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This paper examines infrared spectroscopic effects for the standoff detection of an explosive material, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), inkjet printed on an aluminum surface. Results of a spectroscopic study are described, using multiple optical setups. These setups were selected to explore how variations in the angles of incidence and collection from the surface of the material result in corresponding variations in the spectral signatures. The goal of these studies is to provide an understanding of these spectral changes since it affects standoff detection of hazardous materials on a reflective substrate. We demonstrate that variations in spectral effects are dependent on the relative surface concentration of the deposited RDX. We also show that it is reasonable to use spectroscopic data collected in a standard laboratory infrared spectrometer outfitted with a variable angle reflectometer set at 0° as reference spectra for data collected in a standoff configuration. These results are important to provide a systematic approach to understanding infrared (IR) spectra collection using standoff systems in the field, and to allow for comparison between such data, and data collected in the laboratory. Although the precise results are constrained to a specific material system (thin layers on a reflective substrate), the approach and general discussion provided are applicable to a broad range of IR standoff sensing techniques and applications.

Inverse vulcanization is the method by which molten sulfur can be combined with comonomers to form stable polymers termed “organically modified chalcogenide” or “ORMOCHALC” polymers. One advantage to ORMOCHALC polymers is that they can possess important optical properties, such as high refractive index and strong infrared (IR) transmission, while being easier to fabricate than glass materials with similar optical properties. In the present work, a new ORMOCHALC is fabricated by using tetravinyltin as a comomoner with sulfur. This is the first example of an organometallic molecule being used as a comonomer to develop ORMOCHALCs. The result is an ORMOCHALC polymer that has the highest refractive index reported for a “sulfur and comonomer” polymer and that demonstrates unprecedented transmission in the IR region.

Studies on the reflectance spectra of different mixtures of sand and silt were performed in the infrared spectral region of 7–25 µm to explore the behavior of a second reststrahlen band occurring between 17 and 25 µm with respect to different mixtures of sand and silt. The reflectance spectrum of all samples exhibited reststrahlen bands within both the long wavelength infrared (LWIR) and the very long wavelength infrared (VLWIR) regions of the reflectance spectrum. Results demonstrate that both LWIR and VLWIR reststrahlen bands increase in area as the fraction of sand increases to 80%. More importantly, the data demonstrate that the VLWIR reststrahlen band, like that in LWIR, exhibits a significant and reproducible decrease in reflectivity for mixed soil versus weathered soil.

Mid-wavelength infrared (MWIR, λ = 3 – 5 μm) materials are of great importance due to their applications in optical sensors and devices for military, industry, and non-invasive medical diagnostics. Specifically, MWIR polarimetry are used in biometric recognition and camouflaged detection. Recently, sulfur based organically modified chalcogenides (ORMOCHALC) polymers have been utilized to fabricate MWIR polarizers with competitive extinction coefficient to commercial polarizers, which are mostly made of expensive, brittle, and heavy inorganic materials. On the other hand, to adjust or reinforce the refractive index of the polymer, the chalcogenide content (i.e., sulfur, selenium) needs to be increased, which may result in adverse effects on the thermomechanical characteristics, including Young's modulus and lower glass transition temperature. This reduced thermomechanical stability compromises the structural integrity of ORMOCHALC-based optical devices. In this study, an ORMOCHALC polymer, poly(S–r–DIB), was reinforced with the zinc sulfide (ZnS) nanoparticles to simultaneously improve the refractive index and the thermomechanical properties. The addition of 20 wt% ZnS nanoparticles improves the pure polymer's glass transition temperature (Tg) from 9.6 °C to 31.4 °C, and the refractive index by Δn= 6.58 %. Then, subwavelength wire-grid polarizers were fabricated based on the pure ORMOCHALC first by a simple thermal imprinting method followed by metal deposition. The resulted MWIR polarizer showed a high extinction coefficient which is comparable with commercial polarizers. However, the structural integrity of the polarizers was compromised due to lower glass transition temperature. Finally, finite element analysis was conducted on the possible wire-grid polarizer fabrication utilizing optically and mechanically reinforced composites, as demonstrated in this study. If fabricated, these nanoparticles reinforced polarizers will possess superior structural integrity compared to ORMOCHALC polarizers. Moreover, these polarizers show a spectral selectivity as the transmission curve's resonance wavelength depends on the composite's refractive index, which is tunable by controlling the nanoparticles content. The resonance peak redshifted from 3.12 μm to 3.3 μm for the pure polymer to 20% ZnS reinforced ORMOCHALC polarizer. The extinction coefficient of the polarizer also improved in multiple wavelength bands in the MWIR. These polarizers with superior extinction coefficient, spectral selectivity, and improved thermomechanical stability demonstrate a broader prospect in MWIR optics.

The polymer nanocomposites have attracted increasing attention in the optical components that are miniaturized and integrated with wearable or portable electronics due to the polymer processibility and the tunability of the refractive index (RI) by adding nanoparticles. However, the lack of models predicting the composites’ RI attributed to the morphology, physical properties, as well as volume fraction of the nanoparticles poses difficulties in the design. This study investigates the effect of the size and agglomeration condition of the nanoparticles on the effective RI based on a Finite Element Analysis (FEA) method simulating the Fabry-Perot resonance within the composite film. The result showed that larger particles (or particle clusters) could reinforce the RI of nanocomposites compared with the well-dispersed small particles. The particle-cluster model had lower RI than the single-solid-particle model with the same effective particle diameter, demonstrating that the particle cluster provides less scattering intensity than the single-solid-particle.

We report the latest progress on fabrication of rare earth doped single crystal yttrium aluminum garnet (YAG) core/undoped YAG cladded fibers. Rare-earth doped single crystal core fibers were grown with laser heated pedestal growth methods. In a second step, epitaxial methods were used to grow a single crystalline undoped YAG cladding onto the core fiber. Hydrothermal and liquid phase epitaxy methods utilize the core doped fiber as the seed. X-ray diffraction of cladding reveals an equilibrium (110) morphology. Energy-dispersive X-ray spectroscopy analysis shows there is minimal diffusion of rare-earth dopants into the cladding structure. The use of scandium doping is shown to substitute at the Al3+ site, thereby allowing an additional tunability of refractive index of core structure material besides conventional Y3+ site dopants. The use of these epitaxial growth methods enables material compatibility, tuning of refractive index, and conformal growth of cladding structures onto core fibers for optical devices.

NRL is developing new materials that transmit across wide wavelength ranges and will present recent results. MILTRAN is a new rugged optical ceramic that transmits visible through LWIR and is 3.5 times harder than ZnS and is well suited as an internal lens element. NRL-series moldable glasses transmit SWIR through LWIR and may be bonded to each other in an adhesive-free thermal process. NRL-200-series glasses transmit visible through MWIR and expand the glass map for multispectral lens designs. These new materials enable greater flexibility for designers of lenses for advanced defense applications and potentially reduce the size, weight and cost of next-generation optics.

At the Naval Research Laboratory, we have developed several rugged oxide materials for optics applications. These materials include cubic sesquioxides (Y2O3 and Lu2O3) and spinel ceramics which transmit in the UV, visible, SWIR and MWIR wavelength region. These materials have superior mechanical and optical properties compared to the currently used materials on various Navy / DoD systems. The transparent and rugged ceramics are fabricated by the Hot press/HIP method using ultra-pure powder synthesized in-house and the resulting ceramics typically have excellent optical quality with no birefringence. In this paper, we present the optical properties of these rugged transparent ceramics and lens designs using these materials will be also discussed.

Solid-state laser based on Er-doped gain medium, is a promising approach to designing directly diode-pumped eye-safer pulsed sources in the spectral range ~1.45-1.65 m. This approach allows obtaining spectrally broad laser outputs with high efficiency, if required by application (e.g., anti-speckle), without resorting to nonlinear conversions. All-fiber laser solutions for high PRF pulsed laser sources is deemed desirable, but it may result in high design complexity, especially on the Master Oscillator (MO) end, hence it is not appropriate in cases where low complexity is of major importance. Presented here are the results obtained with pulsed hybrid MOPA comprised of a bulk crystalline Er-based MO and Er-doped fiber amplifier.

We computationally investigate random motheye structures for fused silica from 0.4 to 1.0 μm. We found that increasing the randomness of the diameter and height can further suppress the reflection at the air-glass interface.

We present a method of post-deposition tuning of the optical properties of thin film dielectric filters and mirrors containing chalcogenide glass (ChG) layers by thermally adjusting their refractive index. A common challenge associated with the use of ChG films in practical applications is that they suffer from slight run-to-run variations in optical properties resulting from hard-to-control changes in source material and deposition conditions. These variations lead to inconsistencies in optical constants, making the fabrication of devices with prescribed optical properties challenging. In this paper, we present new work that takes advantage of the large variation of a ChG films’ refractive index as a function of annealing. We have carried out extensive characterization of the thermal index tuning and thickness change of arsenic selenide (As2Se3) ChG thin films and observed refractive index changes larger than 0.1 in some cases. We show results for refractive index as a function of annealing time and temperature and propose a model to describe this behavior based on bond rearrangement. We apply thermal refractive index tuning to permanently shift the resonance of a Fabry-Perot filter and the cutoff wavelength of a Bragg reflector. The Bragg reflector, consisting of alternating As2Se3 and CaF2 layers, exhibits high reflectance across a ∼550 nm band with only five layers. Modeling results are compared with spectroscopic measurements, demonstrating good agreement.

We demonstrate efficient pulse-energy extraction from a partly-quenched erbium-doped aluminosilicate fiber amplifier. This has a high erbium-concentration, which allows for short devices with reduced nonlinear distortions, but which also results in partial quenching and thus significant unsaturable absorption, even though the fiber is still able to amplify. Although the quenching degrades the average-power efficiency, the pulse energy remains high, and our results point to an increasingly promising outcome for short pulses. Furthermore, unlike unquenched fibers, the conversion efficiency improves at low repetition rates, which we attribute to smaller relative energy loss to quenched ions at higher pulse energy. A short (2.6 m) cladding-pumped partly-quenched Er-doped-fiber with 95-dB/m 1530-nm peak absorption and saturation energy estimated to 85 {\mu}J, reached 0.8 mJ of output energy when seeded by 0.2-{\mu}s, 23-{\mu}J pulses. Thus, according to our results, pulses can be amplified to high energy in short highly-Er-doped fibers designed to reduce nonlinear distortions, at the expense of average-power efficiency, Comment: Manuscript submitted to Optics Express. Peer-Review extended up to March, 31

There is a need for robust optical materials with very low reflectivity and high transmission in the visible and infrared wavelength regions for use with high energy lasers. High power continuous-wave (CW) and high pulse energy laser optical systems can suffer damage from untreated optics due to undesirable Fresnel reflections. Sub-wavelength antireflective structured surfaces (ARSS) have been shown to have a higher laser-induced damage threshold than traditional AR coatings, which are necessary for high energy (multi-kW) laser components. Previous optimization of the etching process for ARSS on fused silica, among other optical materials, has yielded controlled and repeatable parameters, resultant from formation of varying depth and density profiles by randomly removing material from the surface. The “grass-like” boundaries result in gradient-index functionalities for incident optical beams, which simultaneously reduces reflection and increases transmission of the substrate. In this study, we investigate resultant optical properties of B270 Superwhite substrates, whose surfaces are modified using reactive ion etching, in order to enhance broadband transmission and lower reflectivity, thus increasing their utility for visible and near-infrared wavelength regions. We report results for ARSS fabricated directly onto the surface of 1” diameter, B270 Superwhite optical windows. Initial trials yielded single-side random ARSS (rARSS) treated samples with transmission enhancement of up to 3% from 400-1800 nm, nearing that of rARSS enhancement of fused silica, with maximum enhancement 80% that of the theoretical value between 700-900 nm.

NRL is developing new glasses that transmit within the visible to LWIR range for use in multispectral imaging. We had previously developed NRL glasses which transmit in 0.9 to > 12 µm in wavelength, with refractive index ranging from 2.38 to 3.17, to expand the glass map and provide compact solutions to multispectral imaging systems. These glasses were specifically designed to have comparable glass molding temperatures and thermal properties so that they can be laminated and co-molded into optics with reduced number of air-glass interfaces (lower Fresnel reflection losses). These NRL glasses also have negative or very low dn/dT, making it easier to athermalize the optical system. A set of NRL glasses can be diffused to make infrared graded index (IR-GRIN) optics. Our multispectral optics designs using these new materials demonstrate reduced size, complexity and improved performance. The glass database compatible with Zemax and CodeV is available for distribution. We are adding n

Rugged optics and window materials with good visible and IR transmission are needed for several sensing and imaging application. Some of the applications require very thick windows or windows in complex geometries, which impose additional limitations on material selection. Magnesium aluminate spinel (MgAl2O4), or spinel for short, is a rugged window material with excellent transmission in ultraviolet to midwave infrared (0.18 – 5.5 m) wavelengths. Spinel is comparable to sapphire and ALON in its ruggedness while having superior transmission near 5 m in wavelength. With its cubic crystal structure, spinel is isotropic unlike sapphire which is birefringent. We have been developing high optical quality spinel ceramics in different shapes and sizes. Current status of these efforts in making spinel windows in large sizes and conformal shapes for several imaging applications will be discussed.

A common challenge in metastructure fabrication is precisely tuning of the frequency of a device’s resonance. Slight variations in device dimensions or material properties can lead to a deviation in resonance frequency in comparison to design. We present a method of tuning a dielectric metastructure’s resonance by thermally adjusting the refractive index of a chalcogenide glass (ChG) material. Several characteristics of ChGs make them good candidates for use in dielectric metastructures. They exhibit high linear refractive indices, enabling high index contrast devices; they have large optical nonlinearities, making them useful for tunable devices and nonlinear frequency conversion; and they have wide transmission windows extending from the visible through the long-wave infrared. Recently, we have carried out extensive characterization of the index tuning of arsenic selenide (As2Se3) ChG thin films and observed refractive index changes larger than 0.1 in some cases. We use this refractive index change to permanently shift the resonance of a Fabry-Perot filter and the cutoff wavelength for a Bragg reflector. We then demonstrate thermal tuning to shift resonance positions for a metasurface (MS). We compare finite element modeling results with measurement results and show good agreement. This tuning mechanism has potential for use in MS devices where a precise resonance frequency is required.

We have studied the in-situ radiation-induced degradation and recovery of the output optical power of actively-pumped erbium-doped fiber amplifiers (EDFAs) pumped at both 980 and 1480 nm during and after radiation exposure. The EDFs have high Er concentrations corresponding to 20 – 60 dB/m peak Er3+ absorptionat 1.532 μm suitable for high power cladding-pumped applications. The fibers were made with a wide range of glass compositions, including the addition of specific dopants to reduce clustering and improve efficiency. Radiation tolerance of nanoparticle doped fibers is superior to those fabricated by solution doping. The effects of photo-annealing have been explicitly studied, and an nth-order kinetic model has been used to predict end-of-life degradation. More robust statistical kinetics modeling of accelerated lifetime data has been used to predict on-orbit performance in more realistic scenarios, such as Geosynchronous Earth Orbit.

We computationally varied the dimensions of cone-shaped motheye structures to optimize the transmission efficiency over a broad wavelength range. We show that optimized cone-shaped motheye structures can theoretically achieve > 99.5% transmission from 0.5 µm–2 µm.

We design and realize chalcogenide-based photonic quasicrystals enabling simultaneous phase matching of an arbitrary number of nonlinear optical processes in a single engineered nanostructure.

This study introduces a method to predict the refractive index (RI) of nanocomposites with the Finite Elements Analysis (FEA) based on the Fabry-Perot interference. The efficacy was verified by comparing the estimated composites’ RI with the available data in the literature. In the experimental verification, the FEA-based prediction showed closer results with the measurement as compared to the effective medium approximation (EMA) approaches, which are prevalently used to predict the physical properties of nanocomposites. Due to the modeling capability, the FEA-method could investigate the effect of the nanoparticle morphology (particle size, shape, and orientation) and distribution. Large particle size, particle agglomeration in high electric-field amplitude region, and particle elongation along the light oscillating direction are found to be the major factors to enhance the RI of composites. The underlying mechanism of RI changing is attributed to the light scattering by embedded nanoparticles, which provides one potential real-time RI tuning schematic.

We computationally study two-layer motheye nanostructures fabricated on MgAl2O4 spinel ceramic windows. We investigated the parameters of the structure, including height, width, and shape, in order to optimize its power transmission efficiency over a broad bandwidth. We found a two-layer motheye structure in which the cones of the upper structure have a concave shape that can theoretically achieve more than 99.8% transmission at normal incidence in the wavelength range between 0.4 μm and 5.0 μm.

Arsenic selenide glass optical fibers typically possess extrinsic absorption bands in the infrared wavelength regions associated with residual hydrogen and oxygen related impurities, despite using 6N purified elemental precursors. Consequently, special additives and refined processing steps are utilized in an attempt to reduce these and other impurities. We investigate the formation of particulate impurities during a purification process based on the addition of 0.1 wt% elemental aluminum (Al) and 0.2 wt% tellurium tetrachloride (TeCl 4 ) during glass synthesis. It was found that during purification and melting steps, Al reacts with TeCl 4 to form AlCl 3 , which in turn reacts with oxygen and hydrogen impurities and the fused quartz (SiO 2 ) ampoule to produce HCl and stable submicron Al 2 SiO 5 compounds in the As-Se glass and fibers. The intensity of the H-Se absorption band centered at 4.57 μm has been significantly reduced from 18 dB/m to 0.8 dB/m. Using thermodynamic data, we have identified stable Al 2 SiO 5 submicron inclusions in the glass and fibers. A two-step gettering process is proposed as a solution to eliminating these inclusions.

Transparent magnesium aluminate spinel (MgAl2O4) ceramic has excellent transmission from the UV to mid-wave IR. It is rugged with strength that is 5x that of glass. Spinel is being developed as a sensor window for numerous military platforms. At the U.S. Naval Research Laboratory (NRL), we have focused on process developments to facilitate wider acceptance of spinel for various applications. These developments include purification of spinel to reduce the absorption and scattering losses for use as an exit aperture on High Energy Laser (HEL) systems and various cost-effective densification methods to reduce manufacturing costs. In this paper, we will provide an update on some of the ongoing spinel activities at NRL.

Detection of explosives on surfaces could potentially be achieved using handheld standoff optical sensors, providing rapid intelligence and safety to the warfighter or first responder. Recent work has shown the capability to discriminate various chemical vapors using a bioinspired or biomimetic detection system modeled on human color vision. This biomimetic system utilizes three overlapping broadband infrared optical filters to discriminate between various chemicals. Preliminary reflectance data of chemicals on surfaces indicate a capability for discrimination of target chemicals and interferents by analysis of biomimetic sensor output using novel analytical methods such as “Comparative Discrimination Spectral Detection” (CDSD). Transitioning this detection method to threats on surfaces at proximate standoff distances (~1 m) requires additional considerations including surface characteristics and angle of detection. This work explores sensor detection parameters for ammonium nitrate residue on aluminum surfaces of various roughnesses. Samples of the explosives component ammonium nitrate, NH4NO3, diluted at 10%, 5%, and 1% in DI water, were prepared by dropcasting onto aluminum coupons with four surface finishes: polished, brushed, extruded, and sandblasted. Surface roughnesses were measured. Single beam reflectance spectra (2 – 20 μm) were collected using a FTIR spectrometer over multiple independent angles of incidence and collection (15° - 80°). Multiple factors were analyzed including albedo, and potential sensor configurations. Characteristics for future MWIR and LWIR sensors, such as illumination power and detector sensitivity, are evaluated which enable chemical spectral identification across range of aluminum surface roughness for proximate standoff distances and different angles of incidence.

High power continuous-wave (CW) and high pulse energy laser optical systems can suffer damage from untreated optics due to undesirable Fresnel reflections. With high energy pulse lasers, traditional anti-reflection (AR) thin-film coated optics are susceptible to localized field enhancement regions, due to multiple boundaries, and experience laser induced damage on both entry and exit interfaces. Sub-wavelength anti-reflective structured surfaces (ARSS) have been shown to have a higher laser-induced damage threshold than traditional AR coatings. Previously published work detailed nanosecond-pulsed laser-induced damage on planar fused silica (FS) substrates with random ARSS (rARSS) treatment (λ=1064 nm). This study details and compares laser damage and fatigue testing of rARSS treated FS via continuous-wave, 2 kW irradiation (λ=1075 nm). Gaussian laser output was focused to ~ 60-μm-diameter (1/e2), to increase incident intensity, yielding a maximum power density of ~ 70.7 MW/cm2. Laser power and duration were controlled while monitoring the window’s optical transmission and entry/exit surface temperatures. CW durations at maximum power up to 5 min were used. Peak temperatures were recorded for untreated FS, as well as double side treated rARSS samples.

New moldable, infrared (IR) transmitting glasses from NRL and graded index (GRIN) optical materials enable simultaneous imaging across multiple wavebands including SWIR, MWIR and LWIR and offer potential for both weight savings and increased performance in optical sensors. Lens designs show the potential for significant SWaP reduction benefits and improved performance using NRL materials and IR-GRIN lens elements in multiband sensors.

ORganically MOdified CHALCogenide (ORMOCHALC) polymers are novel materials that can be synthesized through the recently discovered inverse vulcanization process. Inverse vulcanization requires the heating of chalcogenide comonomers along with compounds that contain available pi electrons. The composition of the polymers presented includes the use of previously unexplored multi-vinyl branching agents, as well as polymer backbones that contain selenium. The crosslinking by unique comonomer species, and the use of selenium as a backbone component are significant in that they have a direct and pronounced effect on the optical properties of the polymers produced. Specific optical benefits of ORMOCHALC polymers include the extensive infrared transmission profile and the unusually high refractive indices these polymers possess. We present the synthesis and optical characterization of unique ORMOCHALC polymers.

Fiber lasers rely on clad pump architectures where double clad designs are used. Currently, polymers are used successfully as pump claddings on Yb-doped laser fibers. In order to transition to resonantly pumped fiber lasers at eye safer wavelengths, polymer pump claddings must have low absorption at those wavelengths. There is currently no suitable low index, low absorption, thermally stable, and high thermal conductivity polymer to act as a low-index coating. This work presents a new class of polymers, termed fluorinated polymer composites (FPCs), which possess improved thermal properties. The FPCs are fabricated by incorporating ceramic nanoparticles into fluorinated polymers. Ultimately, the FPCs could enable further use of fiber lasers at commonly used wavelengths, as well as at eye-safer wavelengths.

Arsenic triselenide (As2Se3) is of interest for use in dielectric metasurfaces for several reasons: It has a high linear refractive index, n=2.8, enabling high index contrast with its surrounding medium; it has an exceptionally high optical nonlinearity (n2 < 900 × that of silica) making it a good candidate for nonlinear metasurfaces; and its band gap of 1.8 eV and wide transmission window, spanning from approximately 1.5-14 μm, make it useful for applications in the shortwave infrared into the long-wave infrared. We discuss recent results in which we showed that unpassivated As2Se3 films degrade significantly under ambient conditions in the presence of light. We discuss the mechanism of this degradation and show that deposition of a thin (~10 nm) Al2O3 passivation layer, deposited via atomic layer deposition, together with preventing exposure to below-band gap light inhibits degradation. Finally, we fabricate initial As2Se3-based dielectric metasurfaces by writing features via a laser direct write system. For mid-wave to long-wave infrared applications, features with relevant sizes can be written using this technique. We measure resonances for these structures and compare to theoretical results.

This paper describes the application of a human color vision approach to infrared (IR) chemical sensing for the discrimination between multiple explosive materials deposited on aluminum substrates. This methodology classifies chemicals using the unique response of the chemical vibrational absorption bands to three broadband overlapping IR optical filters. For this effort, Fourier transform infrared (FT-IR) spectroscopy is first used to computationally examine the ability of the human color vision sensing approach to discriminate between three similar explosive materials, 1,3,5,-Trinitro-1,3,5-triazinane (RDX), 2,2-Bis[(nitrooxy)methyl]propane-1,3,-diyldinitrate (PETN), and 1,3,5,7-Tetranitro-1,3,5,7-tetrazocane (HMX). A description of a laboratory breadboard optical sensor designed for this approach is then provided, along with the discrimination results collected for these samples using this sensor. The results of these studies demonstrate that the human color vision approach is capable of high-confidence discrimination of the examined explosive materials.

Fiber design strategies for increased threshold of SBS and TMI are presented. We report a high efficiency erbium nanoparticle doped fiber incorporating acoustic guiding layers with a significantly reduced SBS gain coefficient.

To date, the performance of all polycrystalline photovoltaics is limited by their open-circuit voltage (Voc), an indicator of charge carrier recombination within the semiconductor layer. Thus, the successful implementation of high-efficiency and low-cost solar cells requires the control and suppression of nonradiative recombination centers within the material. Here, we spectrally and spatially resolve the photovoltage of polycrystalline thin-film Cu(In,Ga)Se2 (CIGS) solar cells. Micro-Raman and energy-dispersive X-ray spectroscopy maps obtained on the same grains showed that the chemical composition of the CIGS layer is very uniform. Surprisingly, we observed concurrent spatial variations in the photovoltage generated across the device, strongly indicating that structural properties are likely responsible for the nonuniform mesoscale behavior reported here. We build a tomography of the photovoltage response at 1 sun global illumination, mimicking the operation conditions of solar cells. Furthermore, we sp...

Quaternary sputtering is a promising alternative to more established deposition methods for the fabrication of Cu(In,Ga)Se2 (CIGS) thin films for photovoltaics (PV). In this technique, a single sputtering target containing all four constituents is employed to deposit the CIGS film. Quaternary sputtering offers several advantages over other deposition methods, including excellent uniformity over large areas, high material usage, and less reliance on toxic Se precursors such as H2Se. Despite these advantages, several drawbacks remain. To date, devices fabricated by quaternary sputtering without additional selenization have been limited in efficiency to about 11%, and realizing bandgap grading in order to match the performance of the best evaporated devices presents a challenge. We discuss the prospects for quaternary sputtering as a fabrication technique for CIGS and highlight areas of research that may result in improved performance. Target fabrication and usage is reviewed. We also present results for films and devices including data for the optical constants of sputtered CIGS. Some recent previously unpublished results, including a study of impurities in CIGS sputtering targets and the first demonstration of a CIGS device on a flexible glass substrate, are discussed

In order to mimic and enhance the properties of moth eye-like materials, nanopatterned fused silica was chemically modified to produce self-cleaning substrates that have anti-reflective and infrared transmissive properties. The characteristics of these substrates were evaluated before and after chemical modification. Furthermore, their properties were compared to fused silica that was devoid of surface features. The chemical modification imparted superhydrophobic character to the substrates, as demonstrated by the average water contact angles which exceeded 170°. Finally, optical analysis of the substrates revealed that the infrared transmission capabilities of the fused silica substrates (nanopatterned to have moth eye on one side) were superior to those of the regular fused silica substrates within the visible and near-infrared region of the light spectrum, with transmission values of 95% versus 92%, respectively. The superior transmission properties of the fused silica moth eye were virtually unchanged following chemical modification.

The chalcogen glasses (i.e., glasses based on the elements S, Se, and Te) are transparent in the infrared (IR), possess low phonon energies, are chemically durable, and can be drawn into fiber. We review our recent research progress at the Naval Research Laboratory (NRL) to develop chalcogenide glass fibers for applications in the mid- and long-wave IR wavelength regions from 2 to 12 μm. Our recent effort in the development of low loss chalcogenide fibers, by describing the synthesis and purification methods, fiber drawing techniques, and highlighting the best results, is summarized. Various applications of these high quality chalcogenide fibers, including multimode beam combiners, mid-infrared supercontinuum sources, fiber Bragg gratings, fiber bundles for IR imaging, anti-reflecting surface structures, and modal filters, are described. Novel infrared (IR) lenses that enable a reduction in the size and weight of IR imaging optics through the use of layered glass structures with broad IR transmission are also presented.

Organically modified chalcogenide (ORMOCHALC) polymers have proven to be alternatives to the conventional inorganic materials for mid-wavelength infrared (MWIR, λ = 3–5 μm) optical components. While the refractive index of ORMOCHALC can be reinforced by the content of chalcogenides such as sulfur (S) and selenium (Se), the increased portion of the S or Se deteriorate the thermomechanical stabilities. As a remedy, this study utilizes ZnS nanoparticles to reinforce both optical and thermomechanical properties of the sulfur-based ORMOCHALC polymer, poly(S-random-1,3-diisopropenylbenzene). The refractive index n and extinction coefficient k of the nanocomposites were characterized by Infrared Variable Angle Spectroscopic Ellipsometry (IR-VASE). The results show a significant increment in the refractive index of Δn = 6.58% at the wavelength of 4 μm by adding 20 wt% ZnS (or 7.29 vol%) in the ORMOCHALC polymer. The low extinction coefficient of the nanocomposites (

New moldable, infrared (IR) transmitting glasses and diffusion-based gradient index (GRIN) optical glasses enable simultaneous imaging across multiple wavebands including short-wave infrared, midwave infrared, and long-wave infrared, and offer potential for both weight savings and increased performance in optical sensors. Lens designs show potential for significant reduction in size and weight and improved performance using these materials in homogeneous and GRIN lens elements in multiband sensors. An IR-GRIN lens with Δn = 0.2 is demonstrated.

There is a strong desire to reduce size and weight of single and multiband IR imaging systems in ISR operations on hand-held, helmet mounted or airborne platforms. NRL is developing new IR glasses that transmit from 0.9 to < 12 µm in wavelength, with refractive index ranging from 2.38 to 3.17, to expand the glass map and provide compact solutions to multispectral imaging systems. These glasses were specifically designed to have comparable glass molding temperatures and thermal properties so that they can be laminated and co-molded into optics with reduced number of air-glass interfaces (lower Fresnel reflection losses). These new NRL glasses also have negative or very low dn/dT, making it easier to athermalize the optical system. Our multispectral optics designs using these new materials demonstrate reduced size, complexity and improved performance. The glass database is now available for distribution. Some of the NRL glasses are also available commercially. This presentation will cover discussions on the new optical materials, multispectral designs, as well fabrication and characterization of new optics.

Graded index (GRIN) optics offer potential for both weight savings and increased performance but have until recently been limited to visible and NIR bands (wavelengths shorter than about 0.9 µm). NRL has developed glass-based IR-GRIN lenses compatible with SWIR-LWIR wavebands. Recent designs show the potential for significant SWaP reduction benefits and improved performance using IR-GRIN lens elements in dual-band, MWIR-LWIR sensors. The SWaP and performance advantages of IR-GRIN lenses in platform-relevant dual-band imagers will be presented.

In this paper, we review the recent progress on chalcogenide negative curvature fibers, including gaps to increase confinement, tube structures to suppress higher-order modes, parameters to decrease bend loss, and the experimental demonstration of chalcogenide negative curvature fibers. The advances in negative curvature fibers using chalcogenide glasses enable many applications in the mid-infrared region.

The global defense community requires new approaches for standoff detection of chemical, biological, radiological, nuclear and explosive (CBRNE) threats. Such standoff detection methods must be capable of discriminating the target hazardous materials from the environmental background. Therefore these sensors must exhibit high selectivity. High selectivity detection of CBRNE threats can be accomplished using infrared (IR) spectroscopy, which produces a unique spectral “fingerprint” of the target chemical, enabling discrimination of the target chemical from other chemicals in the background. Standoff detection using IR spectroscopy however requires that enough of the incident source light may be collected at the detector; therefore a high-power source is needed. Commercially available quantum cascade laser (QCL) sources are capable of projecting high power, coherent laser light at targets down range from the source. In order to collect complete IR spectra throughout the entire fingerprint region, the output of multiple QCL modules are combined into a single exit aperture. This is typically achieved using mirrors and other optics which are susceptible to vibrational and temperature misalignments in field systems. In order to provide a more ruggedized solution to combining the beam output of multiple QCL modules, we developed a unique chalcogenide optical fiber beam combiner which combines the output of four commercial QCL modules. This allows for scanning across a spectral range from 6.01 – 11.20 μm encompassing parts of both the IR functional groups and fingerprint regions. We demonstrate the ability of this QCL system to generate high quality IR spectra of hazardous materials.

Field detection of chemical, biological, radiological, nuclear and explosive (CBRNE) threats requires the development of highly selective sensors with low size, weight, power and cost (SWaP-c). Recent developments have demonstrated that an optical biomimetic sensing approach, based on human-eye color detection can provide high-confidence discrimination of target chemicals while rejecting potential interferents with similar chemical structures. This biomimetic sensing method operates by identifying differences in the overlap between target and interferent chemical infrared absorption bands utilizing three, overlapping, optical bandpass filters. This method is non-spectroscopic and requires only the use of commercially available, off-the-shelf optical components. This approach has been demonstrated for volatile chemical vapors in the mid-wave-infrared (3 – 5 μm). Based on this success, experimental studies of this biomimetic sensing approach have been expanded further into the long wave infrared spectral region (6 – 12 μm) and for detection of explosives on surfaces, including aluminum and plastics. We present discrimination results using this biomimetic sensing approach for explosive samples on surfaces in both the mid- and long- wave infrared. Numerical data, along with experimentally collected data, are discussed. We demonstrate that this method is capable of discriminating between similar explosives on surfaces as well as between these explosives and potential environmental interferents. We present the results of these experiments and discuss potential transition of this approach to future field-ready stand-off devices and applications.

Single crystal fiber composed of rare earth doped YAG offers the potential for high power scaling of fiber lasers due to it lower intrinsic stimulated Brillouin cross-sections and higher thermal conductivity. The use of rare earth doped YAG fibers also mitigates issues associated with photodarkening, as well as issues associated with the silica multiphonon edge absorption and OH- quenching observed in Ho doped silica fiber lasers operating at eye safer wavelengths. To date, small diameter single crystal YAG fibers as small as 17 µm have been grown at Naval Research Laboratory to achieve a core architecture. Recent work has focused on development of cladding structures on the single crystal core material through sputtering, liquid phase epitaxy, and hydrothermal crystal growth to achieve a true core/clad all- crystalline wave guiding structure. Crystalline claddings have been grown with all three approaches with varying degree of quality and crystallinity. In this paper, we will report on our progress in fabricating crystal claddings on YAG single crystal core material.

There is a strong desire to reduce size and weight of single and multiband IR imaging systems in ISR operations on hand-held, helmet mounted or airborne platforms. NRL is developing new IR glasses that transmit from 0.9 to > 12 µm in wavelength, with refractive index ranging from 2.38 to 3.17, to expand the glass map and provide compact solutions to multispectral imaging systems. These glasses were specifically designed to have comparable glass molding temperatures and thermal properties so that they can be laminated and co-molded into optics with reduced number of air-glass interfaces (lower Fresnel reflection losses). These new NRL glasses also have negative or very low dn/dT, making it easier to athermalize the optical system. Our multispectral optics designs using these new materials demonstrate reduced size, complexity and improved performance. The glass database is now available for distribution. Some of the NRL glasses are also available commercially. This presentation will cover discussions on the new optical materials, multispectral designs, as well fabrication and characterization of new optics.

Graded index (GRIN) optics offer potential for both weight savings and increased performance but have until recently been limited to visible and NIR bands (wavelengths shorter than about 0.9 µm). NRL has developed glass-based IR-GRIN lenses compatible with SWIR-LWIR wavebands. Recent designs show the potential for significant SWaP reduction benefits and improved performance using IR-GRIN lens elements in dual-band, MWIR-LWIR sensors. The SWaP and performance advantages of IR-GRIN lenses in platform-relevant dual-band imagers will be presented.

Holmium doped fiber lasers (HoDFL) are attractive candidates for high energy lasers used in directed energy applications because they operate at wavelengths that are safer to the eye. The common solution-doping technique for making HoDFs can result in the incorporation of hydroxyl (OH) impurity in the active fiber core. The HoDFL operational wavelength of ~2.0 µm is near the 2.2 µm combination absorption band of the OH fundamental mode and the SiO4 tetrahedron vibration, so the OH concentration must be

High power laser optical systems can suffer damage from uncoated optics due to undesirable Fresnel reflections. With high power lasers, traditional anti-reflection (AR) thin-film coated optics are susceptible to localized field enhancement regions, due to multiple boundaries, and experience laser induced damage on both entry and exit interfaces. Sub-wavelength random anti-reflective surface structures (rARSS) have been shown to have a higher laserinduced damage threshold than traditional AR coatings. Previously published work detailed nanosecond-pulsed laserinduced damage on rARSS on both single surface and dual surface of optical quality, planar, fused silica substrates at 1064 nm. This study details laser fatigue testing of double-sided rARSS samples via continuous wave, 2 kW ytterbiumfiber- laser irradiation (1075 nm). Laser output was focused to increase incident intensity at the initial interface. The laser spot was focused to ~ 60-μm-diameter (1/e2) yielding a maximum power density of 70.7 MW/cm2. Laser power, test duration, and testing grid were controlled externally via LabVIEW software. Damage testing sites maintained the same laser power density (70.7 MW/cm2) and varied by irradiation time, incrementally from one minute up to one hour. It was determined that double-sided rARSS substrates have a higher damage threshold than thin-film AR coatings, showing no damage of the AR structures at power densities up to 70.7 MW/cm2 and laser irradiation times up to one hour at each interrogation site, while thin-film samples have been reported to fail with power density as low as 2 MW/cm2.

Successful power scaling of the Er-doped fiber laser heavily depends on Er dopant concentration. In order to scale the power up to a kW class, core absorption should be well over the limits defined by current commercial doping techniques. Recently developed nanoparticle (NP) doping technique of fabrication of erbium-doped fibers allows the desired dopant concentration increases while mitigating both Er upconversion and clustering effects. Here we present the latest resonantly-pumped Er fiber laser power scaling results enabled by further development of the NP doping technique of Er-doped fiber fabrication. Using resonantly cladding-pumped (at 1530 nm) large mode area 20/125 µm fiber with the Er-NP-doped core we achieved pump-limited CW power of over 30 W at ~1605 nm with the slope efficiency versus absorbed pump power of ~63%. These are, to the best of our knowledge, the highest power and efficiency demonstrated so far for from the Er-NP-doped fiber. Further considerations on fiber design optimization are presented as well.