This PDF file contains the front matter associated with SPIE Proceedings Volume 10980, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Cultural heritage materials are often fragile, unique and have little documentation about their history and hence what is best for their preservation. Challenges of cultural heritage materials therefore necessitate a focus on non-invasive analytical techniques with a large component of the investigation being a forensic-like approach to recreate the history of use, the impact of various environmental parameters and provenance. The results of these analyses are then used to determine degradation mechanisms and how best to preserve these historic items for future generations. Cultural heritage materials include a range of material types and go from antiquity to yesterday – historic and modern polymers, paper and parchment as substrates, inks and colorants and the media used to capture and record our history, photographic processes. Other collections include glass, metal, wood, ceramics, textiles, magnetic tape, wax cylinders and other sound and storage media. Paper and parchments are prepared and manufactured in many different ways with different sizings, trace metals that can become corrosive at specific levels in both papers and pigments, pigments having a range of binder materials, all impacting the way materials degrade and the decisions needed to stabilize and reduce loss of cultural information, and historically, all requiring sampling for accurate characterization and identification. This diverse composite of characteristics greatly complicates the challenges of preserving our cultural heritage. The Preservation Research and Testing Division (PRTD) at the Library of Congress have created a scientific reference sample collection – the Center for Library Analytical Scientific Samples (CLASS) – with a collection of all the materials found in heritage items. These reference materials have been analyzed with a range of instrumentation and analytical techniques – both non-invasively and through destructive samples – to provide baseline data that can then be correlated with properties found in historic collection items, and with the destructive testing and accelerated aging, can then be used to predict longevity and the impact of various environmental conditions. In addition, often many rare collection items can only be in the research labs for short periods of time, so a go-team of staff has been created to efficiently collect a diverse range of complementary data from the specific collection item in that time. These portable and lab-based instruments include multispectral imaging (reflectance, transmitted and raking) as the baseline mapping of the object, Fourier Transform Infrared spectroscopy (FTIR) – diffuse and reflectance, X-Ray fluorescence spectroscopy (XRF) point and linescan, Fiber Optic Reflectance Spectropscopy (FORS), 3D fluorescence spectroscopy, Raman spectroscopy and other specific techniques that best relate to the material type, and micro-sampling with size exclusion chromatography (SEC) has been correlated to loss of mechanical properties to predict impact of treatments. As a further preservation component, extensive testing of volatile organic compounds (VOCs) with thermal desorption gas chromatography mass spectroscopy (TD-GCMS) to quantify and identify damaging emissions from building and storage materials, Detecting changes due to exposure of historic documents and objects to various environmental conditions, treatments and the assessment of changes during exhibition of light-sensitive materials is critical for preservation and monitoring deterioration or changes due to these conditions assists in understanding the degradation mechanisms. Multivariate image analysis, chemometrics and data fusion processing techniques have been used to analyze these datasets using Solo+MIA, ImageJ, Matlab and ENVI software. Examples of linked spectroscopic datasets will be discussed for a range of historic documents including 15th century Block books, founding father documents, unique historic comic book hero’s, 21st century political cartoons, pre-Columbian ceramics, textiles, historic pigments and sound recordings.

The spatial-spectral compromise inherent in hyperspectral imaging (HSI) refers to one dimension of an HSI detector being devoted to spectral information instead of a second spatial dimension. By dividing light into contiguous bands of energy, HSI systems decrease recorded photon count per pixel from a scene often by a factor of 100 compared to typical RGB or panchromatic imaging systems, and by one to two orders of magnitude for their multispectral imaging (MSI) counterparts. This proves particularly problematic when imaging cultural heritage objects which may be sensitive to damage from intense illumination. A method to counter the signal loss is to sacrifice spatial resolution, increasing a pixel's projected footprint on a target, thus increasing the number of photons striking the pixel per time interval. Panchromatic sharpening is then applied in this research to visually recover the decreased spatial resolution. The increase in ground sample distance (GSD) can be accomplished physically, by increasing the imaging distance. Though this method does not increase the signal itself, as the projected area of the detector does not change, it does allow for quicker scan times over a given area. This allows the user to increase integration time, resulting in a higher integrated signal recorded over similar total scan times than at higher resolutions or lower imaging heights. In this research, a 14th century manuscript was imaged with an HSI detector under museum lighting levels of 50 lux. Here we show that as the spatial resolution was digitally downsampled by factors of approximately two and four, from 333 pixels per inch (ppi) to 161 and 80 ppi, the signal-to-noise ratio (SNR) was effectively increased by a factor of 2.00 and 2.79. Even after being spatially downsampled, the sharpened images were up to 1.5 times spatially sharper than the reference HSI image. Relatively high spectral accuracy was also maintained, with spectral angle mapper (SAM) measurements of 0.0527-0.0963.

A model for standard line-scan imaging spectrometers with circularly symmetric optics is presented. Using reasonable approximations and special cases to maintain clarity, straight-forward analysis demonstrates design options are constrained due to lack of parameters that can readily be engineered. As a result, compromises must be made. This motivates line-scan imaging spectrometers that utilize anamorphic optics. The presented anamorphic design provides an additional degree of engineering freedom and eliminates the need for compromises required in standard systems. Results from a prototype anamorphic line-scan imaging spectrometer are provided, including Signal-to-Noise Ratios and sample imagery.

Hyperspectral imaging in the SWIR wavelength region (1000 nm – 2500 nm) makes it possible to map mechanisms for recombination of photogenerated charge carriers in semiconductors. These mechanisms are linked to imperfections and impurities and lead to decreased performance in solar cells. The hyperspectral camera is mounted with a line laser at wavelength 808 nm, an energy high enough to excite electrons from the valence band to the conduction band in the Silicon material. The camera records radiative photoluminescence from the material, both the Silicon band to band recombination as well as recombinations from trapped electrons due to imperfections. In combination with advanced multivariate techniques for data analysis, this technology have been used to study defects in both multicrystalline and monocrystalline wafers and solar cells. Some of the mapped defects have been linked to well known mechanisms that could previously only be studied by destructive and time consuming methods. So far this technique has only been explored in research laboratories. The goal is however to be able to use this in line in solar cell processing. In this presentation it will also be discussed how this technology can be used to map degradation of outdoor solar panels.

In this project we design and fabricate a hyperbolic metamaterials-based narrowband notch filter for the mid-wave infrared regime with an angle-of-incidence independent center-wavelength for TM polarized incident light. To achieve angle of incidence independence, a subwavelength sized array of copper wires is inserted in a vertical orientation and permeates the three middle layers of the seven layer Bragg stack filter. Analysis using Maxwell- Garnett theory and full-wave electromagnetic modeling, and the fabrication progress to date are presented. Narrowband notch filters have applications in optical communications systems, and remote sensors such as hyperspectral and multi-spectral sensing and imaging.

Metasurfaces, nanophotonic arrays of phase shifting elements, hold promise for the miniaturization of a variety of bulk optical elements, most notably lenses and imaging systems. Owing to the flexibility with which their constituent elements may be engineered, metasurfaces allow for point-to-point polarization control on a subwavelength scale. For this reason, metasurfaces represent an exciting new platform for polarization optics as well. I will discuss how this functionality allows for a new perspective on diffractive optics which explicitly acknowledges the vectorial nature of light. This perspective motivates a theory of unitary polarization gratings; I will derive a few key results concerning these gratings. I will discuss and demonstrate how this perspective allows for the design of metasurfaces with new polarization functionalities. I will describe how, through relatively simple optimization methods, a metasurface producing arbitrarily specified polarization states can be designed. This functionality is equivalent to a traditional diffraction grating with individual waveplate optics on each order; here, all the necessary polarization optics can be integrated into a flat, ultrathin optical element. Moreover, such a metasurface can be used in a reverse configuration as a parallel snapshot polarimeter with no need for additional polarization optics. I present a detailed experimental characterization of this device in the visible spectral region and a comparison of the performance of the metasurface to a commercially available rotating waveplate polarimeter. Finally, I will discuss the extension of these concepts to compact polarization imaging systems and will provide a broad outlook on metasurfaces in polarization optics, polarization sensing systems, and polarization instrumentation more generally.

Fungal pathogens constitute the greatest economical concern to corn farmers; they result in yield losses, grain quality reduction and production of mycotoxins. Improvement of detection methods are imperative. This work aimed to examine corn fungal pathogens with HSI. Isolates of Fusarium spp. and Stenocarpella spp., were plated on growth media in glass Petri dishes in triplicate, and incubated at 25°C for 9 days. Images were acquired with a SisuChema short-wave infrared pushbroom imaging system in the spectral range 920 – 2514 nm. Principal component analysis (PCA), with various pre-processing methods, and multivariate curve resolution (MCR) were used to explore the data. PCA with or without pre-processing, revealed chemical differences within and between fungal isolates. Differences were amplified with time. Examination of the mean spectra and PC loadings after spectral pre-treatment indicated variation primarily around bands associated with water/moisture (1450 & 1930 nm), protein (2180 & 2242 nm) and carbohydrates/starch (1090, 1360 & 2100 nm). This is expected since fungi are mainly comprised of these constituents and as the mycelium grows and ages, there is a change in carbohydrate (content or structure), moisture and protein. This was apparent in higher order components (PCs 4-6) and appeared as textured information. MCR revealed similar results, however the concentration maps were clearer than PCA score images. In addition, these maps were textured illustrating the physical changes of the mycelium with time. These were due to the growing hyphae and possible spore formation. In addition, it is likely that these concentration maps indicated presence of mycotoxins.

Since hyperspectral imaging has been developed as a powerful technique in earth remote sensing, this technique continues being utilized in medical, biological, agricultural, and industrial areas. This technique is capable of providing radiometric measurements over contiguous spectral ranges for every pixel of an image. Thus, data from hyperspectral images contain two–dimensional spatial information and spectral information as well. Our research group has been working on development of hyperspectral imaging technology for food quality and safety evaluation. For the detection of contaminants on broiler carcasses in poultry industry, real-time hyperspectral imaging system with line-scan platform has been developed and tested at processing line speed. In particular, hyperspectral imaging methods at microscopic level have been developed for food safety applications, because early and rapid detection of foodborne pathogen during food processing are crucial to protect foodborne illness and prevent outbreaks to the public. In this presentation, the state-of-the-art hyperspectral microscope imaging (HMI) methods to identify and classify foodborne bacteria are reported. More detail, research findings regarding development of HMI methods including immobilization of live bacterial samples on microscopic glass slide for HMI scan, dark-field illumination for scattering intensity from bacterial images at cellular level, acousto-optic tunable filters (AOTF)-based HMI image acquisition from live bacteria, HMI image processing and analysis at both spatial and spectral domains, development of classification models with deep-learning as well as various chemometric methods, and finally, early and rapid detection of foodborne pathogens with HMI technique for food industry applications are presented.

Sustained and enhanced land imaging is crucial for providing high-quality science data on change in land use, forest health, environment, and climate. Future thermal land imaging instruments operating in the 10-12 micron band will provide essential information for furthering our hydrologic understanding at scales of human influence, and producing field-scale moisture information through accurate retrievals of evapotranspiration (ET). To address the need for cost-effective future thermal land imaging missions we are developing novel uncooled doped-silicon thermopile detectors, an extension of a detector design concept originally developed at NASA-Goddard for planetary science applications. These doped-Si thermopile detectors have the potential to offer superior performance in terms of sensitivity, speed, and customization, when compared to current commercial-off-the-shelf uncooled detector technologies. Because cryocooler technology does not need to be fielded on the instrument, these and other uncooled detectors offer the benefit of greatly reduced instrument cost, mass, and power at the expense of some acceptable loss in detector sensitivity. We present the motivation for an uncooled thermal imaging instrument, our doped-Si thermopile detector concept, and performance expectations and comparisons. We also provide an update on the current status of this detector technology development.

Currently at NIST, there is an effort to develop a black array of broadband absolute radiometers (BABAR) for far infrared sensing. The linear array of radiometer elements is based on uncooled vanadium oxide (VO_x) microbolometer pixel technology but with the addition of two elements: vertically aligned carbon nanotubes (VACNTs) and an electrical substitution heater. Traditional microbolometer pixels use a thermistor film as an absorber, which is placed a quarter wavelength above a reflector, typically limiting absorption to a narrow band from 8 μm to 15 μm. To extend the sensing range of the imaging array into the far infrared (20 μm to 100 μm), we are replacing the cavity with a single absorber of VACNTs. In addition, each pixel has an electrical substitution heater which can be used to determine equivalent incident optical power when the device is non-illuminated. This device forms the basis of an absolute radiometer eliminating the need for an external reference (e.g. blackbody source).

This paper reports the recent progress on the development of GaSb-based vertical-cavity surface-emitting lasers (VCSELs) with a record-long emission wavelength of above 4 μm using type-II quantum wells. Mid-wave infrared (MWIR) spectral region, covering the 3-6 μm wavelength range, is technologically very interesting for enabling two major application areas such as sensing and defense/security. Among several types of diode lasers, electricallypumped continuous-wave operating VCSELs seem to be the most attractive choice owing to their low-power consumption, inherent longitudinal single-mode emission, and simple electro-thermal wavelength tunability. The applicability of MWIR VCSELs for these two major areas are also discussed in this paper. Single-mode low-power (a few mWs) VCSEL operating at room-temperature with reasonable tunability is essential for the sensing application. For the advanced military application, high optical power (with at least a few watts), high-efficiency and high-brightness (>1 W/mm²) MWIR lasers are important. Given that the MWIR wavelength regime is eye-safe and has a low-loss atmospheric window, the development of next-generation MWIR laser sources is currently in high demand.

Uniformity from Lambertian optical sources such as integrating spheres is often trusted as absolute at levels of 98% (+/- 1%) or greater levels. In the progression of today’s sensors and imaging system technology that 98% uniformity level is good, but not good enough to truly optimize pixel-to-pixel and sensor image response. The demands from industry are often for “perfect” uniformity (100%) which is not physically possible, however, perfectly understood non-uniformity is possible. A barrier to this concept is that the definition and measurement equipment of uniformity measurements often need to be very specific to the optical prescription of the unit under test. Additionally, the resulting data are often a relativistic data set, assigned to an arbitrary reference, but not actually given an expression of uncertainty with a coverage factor. This paper discusses several optical measurement methods and numerical methods that can be used to quantify and express uniformity so that it has meaning to the optical systems that will be tested, and ultimately, that can be related to the Guide to the Expression of Uncertainty in Measurement (GUM) to provide an estimated uncertainty. The resulting measurements can then be used to realize very accurate flat field image corrections and sensor characterizations.

Long-wavelength infrared (LWIR) focal plane arrays (FPAs) needed for Earth Science imaging, spectral imaging, and sounding applications have always been among the most challenging in infrared photodetector technology due to the rigorous material growth, device design and fabrication demands. Future small satellite missions will present even more challenges for LWIR FPAs, as operating temperature must be increased so that cooler (and radiator) volume, mass, and power can be reduced. To address this critical need, we are working on following three technologies. 1) Type-II superlattice (T2SL) barrier infrared detector (BIRD), which combines the high operability, spatial uniformity, temporal stability, scalability, producibility, and affordability advantages of the quantum well infrared photodetector (QWIP) FPA with the better quantum efficiency and dark current characteristics. A mid-wavelength infrared (MWIR) T2SL BIRD FPA is a key demonstration technology in the (6U) CubeSat Infrared Atmospheric Sounder (CIRAS) funded under the ESTO InVEST Program. A LWIR T2SL BIRD FPA is also being developed under the ESTO SLI-T Program for future thermal infrared (TIR) land imaging needs. 2) The resonator pixel technology, which uses nanophotonics light trapping techniques to achieve strong absorption in a small detector absorber volume, thereby enabling enhanced QE and/or reduced dark current. 3) High dynamic range 3D Readout IC (3DROIC), which integrates a digital reset counter with a conventional analog ROIC to provide a much higher effective well capacity than previously achievable. The resulting longer integration times are especially beneficial for high flux/dark current LWIR applications as they can improve signal-to-noise ratio and/or increase the operating temperature. By combining the aforementioned technologies, this project seeks to demonstrate a cost-effective, high-performance LWIR FPA technology with significantly higher operating temperature and sensitivity than previously attainable, and with the flexibility to meet a variety of Earth Science TIR measurement needs, particularly the special requirements of small satellite missions.

Linear-mode avalanche photodiode arrays, LmAPDs, based on bandgap engineered HgCdTe, grown by Metal Organic Vapour Phase Epitaxy, MOVPE, can produce virtually noise-free infrared sensors. These are required for applications in big science, security and biochemistry. A custom device called SAPHIRA (320x256/24μm) has been designed specifically for LmAPDs. SAPHIRA has been deployed as a wavefront sensor for adaptive optic systems in nine major telescopes and notably five devices control the four 8.2 metre telescopes of the VLT interferometer. These demanding applications have driven frames rates up to 200 kframes/s and avalanche gains to x600 achieving read noise as low as 0.26 e- rms and enabling single photon counting. The detector is baselined for time-domain astronomy vital for exoplanet spectroscopy and understanding the physics of active stellar objects. The three 30 metre class telescopes currently under construction and the three candidate space telescopes, HabEx, Luvoir and EZE will depend on noise-free infrared detectors at very low dark current. Work at the University of Hawaii and European Southern Observatory has demonstrated dark currents in the 4-10 electrons/hour range and with avalanche gain offers the prospect of higher science return from these instruments. A 1kx1k/15μm 3-side buttable array is currently in development to service extreme low background applications especially spectroscopy. A 512x512/24μm SAPHIRA array with 64 parallel video outputs is in development for pyramid wavefront sensing on the European Extremely Large telescope, ELT, mirror co-phasing and rapid time-domain astronomy.

We have developed a new approach for rapid die-level hybridization of backside-illuminated silicon avalanche photodiode (APD) arrays to CMOS readout integrated circuits (ROICs). APD arrays are fabricated on a custom silicon-on-insulator (SOI) wafer engineered with a built-in backside contact and passivation layer. The engineered APD substrate structure facilitates uniform APD substrate removal by selective etching at the die level after bump bonding. The new integration process has the following advantages over wafer-level 3D integration: 1) reduced cost per development cycle since a dedicated full-wafer ROIC fabrication is not needed, 2) compatibility with existing ROICs that are in chip-format from previous fabrication runs, and 3) accelerated schedule. The new approach is applied to produce 32×32 100-μm-pitch silicon GmAPD arrays. Electrical performance of the APD arrays show 100% pixel connectivity and excellent yield before and after substrate removal.

Detection of ultraviolet (UV) bands provides distinct advantages for NASA, defense, and commercial applications, including increased spatial resolution, small pixel sizes, and large format arrays. Al_xGa₁-_xN semiconductor alloys have attracted great interest for detection in the UV spectral region because of their potential for high optical gain, high sensitivity, and low dark current performance in ultraviolet avalanche photodiodes (UV-APDs). We are developing GaN/AlGaN UV-APDs that demonstrate consistent and reliable UV-APD performance and operation. For these UV detectors we have measured gains of above 5×10⁶ and high quantum efficiencies at ~350 nm enabled by a strong avalanche multiplication process. These UV-APDs are fabricated through high quality metal organic chemical vapor deposition (MOCVD) growth on lattice-matched, low dislocation density GaN substrates with optimized GaN/AlGaN UV-APD material growth and doping parameters. The high performance, variable-area GaN/AlGaN UV-APD detectors and arrays can be customized to a wide variety of sizes including large-area formats to enable sensing and high-resolution detection over UV bands of interest.

This paper discusses design, fabrication, and experimental demonstration of very large-scale silicon photonic integrated circuits (VLSPIC) that include spectrometers, interferometers, and phase tuners to reconstruct spectrally resolved images. Recently-fabricated VLSPICs included 18 spectral bins and 12 baselines, successfully reconstructing reference images.

Infrared imaging systems operating without cooling is a big challenge duo to the noise originated from the semiconductor materials used for the photodetectors in the imaging system. In this paper, we report significant enhancement of plasmonic structures to photodetection of midwave infrared radiation, which make the mid-wave imaging system possible for room temperature operation. By integrating a two-dimension metallic array with InAsSbbased heterojunction photodiode, the room temperature detectivity can be enhanced to about 1010 Jones with a response speed of around 600 ns. We also achieve a dual-band enhanced photodetection by integrating a hole array atop of a GaSb/InAsSb heterostructure. By making use of localized surface plasmons in semiconductors, we extend highsensitivity photodetection to millimetre and terahertz waves. A room-temperature noise equivalent power of about 10-13 WHz-1/2 is demonstrated. Our work on surface plasmon assisted infrared photodetection for room-temperature operation make future uncooled IR systems possible.

Remote sensing systems of various kinds are the most widely used tools for acquiring information within all areas of our life. However, those optical sensors, cameras and imaging systems are susceptible to blindness and/or damages due to high power signal entering their optics. This paper presents a family of non-linear, solid-state passive wideband smart protection filters. These filters have advantages over fixed spectral filters, which permanently block only specific wavelengths, in that the wideband filter is transparent at all wavelengths until it is hit by damaging light. At input powers below threshold, the filter has high transmission over the whole spectral band. However, when the input power exceeds the threshold power, transmission is decreased dramatically. We present a novel technology for protection of any imaging system, sensors and the human eye against sun blindness as well as laser threats from the visible and up to the infrared (IR).

We describe development of spectrally tunable micro-engineered filters operating in the longwave infrared (LWIR) from 8 to 12 micron based on the guided mode resonance (GMR) phenomenon. The device structure consists of a subwavelength dielectric grating on top of a planar waveguide using high index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) with refractive index of 2.4. The filters are designed to reflect incident broadband light at one (or more) narrow spectral band while fully transmitting the other wavelengths. Tuning of the reflection wavelength is achieved by changing the angle of incidence of light by mechanically tilting the filter. Filters based on one dimensional (1-D) gratings are polarization dependent and those based on two dimensional (2-D) gratings are less sensitive to incident polarization of light. Both filters were fabricated using standard dielectric deposition and photolithographic techniques and characterized. Simple 2-layer anti-reflection coatings were applied to improve transmission over the entire spectral region. Our experimental setup consists of a modified commercial Fourier Transform Infrared Spectrometer (FTIR) system with a separate chamber for a collimated incident beam of light, focusing optics, a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector, a wire grid polarizer and a micro-engineered filter. We will present detailed characterization experiment and compare the theoretical and experimental results.

Recently, highly-integrated optical phase-locked loops (OPLLs) have been demonstrated for a number of potential applications including coherent optical communications, light detection and ranging (LiDAR) and frequency metrology. Another particularly interesting application is an optical frequency synthesis (OFS) for which OPLL-based offset locking is recently considered to be one of the most attractive techniques. There have been extensive ongoing research efforts to develop low-cost, compact, robust and power-efficient OFS systems using this OPLL-based technology. In this talk, I will discuss about a power-efficient and highly-integrated photonic system, producing low phase-noise coherent optical signal with a wavelength range of 23 nm in the C-band. In fact, the experimental results on the recently-developed highly­integrated OFS based on OPLL technology will be presented. The system includes novel InP-photonic integrated coherent receiver circuits that consume record-low (approximately 184 mW) electrical power. By employing a combination of photonic and electronic integration, this low-cost highly-integrated InP-based OFS with low-energy consumption exhibits both compact size and exceptional stability. This work is a major step towards demonstration of the true chip-scale optical frequency synthesizer with programmable <1 Hz frequency resolution, <1 cm3 volume, and <1 W electrical power consumption.

Image-based wavefront sensing uses a physical model of an aberrated pupil to simulate a point-spread function (PSF) that attempts to match measured data. Nonlinear optimization is used to update parameters corresponding to the wavefront. If the starting guess for the wavefront is too far from the true solution, these nonlinear optimization techniques are unlikely to converge. We trained a convolutional neural network (CNN) based on Google's Inception v3 architecture to predict Zernike coefficients from simulated images of PSFs with simulated noise added. These coefficients were used as starting guesses for nonlinear optimization techniques. We performed Monte Carlo analysis to compare these predicted coefficients to 30 random starting guesses for total root-mean-square (RMS) wavefront errors (WFE) ranging from 0.25 waves to 4.0 waves. We found that our CNN's predictions were more likely to converge than 30 random starting guesses for RMS WFE larger than 0.5 waves.

Sensors and imaging systems operating from visible to long-wave infrared (LWIR) spectrum are being developed for a variety of defense and commercial systems applications. Signal losses due to the reflection of incident signal from the surface of sensors and optical components limits the performance of image sensing systems. Antireflection (AR) coating technology overcomes this limitation and enhance the performance of image sensing systems. Magnolia is actively working on the development and advancement of ultra-high-performance AR coatings for a wide variety of defense and commercial applications. Nanostructured AR coatings fabricated via a scalable self-assembly process are shown to enhance the optical transmission through transparent optical components and sensor substrates by minimizing reflection losses in the spectral band of interest to less than one percent, a substantial improvement over conventional thin-film AR coating technology. Step-graded AR structures also exhibit excellent omnidirectional performance and have recently been demonstrated in various IR spectral bands.

HgCdTe is one of the most important materials for fabrication of infrared detectors and focal plane arrays (FPAs) deployed in environments where high energy particle, such as protons and neutrons, are present. We designed and fabricated HgCdTe-based FPAs that can be used in high neutron radiation environment and we measured their characteristics. The influence of the radiation on the infrared FPAs and cameras are presented. HgCdTe material and devices are capable of maintaining high performances under high energy neutron irradiation environment. For the MWIR FPA directly facing 2.59×10⁸ n/cm²·s neutron flux beam (with the highest energy 66 MeV) for 1 hour, the noise equivalent differential temperature (NEDT) increased ~ 8 times after irradiation. However NEDT decreased to 33 mK (compared to the original value of 21 mK) after one warming-up (to room temperature) and cooling-down cycle. The NEDT for the MWIR FPAs mounted parallel to the beam did not degrade (16 mK and 28 mK before irradiation, changed to 18 mK and 26 mK after irradiation, respectively).

Lead sulfide (PbS) nanocrystals have been used as the active material in high performance, solution-processed, room temperature devices, such as photodetectors, light-emitting diodes, and solar cells. The addition of a zinc sulfide (ZnS) shell to PbS nanocrystals could be advantageous for these devices because it could lead to higher/more stable photoluminescence quantum yields and reduced non-radiative recombination from electron-phonon coupling. However, while ZnS shells have been successfully added to several nanocrystals such as CdS and CdSe it has never been added directly (without a spacer layer) to PbS nanocrystals. This is because it is difficult to add shells to Pb chalcogenide nanocrystals due to their tendency to Ostwald ripen at even moderate temperatures. We have overcome this roadblock and are the first to demonstrate the synthesis of PbS/ZnS core/shell nanocrystals using a “flash” type synthesis with Zn oleate and thioacetamide as the ZnS precursors. We have found that the reaction is self-limiting and deposits a single monolayer of ZnS per shell reaction without causing the PbS nanocrystals to Ostwald ripen. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) verified the presence of the ZnS shell. Furthermore, the absorbance and photoluminescence peak energies were found to redshift upon adding the ZnS shell due to the relaxation of a ligand-induced tensile strain, as well as wave function leakage into the ZnS shell.

Several methods of additive manufacturing are being investigated for construction of microwave and millimeter-wave antennas and electromagnetic structures. These include methods such as stereo lithography and fused deposition modeling. The impacts of surface roughness, performance of dielectric lenses, and 3D printed aperture antennas are studied. The limits and process of stereo lithography printing is assessed through experimentation using the Formlabs Form 2 printer to develop WR10 waveguide and aperture antennas operating at 94 GHz. Performance and pitfalls are reported for future consideration in exploiting and extending additive manufacturing technology for fabrication of such structures in the optical regime.

Design, fabrication and assembly of curved imaging arrays is challenging. Optical design theory reveals the design challenge of a well-corrected image across a planar surface. As the field of view (FOV) is increased, deviation from the plane of best focus (Petzval surface) is increased. Reducing that deviation drives the power of lens elements higher, resulting in the increase of residual aberrations, especially zonal spherical aberrations. Wide FOV systems having customary planar focal plane arrays (FPA) require additional lenses to correct for distortions. The curved nature of a hemispherical imager minimizes off-axis aberrations and distortions throughout the FOV, and thus is of significant interest for increasing system performance while also reducing complexity, SWAP and cost. This paper will present an overview of hemispherical array development enabled by Quilt Packaging (QP) chip integration technology for creation of piece-wise curved arrays. QP is a direct edge-to-edge chip interconnection approach that can be implemented to enable hemispherical imagers created from the piecewise integration of 2D array ICs of varying geometries. The unique attributes of QP allow for an interconnection that is both robust mechanically and very low loss, high bandwidth electrically. Utilizing this approach allows for straightforward 2D wafer processing and design, reduces or eliminates the need for thinning/flexible chips, and can improve fabrication yields by reducing individual chip area. Indiana Integrated Circuits, LLC and Northrop Grumman Corporation are evaluating the potential performance, reliability and SWAP benefits of extending earlier Quilt Packaging imager work into the 3rd dimension for curved arrays.

The Fabry-Perot interferometer (FP) is a well-developed and widely used tool to control and measure wavelengths of light. In optical imaging applications, there is often a need for systems with compact, integrated, and/or widely tunable spectral filtering capabilities. We evaluate the performance of a novel integrated FP filter device consisting of an array of individually tunable MEMS FP etalons designed to operate across the visible light spectrum from 400-750 nm. This design can give rise to a new line of compact spectrometers with fewer moving parts and the ability to perform customizable filtering schemes at the hardware level. The original design was modeled, simulated, and fabricated but not tested and evaluated. We perform optical testing on the fabricated devices to measure the spectral resolution and wavelength tunability of these FP etalons. We collect the transmission spectrum through the FP etalons to evaluate their quality, finesse, and free spectral range. We then attempt to thermally actuate the expansion mechanisms in the FP cavity to validate tunability across the 400 to 750 nm spectrum. The simulated design materials set was modified to create a more practical device for fabrication in a standard CMOS/MEMS foundry. Unfortunately, metal thin film stress and step coverage issues resulted in device heater failures, preventing actuation. This FP filter array proves to be a viable design for an imaging focal plane with individually tunable pixels. However, it will require more optimization and extensive electrical, optical, thermal, and mechanical testing when integrated with a detector array.

A CMOS image sensor with off-center circular apertures for two-dimensional (2D) and three-dimensional (3D) imaging was fabricated, and its performance was evaluated, including the results of 2D and 3D images. The pixel size, based on a four-transistor active pixel sensor with a pinned photodiode, is 2.8 μm × 2.8 μm. Disparate images as well as focused images for depth calculation can be obtained using the designed pixel pattern. The pixel pattern is composed of one white subpixel with a left-offset circular aperture, a blue pixel, a red pixel, and another white subpixel with a right-offset circular aperture. The proposed technique was verified by simulation and measurement results using a point light source. In addition, the depth image was implemented by calculating the depth information from the 2D images.

This paper performs Compressed Sensing (CS) super-resolution for images sampled using pixel binning. It extends Image Sensor Engineering Toolbox (ISET) to accurately simulate the CS acquisition and reconstruction process. Using ISET allows designers to easily explore large design space and measure the performance of CS under different conditions such as lighting, noise etc. without going through the cycle of full system development. CS using binning can make CS programmable and reduce power consumed in image acquisition process by half while maintaining good reconstruction quality. This work shows the results of CS reconstruction with different sensor noise and lighting conditions. It shows that even at low Peak Signal to Noise Ratio (PSNR) of approximately 26 dB and Structural Similarity (SSIM) index of .7, the perceptual quality of image is quite good.

In this study, a methodology is developed to enhance additively manufactured surfaces for use as 3D printed optical mirrors. Utilizing vacuum deposition and pulse-reverse-current electroplating, a grain size smaller than one-tenth the wavelength can be achieved for mmWave, IR, visible, and UV. A shared-aperture, multispectral imaging system consisting of 3D printed optical mirrors is proposed for military and security applications. Being centered and aligned along the same optical axis provides the advantage of exploiting multi-beam target illumination while maintaining a consistent reference for image processing. With the use of additive manufacturing and surface treatment techniques, complex designs can be achieved to develop passive apertures with predictable resolution and dimensional tolerance. Optimization and integration of this surface methodology would enable the ability to additively manufacture multispectral optical systems.

Calculations are presented of vibrational absorption spectra for PCE, TCE, DCE and VC molecules using density function theory (DFT). These ground water contaminants are among the most widely spread carcinogens in the environment of industrial countries. DFT calculated absorption spectra of isolated molecules represent quantitative estimates that can be correlated with additional information obtained from laboratory measurements. The chlorohydrocarbons, whose spectra are calculated, are characterized by an interconnected equilibrium network. The simultaneous knowledge of the spectrum of each molecule within this network will be useful for detection and monitoring of the water contaminants. The DFT software GAUSSIAN was used for the calculations of the infrared (IR) spectra presented here.

High-speed gated imaging methods such as time-of-flight or fluorescence lifetime imaging are key enablers for various applications such as gesture recognition, safety instrumentation, health monitoring and materials characterization. In these applications, short light pulses are used to generate and accumulate a photocurrent. Assuming linearity and timeinvariance, this system can be modeled by a convolution of the incoming photon stream with an impulse response function (IRF) followed by a time-gated integration. Knowing the IRF allows for further improved signal analysis and sensor design. The IRF can be measured by employing light sources resembling a delta distribution or broadband-tunable sinusoidal waveforms. Both these methods are difficult to realize for increasingly fast detectors. This paper discusses a deconvolution-based approach where the signal shape of the employed light source is considered and corrected for. The IRF reconstruction schemes introduced in this paper are based on a preprocessing step to invert the integration and followed by denoising and deconvolution. Different deconvolution algorithms have been investigated and compared. In particular, we investigated direct deconvolution, Wiener deconvolution and parametric estimation of a pre-defined IRFmodel using optimization. In order to evaluate the error of the different reconstruction methods in the presence of jitter and shot noise, a ground truth needs to be generated against which the deconvolution result can be compared. For this, example IRFs that resemble typical sensor behavior were defined using analytical models. Low normalized root-meansquare error (< 0.05) can be achieved with the parametric estimation. The advantages and disadvantages of each schemes are also discussed.

A hyperspectral beam-scanning microscope operating in the long wave infrared (LWIR) is demonstrated for future application to stand-off imaging platforms. A 32-channel quantum-cascade laser (QCL) array enables rapid wavelength modulation for fast hyperspectral imaging through sparse sampling in position and wavelength, which when coupled with image reconstruction techniques can enhance frame rate. Initial measurements of dichloromethane and water mixtures are shown, utilizing spectral information for classification across the field of view. Ongoing efforts aim to utilize copropagating visible and IR beams to enhance spatial resolution for the IR measurements by combining spatial information retrieved from visible images obtained concurrently. Future work will leverage Lissajous trajectories for sparsely-sampled beam-scanning and extend the image interpolation algorithms to arbitrary dimension for sparse sampling in the spectral domain. Simulations of the error associated with various sparse-sampling methods are also presented herein which support the use of Lissajous trajectories as a sparse-sampling method in beam-scanning microscopy.

This paper presents the latest developments on filter-wheel based multispectral imaging systems as well as their extension to making 3D images. The system, capable of producing high spatial resolution images on a spectrum spanning from 400nm to 1050nm (in 12 steps of 50nm (configurable) with 50nm or less bandwidth) can be used, without hardware change. To produce 3D image stacks where the height resolution is given by the numerical aperture of the optics used and the reproducibility of the image plane moving motor is also possible. This paper introduces the reader to spectral imaging and to 3D measurement techniques. The main parameters and relevant publications of/about the industrial monocular multispectral 3D-Imager are then presented. Correction of chromatic aberration on filter wheel system, a key idea for 3D image reconstruction, is revised. 3D imaging capabilities of the system as well as proper calibration are introduced. Selected applications and algorithms are presented towards to the end of the paper.

Broadband photodetection is crucial for various defense and scientific applications such as biomedical imaging, communications, and environmental and spectral monitoring. In recent years, transition metal dichalcogenide semiconductors, from the two-dimensional layered materials family, have attracted special attention for their application in photodetection due to their outstanding optoelectronic properties and large optical absorbance for their atomically thin thicknesses. Here, we present a CVD-synthesized MoS₂ phototransistor with Au/Ti contacts enhanced by Au nanoparticles via surface nanoplasmonics. From electronic and optoelectronic characterizations, intrinsic device parameters were extracted and analyzed including the field-effect mobility of 37.4 cm²V^-1s^-1, a high ON/OFF ratio of 10⁶. Next, the optoelectronic characterization was carried out before and after Au nanoparticles using a tunable laser with a wavelength absorption range from 400 nm to 1100 nm under vacuum conditions. The spectral photoresponse was improved from a cutoff wavelength of ~ 975 nm before the Au nanoparticles to a broadband spectral detection with a minimum standard deviation of 0.56 μA at from near-ultraviolet to near-infrared and a maximum photocurrent of 7.61 μA at an incident optical power density of ~ 2 μWcm^-2. In addition, the photocurrent has been increased 5-fold after decorating the MoS₂ photodetector with Au nanoparticles. The improvement of the light-matter interaction of MoS₂ nanosheet, described before, is attributed to the localized surface plasmon in gold nanoparticles.

There is a strong need for rad hard and high operating temperature IR detectors for space environment. Heavy metal Selenides (high Z and large density) have been investigated for more than half century for high operating temperature mid wave infrared (MWIR) applications. Most of the efforts have been devoted to make detector arrays on high-resistivity Si substrates for operating wavelengths in the 1.5 to 5.0 μm region using physical vapor transport grown poly crystalline materials. For most of the biological spectral and imaging applications, short wave infrared (SWIR) detectors have shown better performance. Recent growth materials have shown variation in morphology with slight change in growth conditions and hence variation in performance parameters such as bandgap, mobility and resistivity from sample to sample. We have performed growth and optical characterization of binary materials CdSe-PbSe to determine the suitability for IR detector. We have determined bandgap using several theoretical models for different morphologies observed during growth on silicon wafers.

Comparison of optical propagation properties in different wavelength regimes are difficult to measure, especially in continuous varying environment. Detector and measurement equipment are typically optimized for specific spectral band and utilize matching amplifiers with have different response and process time making it hard or impossible to match time sensitive data. In this paper we propose the use of a wavelength conversion imaging system which has a broad spectral response and large detector size for synchronous multi-wavelength measurement. We showcase the strength of this approach in comparing the effect of turbulence media on a 1.55 μm and a 4 μm lasers at ambient (20 °C), 45 °C and 75 °C air temperature. Recording the movement and size of the lasers spot and comparing it frame to frame allows for extraction of the effect of turbulence on the individual lasers. Furthermore using cross correlation technique the conditions under which a good comparison can be identified. The result shows peak at 0s time shift for 45 °C and 75 °C which confirms the correlated measurement while the beam wander shows no correlation for ambient temperature as it is random.

Ground penetrating radar (GPR) is a remote geophysical sensing method that has been applied in the localization of underground utilities, bridge deck survey, localization of landmines, mapping of terrain for aid in driverless cars, etc. Multistatic GPR can deliver a faster survey, wider spatial coverage, and multiple viewpoints of the subsurface. However, because of the transmit and receive antennas spatial offset, formation of 3D GPR image by simple stacking of the acquired A-scans is inaccurate. Also, averaging of different receivers data may lead to destructive interference of back-scattered waves due to different time delays implied by the spatial offset, so averaging does not lead to higher SNR in general. Furthermore, the energy back-scattered by scatter points are spread in hyperbolas in the GPR raw data. Migration or imaging algorithms are employed to increase SNR by focusing the hyperbolas. This focusing process also leads to better accuracy in target localization. In this paper, a computationally efficient synthetic aperture radar (SAR) imaging algorithm that properly integrates multistatic GPR data in both ground and air-coupled cases is presented. The algorithm is successfully applied on two synthetic datasets.

Metamaterials, or synthetic materials that have engineered material parameters, have been utilized to demonstrate extraordinary usefulness in the control of electromagnetic waves. In this paper, a novel metalens design in the near infrared band from 1.5μm to 3μm is presented. It consists of a Fresnel zone plate (FZP)-type metasurface which is called a plasmonic waveguide coupler (PWC), situated on a slab of type I hyperbolic metamaterial (HMM) that lies on a silicon substrate which has a silver nanodipole embedded within it. The PWC is made out of rings of Indium Tin Oxide (ITO) and the type I HMM is constructed using a periodic stack of ITO and silicon layers that, through effective medium theory (EMT), act as a slab of type I HMM. Together the PWC and the HMM slab serve to focus incoming radiation onto a focal point marked by the location of the silver (Ag) nanodipole. The Ag nanodipole allows for high subwavelength confinement of optical modes because the in-focal point component of the electromagnetic field vector couples to the plasmonic resonance of the dipole. The maximum achieved resolution is up to 0.01667 λ at an operating wavelength of 1.53 μm and electric field intensity enhancement of 10⁶ is achieved. The enhancement in field is related to the principal localized surface plasmon resonances (LSPRs) around the resonator’s edges on the interface between the Ag nanodipole and the substrate.