter UAV, acquiring hyperspectral data cubes at 1/100 second. Snapshot mode and short integration times mitigate motion artifacts. The low size, weight, and power consumption can offer longer and higher flights at smaller drone sizes. These improvements augment the potential for additional instrument incorporation (i.e. LiDAR, Multi-spectral IR) in the future. Imaging results and system description are presented and discussed.

Ideally, a chemical analysis instrument should to be brought to the sample for (near) real-time analysis onsite (rather than bringing a sample to a lab for analysis, as is usually done). In this paper, this paradigm shift is addressed using battery-operated microplasmas. But, how does one introduce an initially ambient temperature sample into a low-power (~10 W) gas-phase microplasma? One way is by using an eletrothermal vaporization sample introduction and a vaporization chamber for introduction of micro- (and nano-size) samples into a microplasma. But then, how does one develop an “optimized” vaporization chamber? To reduce cost and time-delays, rapid prototyping (via 3D printing) and smoke experiments were used, as detailed in this paper. In the future, candidate designs will be evaluated using CFD simulations.

Despite massive international search efforts, the detection and identification of hazardous materials is still a persisting challenge. Here, a modular, compact Raman spectrometer for short distance detection and imaging of traces of explosives and related compounds was developed. It utilized green excitation sources (a laser pointer, or a hand-held laser) and the resultant beam was focused as point or line on particular samples. The ensuing signals were detected and employed for sensitive and selective detection of different liquids and individual particles, placed on a variety of substrates. Synchronization of sample movement with detector readout, allowed obtaining not only characteristic Raman signals but also Raman images of traces of different compounds. Imaging was carried out by point-mapping, photo-guided sampling/point-mapping and line-mapping, where the latter two approaches allowed detection and mapping of individual particles at considerably reduced sampling times. It is anticipated that realization of these operation modes can lead to improved performance and be applied for examination and identification of objects contaminated with harmful compounds. This suggests the possibility for future use of the setup as a fast, compact, and relatively low-cost system for detecting and mapping a variety of samples under lab conditions and, following additional improvements, for field measurements.

Raman spectroscopy is routinely used in the laboratory for detection, chemical identification, and quantitative measurements of complex molecular compounds. One key advantage of the method is that a single laser wavelength can be used to identify and measure several different molecular compounds simultaneously. While Raman spectroscopy is a powerful technique, it is a very inefficient process where only one in 1011 scattered photons contain the desired vibrational information. Several techniques have been developed to enhance Raman scattering, which are typically applied to liquids and solids such as surface enhanced Raman spectroscopy and coherent anti-Stokes Raman spectroscopy. For gas phase measurements, photonic crystals, cavity enhanced Raman spectroscopy and functional waveguides have been developed to provide Raman enhancement. However, Raman spectroscopy has seen limited use in commercial and military applications due to instrument complexity, sample preparation, acquisition time, and spatially localized point measurements.

A recently developed technique to enhance spontaneous Raman scattering utilizing a highly reflective integrating cavity is presented. Elastically scattered light circulates within the cavity volume continuously interacting with the sample, whether a bulk sample or gas, resulting in significant Raman enhancement. In addition, the Raman scattered light is collected from all directions before being coupled out of the cavity. Enhancements of 10⁷ have been realized with the use of inexpensive low power diode lasers and a modest CCD based spectrometer. Application of the iCERS technique operating near 400 nm providing near real-time detection and measurement of trace gases, chemicals, and biological compounds is discussed.

Portable hyperspectral imagers enable real time decision-making in application areas such as threat detection, forensics, environmental and agricultural monitoring and biomedical screening. High spectral and spatial resolution provide far more actionable information than obtainable by multispectral imagers. High-resolution VNIR reflectance imagers characterize photosynthetic productivity via solar-induced fluorescence (SIF) monitoring, representing a substantial improvement over earlier multispectral analytics such as normalized difference vegetation index (NDVI). Handheld Raman instruments enable high speed screening for explosives, narcotics and hazardous materials. A line scan/area imaging approach detects trace quantities of target materials, such as small particles of explosives or narcotics, within a bulk sample.

Optical (visible) microscopy has established itself as an invaluable tool in Materials Science, and perhaps the canonical technique in Biology. Unfortunately, an optical image provides extremely limited information regarding chemical composition. A great deal of effort has been spent to circumvent this limitation through the use chemical dyes - most notably fluorescent markers - to achieve contrast between different chemical species. Infrared spectroscopy is an ideal technique to chemically fingerprint materials. Specifically, infrared microspectroscopy holds great promise as a labelless technique to achieve chemically specific microscopy. Unfortunately, the long wavelengths of the infrared lead to low spatial resolution in infrared microscopy, on the order of several microns. Traditionally, this limitation has been circumvented via scanning probe techniques such as s-SNOM and AFM-IR. While the scanning probe techniques provide excellent resolution, their contact nature and low signal levels limit the speed at which images can be acquired. We have developed a new technique to collect infrared hyperspectral images below the IR diffraction limit. Optically Sensed photothermal InfraRed Imaging micro-Spectroscopy (OSIRIS) permits the construction of infrared images on a resolution limited by the wavelength of the probe beam. In this technique, an infrared laser is used to excite the sample, while a short wavelength probe beam senses the resultant change in temperature. With this technique, hyperspectral images can be acquired orders of magnitude faster than the scanning probe techniques. Furthermore, a confocal setup permits tomography, which is extremely limited in the scanning probe techniques due to their surface nature.

Hyperbolic metamaterials (HMM), a class of artificially engineered materials with a highly anisotropic permittivity response originating from opposite signs of the principal components of the electric tensor, have attracted significant interest in recent years due to their ability to manipulate the propagation light in exotic ways. Such materials enable distinctive optical phenomena such as negative refraction, super-resolution imaging, and enhanced spontaneous emission. Here we exploit the hyperbolic iso-frequency characteristic of a planar type-II HMM (composed of alternating, sputtered films of Ag and SiO2) to achieve high-sensitivity proximity detection of metallic and dielectric nanoparticles in a transmission dark-field configuration. The iso-frequency surface is unique in that propagation of light inside the HMM over the entire visible-range is allowed only for electromagnetic modes having tangential spatial frequencies kx exceeding the free-space wavevector k0 by over a factor of two (kx > 2k0). The nanoparticle detector consists of a 500-nm thick slab of HMM having an input side coated with ≈ 6-nm-thick Ag nano-islands (to couple light into the HMM) and an exit side consisting of a template-stripped ultra-smooth Ag surface. As a result of the optical bandgap of HMM, light illuminating the input surface at any angle (intensity I0) is effectively blocked from transmitting through the slab; only a vanishingly small evanescent-amount (intensity I1) leaks through, corresponding to an optical density OD = log(I1 / I0) ≈ 8 at λ0 = 633 nm. Bringing nanoparticles into deep-subwavelength proximity or contact with the pristine exit-surface of the detector opens up efficient transmission channels, corresponding to out-scattering of high-spatial frequencies into free space propagating modes (yielding an intensity for a given nanoparticle density, and an optical contrast ratio for detection defined as γ. FDTD simulations for an HMM structure having a perfectly flat exit-surface decorated with spherical gold particles of diameter 100 nm predict values of γ as high as ≈ 890. Two-dimensional finite-difference-time-domain (FDTD) simulations predict high-contrast detection of particles of diameter down to ≈ 10 nm whether composed of metals (Ag, Cr) or dielectric (SiO2). We experimentally demonstrate that this HMM-based structure is capable of revealing in transmission spherical Au nanoparticles of diameter ≈ 40 nm deposited on its template-stripped surface. Incoherent light is used to illuminate the Ag nano-islands side of the detector, and a 100x objective lens (NA = 0.75) is used to collect the light exiting the template-stripped side of the device. The incoherent light is obtained by filtering a white-light LED source with a bandpass filter centered at λ0 = 633 nm (bandwidth = 92 nm). The use of broadband incoherent light, rather than a coherent laser source, minimizes the sizes of speckles created by the Ag nano-islands. Effective medium theory (EMT) predicts relatively constant type-II hyperbolic iso-frequency characteristic throughout this frequency band. The Au nanoparticles are randomly dispersed onto the surface, yielding an average inter-particle distance of ≈ 5 μm, as measured by scanning electron microscopy. The optical transmission images clearly indicate the presence of the Au nanoparticles on the detector surface, which appear as bright spots. The average contrast ratio γ for single Au nanoparticles is measured to be ≈ 22, with it being presently limited by residual roughness of the template-stripped Ag surface. In conclusion, we exploit the hyperbolic iso-frequency characteristic of a planar type-II HMM to achieve optical proximity detection of nanoparticles, using a simple transmission scheme not requiring the use of dark-field optics. In this talk, we will further discuss recent results of fluorescence imaging performed under the same HMM platform, and efforts on expanding this work towards single-molecule imaging in a wide-field configuration. Due to its high sensitivity in deep-subwavelength proximity to a surface, this HMM-based device hints at promising applications in bio-chemical sensing, particle tracking and contamination analysis. References: [1] A. Poddubny, I. Iorsh, P. Belov, P. & Y. Kivshar, “Hyperbolic metamaterials,” Nat. Photon. 7, 958–967 (2013). [2] H.N. Krishnamoorthy et al. “Topological transitions in metamaterials,” Science 336, 205–209 (2012). [3] M.Y. Shalaginov et al. “Broadband enhancement of spontaneous emission from nitrogen-vacancy centers in nanodiamonds by hyperbolic metamaterials,” Appl. Phys. Lett. 102, 173114 (2013). [4] T. Xu & H.J. Lezec, “Visible-frequency asymmetric transmission devices incorporating a hyperbolic metamaterial,” Nat. Comm. 5, 4141 (2014).

Incorporating wavelength information into a monochrome image is of great interest for imaging spectroscopy. Here, we show that by exploiting the properties of multiple scattering materials, we can encode spectral information in a CMOS image, where the spectral resolution obtained is only limited by the scattering strength of the material. As a proof-of-concept, we demonstrate this technique using a thin multiple scattering layer of gallium phosphide nanowires and a microlens array. We achieve a spectral resolution of approximately 4 nm and a reconstructed image containing 64 pixels. We demonstrate that a computational technique which is commonly used in compressive sensing can be used to reconstruct both sparse and dense spectra, when undersampling and oversampling a signal, respectively. This method provides an ultra-compact solution to obtaining both spatial and spectral information in one measurement, for potential use in portable spectroscopy.

Due to complexity and bulkiness of instrumentation, it has been highly difficult to make hyperspectral measurements on the location where the actual samples are. Also, demanding off-line data processing has prevented in obtaining immediate results for decision making and actions on the measurement spot. To solve these issues Specim Ltd has introduced a mobile hyperspectral camera operating in the 400-1000nm spectral range. Based on a spectrometer with push-broom technology, this new mobile camera collects 10-20 times more light from the target than filter based cameras. It results in higher SNR and/or quicker image acquisition in similar illumination conditions. In addition to the portability, the camera has integrated data processing capability and easy user interface. For the first time, the applications can be uploaded in the camera and obtain push-a-button results. The applications change the camera operations, and data processing to match the user requirements. The software included to the camera is used to generate the applications with the graphical user interface without a need for programming. The camera offers a ready-to-go hyperspectral platform for various OEM industries in agriculture, food safety, law enforcement, and health care to rapidly develop application solutions to their end users. Similarly, researchers and professionals in several application fields can quickly adopt the camera to their everyday work. This paper illustrates the design targets and challenges, as well as the full features in the mobile hyperspectral imager. In addition to the technological description, the results from the pilot phase field and laboratory tests are shown.

The detection of threats from a distance within a complex background is a valuable and often lifesaving utility. Hyperspectral Imaging (HSI) allows for the detection of threat materials in convoluted environmental scenes, where the target of interest is typically low in concentration and co-located with interferrants and varying substrates. The ideal HSI sensor is able to automatically assess scenes in near real-time and at a safe standoff distance to avoid risk of harm to the operator and equipment. Unfortunately, many current generation HSI sensors have limitations that hinder the ability to effectively handle these situations. These systems often have an inefficient area search rate, do not provide near real-time measurements, or require the user to be in close proximity to the material in question.

ChemImage Sensor Systems (CISS) has developed a wide-field, portable shortwave infrared hyperspectral imaging (SWIR HSI) sensor called VeroVision™ Threat Detector capable of detecting threat materials (i.e. explosives, narcotics) at standoff distances for a variety of applications. The portable HSI sensor has been designed to analyze complex scenes at near real-time speeds. The sensor utilizes SWIR absorbance spectroscopy to provide molecular discrimination of the target area at standoff distances from 1 to 20+ meters. The sensor has a custom developed software package that provides the operator the ability to adapt its spectral library to incorporate new threat materials or change the type of target of interest.

This paper will provide an overview of VeroVision as well as a discussion on how it may be effectively applied to room and vehicle clearing applications.

There is a growing demand for effective detection of hazardous materials at safe distances in real-time with a high degree of autonomy. In an effort to address this need, ChemImage Sensor Systems (CISS) in collaboration with the Carnegie Mellon Robotics Institute has developed a novel, adaptable, short-wave infrared (SWIR) hyperspectral imaging system for real-time standoff detection of hazardous materials (e.g., explosives, narcotics, etc.). At the heart of this system is the Conformal Filter (CF), which is a liquid crystal (LC)-based tunable filter that transmits multi-band waveforms. Building on concepts of multivariate optical computing, the CF is tuned electro-optically and dynamically to mimic the functionality of a discriminant vector for classification. The resulting integrated detector response approximates the detection response of conventional hyperspectral imaging with only two discrete measurements instead of hundreds to thousands. Real-time detection is achieved by operating two CFs in tandem within a dual polarization (DP) system, which exploits the polarization sensitivity of the LC filters and allows for simultaneous acquisition of the compressed hyperspectral imagery. This improved sampling rate coupled with advanced object recognition, semantic scene understanding, and image reconstruction algorithms enables real-time (i.e., >10 detection fps), on-the-move detection of targets.

This paper will discuss the development, characterization, and test results of the first generation SWIR DP-CF imaging sensor, with a focus on its application to explosives and narcotic threat detection.

Next generation sensors will all share in common the requirement to move increasingly massive amounts of data. As such, an infrastructure problem becomes apparent. Even if instruments can produce quality data, it is not necessarily feasible to collect and move it. With the rapidly growing number of sensors, basic data movement becomes an integral system-engineering problem. Wireless networks are being increasingly employed as part of that infrastructure, but may be rapidly overwhelmed, particularly in currently regulated frequency bands. These facts compel the development of terahertz wireless systems, which if implemented correctly could support the massive flow of `cloud', IoT, and distributed data. While such terahertz systems are continually growing closer to practical reality, they are still very immature. From a system-engineering perspective, it is apparent that there are even many fundamental gaps in knowledge that prevent reliable operations. Indeed, terahertz absorption through the atmosphere is still not fully understood, nor even correctly tabulated in some cases. We present new studies on terahertz propagation using comparisons to previous data and the international standards that commonly underpin system-level engineering of wireless systems. In particular, we examine the role of continuum absorption and high frequency absorption wings, and the method by which they are accounted for in engineering standards between 0-1 THz. These studies reveal a need for greater accuracy in atmospheric measurements.

In recent years, the THz field has gained considerable interest in the scientific community due to a multitude of potential applications, ranging from spectroscopy to communications. However, there is a lack of fundamental device components that are needed to propel the field from mere scientific curiosities to real-world applications. And, the quest for high performance, energy efficient, and low cost device architectures that could manipulate THz radiation is an on-going endeavor. Here, we review recent work on how to fabricate several fundamental THz devices by revitalizing an age old, little known, and unconventional material-design technology called artificial dielectrics. These are man-made media that mimic the properties of naturally occurring dielectric media, or even manifest properties that cannot generally occur in nature. For example, the well-known dielectric property, the refractive index, which usually has a value greater than unity, can have a value less than unity in an artificial dielectric. Using artificial dielectrics, we demonstrate a lens that can focus THz radiation, a polarizing-beamsplitter that can split an arbitrarily polarized beam into two linearly polarized orthogonal components, a quarter-waveplate that can change a linearly polarized beam into a circularly polarized one, and an isolator that can minimize harmful back-reflections. These artificial-dielectric devices exhibit remarkable performance characteristics, exceeding what has been reported in the literature, in some cases by several orders of magnitude. Indeed, our device specifications rival those of similar functional devices commercially available for optical wavelengths. Furthermore, the inherent simplicity of the device geometry makes these devices inexpensive to fabricate.

Dipole induced quadrupole resonance leads to transmission peak within the broad dipole absorption as shown in plasmonic metamaterials. We show quadrupolar interactions as a new paradigm to control this classical analogue of electromagnetically induced transparency (i.e. plasmon induced transparency, PIT) in metamaterials. While the asymmetry factor of the resultant Fano line shape of a EIT spectrum limits the quality factor (Q) of the resonance and thus the sensing application, we show that quadrupolar interactions give a handle on the Q factor. A Q as high as 600 is seen in simulations at about 0.977 THz. Limited by the experimental resolution, a Q of about 100 is observed. Further plasmonic structures can be designed to make use of the quadrupolar interactions for high sensitive devices at THz frequencies.

Although unique potentials of terahertz waves for chemical identification and material characterization have been recognized for quite a while, the relatively poor performance of current terahertz spectroscopy and spectrometry systems continue to impede their deployment in field settings. This talk describes some of the recent advancements in terahertz spectroscopy systems by using optically-pumped plasmonic photoconductors for terahertz wave generation and detection. Incorporating plasmonic nanostructures inside the active area of photoconductive terahertz sources and detectors offers enhanced quantum efficiencies while maintaining ultrafast operation. This enhancement is due to the unique capability of plasmonic nanostructures to significantly increase the concentration of photo-induced carriers inside the device active area, where they interact with a bias/terahertz electric field to generate/detect terahertz radiation. By the use of this powerful technique, broadband terahertz spectroscopy with record-high signal-to-noise ratio levels exceeding 110 dB and broadband terahertz spectrometry with quantum-level sensitivities are demonstrated. Such high-sensitivity terahertz spectroscopy and spectrometry systems could offer numerous opportunities for e.g., biomedical sensing, atmospheric studies, space explorations, pharmaceutical quality control, and security screening applications.

Long-distance propagation of THz pulses is important to THz communication and monitoring pollutants and dangerous gases. The THz pulses are propagated through 186 m and 910 m distance which are approximately equal to 52 and 255 round-trips of the circulating 50 fs optical pulse within the mode-locked ring laser, respectively. When the humidity and temperature are continuously varied during the measurement, the THz pulses also continuously shifted with 2.36 and 15.53 ps/(g/m3) ratio for a 186- and 910-m propagation, respectively. The complexity of the atmosphere required the use of the complete theory of Essen and Froome to compare the measured time shifts due to both the dry atmosphere and water vapor with theoretical calculations. When the THz pulses were sequentially measured for a distance of 186 and 910 m at the same weather condition, the time variation due to atmospheric turbulence between the two pulses of the 910 m measurement was up to 4 times larger than that between the two pulses of the 186 m measurement. Meanwhile, the characteristics of the N2O and CO gases are investigated 93 meters away from the THz transmitter using the THz pulse which propagates an outside environment of 79 m between two buildings. The natural resonances of the gases are detected in the 0.5 THz bandwidth except water vapor resonances.

Terahertz (THz) spectroscopy is a nondestructive method that has the ability to identify many hazardous materials by investigating their low frequency vibrational modes (0.1-6.0 THz). Ammonium nitrate (AN), often used in improvised explosives, exhibits featureless reflection/transmission spectrum at THz frequencies at room temperature. However, the low frequency vibrational modes exhibit strong temperature dependence below room temperature (<240k) due to the polymorphic phase transitions. In this work, we study the effective dielectric properties of AN embedded in a polytetrafluoroethylene (PTFE) host medium using terahertz time domain spectroscopy in the temperature ranging from 5K to 300K. The dielectric properties of pure AN were extracted using three different effective medium theories (EMT): (i) the simple effective medium approach, (ii) the Maxwell-Garnett (MG) model, and (iii) the Bruggeman (BR) model. The dielectric properties obtained from theoretical approximations agree well with the experimental values. We identified six lattice vibrational modes between 0.2-3.0 THz that are associated with the polymorphic phase transitions at low temperature.