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Many optical applications, including free-space optical communications, lidar, and astronomical measurements, are impacted by the presence of light-scattering particles also known as obscurants. Scattering from particles consisting of sand, dust, dirt, and other substances can significantly degrade optical signals. For many obscurants, the index of refraction is dependent on the wavelength of light, and there exists a Christiansen wavelength (λ c ) at which scattering is at a minimum. At λ c the index of refraction of the scattering particles (n s ) matches that of the surrounding medium, in this case air (with refractive index n a ). This condition makes the scattering particulates almost invisible to the propagating light, minimizing scattering and increasing transmission at λ c . Previously, the authors showed a technique for measuring the index of refraction n (λ) and the extinction coefficient k (λ) using spectroscopic ellipsometry for various sand samples. Spectroscopic measurements on static sand samples demonstrated good agreement with the predicted spectral properties and highlighted the presence of a Christiansen feature near 8 µm. However, in outdoor environments, the scattering particles are never stationary but in a constant state of motion. In this work, spectroscopic measurements on dynamic sand samples (sand that is falling through the optical beam path) show two Christiansen features seen previously in predicted and observed static sand measurements. Additionally, we characterize, for the first time, transmission around a Christiansen feature using a tunable laser and show results consistent with other spectroscopic measurements., Competing Interests: Declaration of Conflicting InterestsThe authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Passive infrared (IR) systems enable rapid detection of chemical vapors but are limited by size, weight, cost, and power. Previously, the authors reported a novel passive sensor that utilizes multiple IR filter/detector combinations to discriminate between different chemical vapors based on their unique IR absorption spectra in the same manner the human eye uses to generate colors. This approach enables a very small, compact, and low-power sensor system with the capability to discriminate between chemical vapors of interest and background chemicals. All previous work showed the capability of this sensor system in discriminating chemical vapors against a hot blackbody in a laboratory environment. Now the authors demonstrate the ability of this sensor system to discriminate between the chemical vapor agent simulant dimethyl methylphosphonate and ethanol against the cold sky in an outdoor environment., Competing Interests: Declaration of Conflicting InterestsThe authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

In order to model the propagation of light through a sand cloud, it is critical to have accurate data for the optical constants of the sand particles that comprise it. The same holds true for modeling propagation through particles of any type suspended in a medium. Few methods exist, however, to measure these quantities with high accuracy. In this paper, a characterization method based on spectroscopic ellipsometry (SE) that can be applied to a particulate material is presented. In this method, a polished disc of an adhesive compound is prepared, and its optical constants are measured. Next, a mixture of the adhesive and a sand sample is prepared and processed into a polished disc, and SE is performed. By treating the mixture as a Bruggeman effective medium, the optical constants of the particulate material are extracted. For verification of the proposed method, it is first applied to pure silica powder, demonstrating good agreement between measured optical constants and literature values. It is then applied to Arizona road dust, a standard reference material, as well as real desert sand samples. The resulting optical constant data is input into a rigorous scattering model to predict extinction coefficients for various types of sand. Modeling results are compared to spectroscopic measurements on static sand samples, demonstrating good agreement between predicted and measured spectral properties including the presence of a Christiansen feature near a wavelength of 8 µm., Competing Interests: Declaration of Conflicting InterestsThe authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

There are limited fiber-based single-mode laser sources over the visible and near infrared range. Nonlinear conversion through four-wave mixing in photonic crystal fibers allows for the generation of new wavelengths far from a pump wavelength. Utilizing an all-fiber spliced configuration, we convert 1064 nm light into a W-level signal in the 750 nm - 820 nm spectral region. We demonstrate over 7.9 watts in the signal band, out of a custom photonic crystal fiber with M 2  < 1.15. The input peak power as well as fiber length can be selected to keep the converted power in a 0.6 nm narrow emission band or broaden the output to 45 nm spectral band with spectral density greater than 50 mW/nm by pumping with higher peak powers.

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.

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 (As 2 Se 3 ) 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 As 2 Se 3 and CaF 2 layers, exhibits high reflectance across a ∼550 nm band with only five layers. Modeling results are compared with spectroscopic measurements, demonstrating good agreement.

Color vision results from the interaction of retinal photopigments with reflected or transmitted visible light. The International Commission on Illumination (CIE) developed the CIE color-matching chart, which separates colors on the basis of the interaction of their spectral profiles with three retinal photopigments in the human eye. We report the development of an infrared chromaticity (CIE-IR) chart, which mimics the CIE chart, in order to discriminate between different chemicals on the basis of the interactions of their IR signatures with three different IR optical filters, instead of the retinal photopigments in the human eye. Our results demonstrate that the CIE-IR chart enables separation of different classes of chemicals, as the visible CIE chart does with color, except for those in the IR spectral region. Such results clearly show that the biomimetic sensing method based on human color vision is in fact a true analogue to color vision and that the proposed CIE-IR chart can be used as a classification method unique to this biomimetic sensing modality.

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.

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.

Ultra-compact, low-loss, fast, and reconfigurable optical components, enabling manipulation of light by light, could open numerous opportunities for controlling light on the nanoscale. Nanostructured all-dielectric metasurfaces have been shown to enable extensive control of amplitude and phase of light in the linear optical regime. Among other functionalities, they offer unique opportunities for shaping the wave front of light to introduce the orbital angular momentum (OAM) to a beam. Such structured light beams bring a new degree of freedom for applications ranging from spectroscopy and micromanipulation to classical and quantum optical communications. To date, reconfigurability or tuning of the optical properties of all-dielectric metasurfaces have been achieved mechanically, thermally, electrically or optically, using phase-change or nonlinear optical materials. However, a majority of demonstrated tuning approaches are either slow or require high optical powers. Arsenic trisulfide (As 2 S 3 ) chalcogenide glass offering ultra-fast and large χ (3) nonlinearity as well as a low two-photon absorption coefficient in the near and mid-wave infrared spectral range, could provide a new platform for the realization of fast and relatively low intensity reconfigurable metasurfaces. Here, we design and experimentally demonstrate an As 2 S 3 chalcogenide glass based metasurface that enables reshaping of a conventional Hermite-Gaussian beam with no OAM into an OAM beam at low intensity levels, while preserves the original beam's amplitude and phase characteristics at high intensity levels. The proposed metasurface could find applications for a new generation of optical communication systems and optical signal processing.

An optical-filter-based sensor that was designed to mimic human color vision was recently developed. This sensor uses three mid-infrared optical filters to discriminate between chemicals with similar, strongly overlapping mid-infrared absorption bands. This non-spectroscopic technique requires no spectral scanning. This paper defines the selectivity and specificity of this biomimetic sensor. Receiver operating characteristic curves are presented for each target chemical. These results demonstrate that the sensor is highly selective and can provide discrimination with no false positives for three similar target chemicals-acetone, hexane, and fuel oil-while rejecting potential interferents.

A novel method combining elemental sulfur and selenium was developed, yielding crystalline sulfur-selenium compounds. The compounds were melted, and an organic comonomer added. Once the organic comonomer was consumed, the viscous compound was vitrified and allowed to cool yielding organic-inorganic hybrid polymers that are termed Organically Modified Chalcogenide (ORMOCHALC) polymers.

Optical-filter-based chemical sensors have the potential to dramatically alter the field of hazardous materials sensing. Such devices could be constructed using inexpensive components, in a small and lightweight package, for sensing hazardous chemicals in defense, industrial, and environmental applications. Filter-based sensors can be designed to mimic human color vision. Recent developments in this field have used this approach to discriminate between strongly overlapping chemical signatures in the mid-infrared. Reported work relied on using numerically filtered FTIR spectra to model the infrared biomimetic detection methodology. While these findings are encouraging, further advancement of this technique requires the collection and evaluation of directly filtered data, using an optical system without extensive numerical spectral analysis. The present work describes the design and testing of an infrared optical breadboard system that uses the biomimetic mammalian color-detection approach to chemical sensing. The set of chemicals tested includes one target chemical, fuel oil, along with two strongly overlapping interferents, acetone and hexane. The collected experimental results are compared with numerically filtered FTIR spectral data. The results show good agreement between the numerically filtered data model and the data collected using the optical breadboard system. It is shown that the optical breadboard system is operating as expected based on modeling and can be used for sensing and discriminating between chemicals with strongly overlapping absorption bands in the mid-infrared.

Negative curvature fibers have been gaining attention as fibers for high power infrared light. Currently, these fibers have been made of silica glass and infrared glasses solely through stack and draw. Infrared glasses' lower softening point presents the opportunity to perform low-temperature processing methods such as direct extrusion of pre-forms. We demonstrate an infrared-glass based negative curvature fiber fabricated through extrusion. The fiber shows record low losses in 9.75 - 10.5 µm range (which overlaps with the CO 2 emission bands). We show the fiber's lowest order mode and measure the numerical aperture in the longwave infrared transmission band. The possibility to directly extrude a negative curvature fiber with no penalties in losses is a strong motivation to think beyond the limitations of stack-and-draw to novel shapes for negative curvature fibers.

We computationally investigate fabrication tolerances in As 2 S 3 negative-curvature antiresonant tube-lattice fibers. Since the dominant loss mechanisms for silica in the mid-infrared (mid-IR) is material absorption, As 2 S 3 , which offers a reduced loss over that wavelength range, is a natural candidate for mid-IR antiresonant fibers. However, any fiber fabrication technology, including for soft glasses, will have imperfections. Therefore, it is important to know how imperfect fabrication will affect the results of a fiber design. We study perturbations to the fiber, including a nonconstant tube-wall thickness, a single cladding tube with a different radius, a single cladding tube with a different tube-wall thickness, and "key" sections in the jacket.

We computationally investigate moth-eye anti-reflective nanostructures imprinted on the endfaces of As 2 S 3 chalcogenide optical fibers. With a goal of maximizing the transmission through the endfaces, we investigate the effect of changing the parameters of the structure, including the height, width, period, shape, and angle-of-incidence. Using these results, we design two different moth-eye structures that can theoretically achieve almost 99.9% average transmisison through an As 2 S 3 surface.

Random anti-reflection structured surfaces (rARSS) have been reported to improve transmittance of optical-grade fused silica planar substrates to values greater than 99%. These textures are fabricated directly on the substrates using reactive-ion etching techniques, and often result in transmitted spectra with no measurable interference effects (fringes) for a wide range of wavelengths. The inductively coupled reactive-ion plasma (ICP-RIE) used in the fabrication process to etch the rARSS is anisotropic and thus well suited for planar components. The improvement in spectral transmission has been found to be independent of optical incidence angles for values from 0° to ±30°. Qualifying and quantifying the rARSS performance on curved substrates, such as convex lenses, is required to optimize the fabrication of the desired AR effect on optical-power elements. In this work, rARSS was fabricated on fused silica plano-convex lenses using a planar-substrate optimized ICP-RIE process to maximize optical transmission in the range from 500 to 1100 nm. Results are presented from optical transmission tests of rARSS lenses for both TE and TM incident polarizations at a wavelength of 633 nm and over a 70° full field of view. These results suggest optimization of the fabrication process to account for anisotropy is not required, mainly due to the wide angle-of-incidence AR tolerance performance of the rARSS lenses.

We present recent advancements in structured, antireflective surfaces on optics, including crystals for high-energy lasers as well as windows for the infrared wavelength region. These structured surfaces have been characterized and show high transmission and laser damage thresholds, making them attractive for these applications. We also present successful tests of windows with antireflective surfaces that were exposed to simulated harsh environments for the application of these laser systems.

The infrared range of the optical spectrum is attractive for its use in sensing, surveillance, and material characterization. The increasing availability of compact laser sources and detectors in the infrared range stands in contrast with the limited development of optical components for this optical range. We highlight developments of infrared components with a particular focus on fiber-based components for compact optical devices and systems.

We demonstrate a low-loss, repeatable, and robust splice between single-mode silica fiber and single-mode chalcogenide (CHG) fiber. These splices are particularly difficult to create because of the significant difference in the two fibers' glass transition temperatures (∼1000°C) as well as the large difference in the coefficients of thermal expansion between the fibers (∼20×10(-6)/°C). With 90% light coupled through the silica-CHG fiber splice, predominantly in the fundamental circular-symmetric mode, into the core of the CHG fiber and with 0.5 dB of splice loss measured around the wavelength of 2.5 μm, after correcting only for the Fresnel loss, the silica-CHG splice offers excellent beam quality and coupling efficiency. The tensile strength of the splice is greater than 12 kpsi, and the laser damage threshold is greater than 2 W (CW) and was limited by the available laser pump power. We also utilized this splicing technique to demonstrate 2 to 4.5 μm ultrabroadband supercontinuum generation in a monolithic all-fiber system comprising a CHG fiber and a high peak power 2 μm pulsed Raman-shifted thulium fiber laser. This is a major development toward compact form factor commercial applications of soft-glass mid-IR fibers.

It has been experimentally observed that moth-eye antireflective microstructures at the end of As2S3 fibers have an increased laser damage threshold relative to thin-film antireflective coatings. In this work, we computationally study the irradiance enhancement in As2S3 moth-eye antireflective microstructures in order to explain the increased damage threshold. We show that the irradiance enhancement occurs mostly on the air side of the interfaces and is minimal in the As2S3 material. We give a physical explanation for this behavior.

Optical filter-based chemical sensing techniques provide a new avenue to develop low-cost infrared sensors. These methods utilize multiple infrared optical filters to selectively measure different response functions for various chemicals, dependent on each chemical's infrared absorption. Rather than identifying distinct spectral features, which can then be used to determine the identity of a target chemical, optical filter-based approaches rely on measuring differences in the ensemble response between a given filter set and specific chemicals of interest. Therefore, the results of such methods are highly dependent on the original optical filter choice, which will dictate the selectivity, sensitivity, and stability of any filter-based sensing method. Recently, a method has been developed that utilizes unique detection vector operations defined by optical multifilter responses, to discriminate between volatile chemical vapors. This method, comparative-discrimination spectral detection (CDSD), is a technique which employs broadband optical filters to selectively discriminate between chemicals with highly overlapping infrared absorption spectra. CDSD has been shown to correctly distinguish between similar chemicals in the carbon-hydrogen stretch region of the infrared absorption spectra from 2800-3100 cm(-1). A key challenge to this approach is how to determine which optical filter sets should be utilized to achieve the greatest discrimination between target chemicals. Previous studies used empirical approaches to select the optical filter set; however this is insufficient to determine the optimum selectivity between strongly overlapping chemical spectra. Here we present a numerical approach to systematically study the effects of filter positioning and bandwidth on a number of three-chemical systems. We describe how both the filter properties, as well as the chemicals in each set, affect the CDSD results and subsequent discrimination. These results demonstrate the importance of choosing the proper filter set and chemicals for comparative discrimination, in order to identify the target chemical of interest in the presence of closely matched chemical interferents. These findings are an integral step in the development of experimental prototype sensors, which will utilize CDSD.

We computationally investigate cascaded amplification in a three-level mid-infrared (IR) Pr(3+)-doped chalcogenide fiber amplifier. The overlap of the cross-sections in the transitions (3)H(6)→(3)H(5) and (3)H(5)→(3)H(4) enable both transitions to simultaneously amplify a single wavelength in the range between 4.25 μm and 4.55 μm. High gain and low noise are achieved simultaneously if the signal is at 4.5 μm. We show that 45% of pump power that is injected at 2 μm can be shifted to 4.5 μm. The efficiency of using a mid-IR fiber amplifier is higher than what can be achieved by using mid-IR supercontinuum generation, which has been estimated at 25%. This mid-IR fiber amplifier can be used in conjunction with quantum cascade lasers to obtain a tunable, high-power mid-IR source.

We computationally investigate supercontinuum generation in an As ₂ S₃ solid core photonic crystal fiber (PCF) with a hexagonal cladding of air holes. With a goal of obtaining a supercontinuum output spectrum that can predict what might be seen in an experiment, we investigate the spectral and statistical behavior of a mid-infrared supercontinuum source using a large ensemble average of 10⁶ realizations, in which the input pulse duration and energy vary. The output spectrum is sensitive to small changes (0.1%) in these pulse parameters. We show that the spectrum can be divided into three regions with distinct characteristics: a short-wavelength region with high correlation, a middle-wavelength region with minimal correlation, and a long-wavelength region where the behavior is dominated by a few rare large-bandwidth events. We show that statistically significant fluctuations exist in the experimentally expected output spectrum and that we can reproduce an excellent match to that spectrum with a converged shape and bandwidth using 5000 realizations.

We report on a proof of concept for a compact supercontinuum source for the mid-infrared wavelength range based on a microchip laser and nonlinear conversion inside a selenide-based optical fiber. The spectrum extends from 3.74 to 4.64 μm at -10  dB from the peak and 3.65 to 4.9 μm at -20  dB from the peak; emitting beyond the wavelength range that periodically poled lithium niobate (PPLN) starts to display a power penalty. Wavelength conversion occurs inside the core of a single-mode fiber, resulting in a high-brightness emission source. A maximum average power of 5 mW was demonstrated, but the architecture is scalable to higher average powers.

Barium copper sulfur fluoride (BaCuSF) is a p-type transparent conductor (p-TC) that, when doped with potassium, exhibits exceptionally high conductivity. The results of a detailed optical and electronic characterization of BaCuSF thin films deposited at a substrate temperature of 100 °C are presented. X-ray diffractometry shows the presence of a cubic BaCuSF phase. Spectroscopic measurements demonstrate that the films transmit from the visible through the mid-infrared with a band gap of 1.8 eV. Hall measurements indicate that the material is a degenerate semiconductor. As deposited, the films exhibit conductivity at room temperature of approximately 260 S/cm - among the highest reported room temperature conductivities for p-TCs. After post-deposition treatment in water, their conductivity increases to as high as 800 S/cm, and their band gap is reduced to 1.5 eV. The potential for low temperature deposition of p-type films with high conductivity and optical transmittance makes BaCuSF promising for several applications including flexible electronics and photovoltaics.

Efficient inscription of Fiber Bragg gratings (FBGs) in single-mode, thin cladding As(2)S(3) fibers is demonstrated by using near bandgap light at 532 nm. The FBGs with the reflectivity of over 80% can be induced in only 80-90 s, substantially faster than in previous reports. The dynamics of the grating growth are investigated in the photosensitivity process, showing a fast blue shift of the Bragg wavelength and then a somewhat slower red shift. The aging of the grating after fabrication is also reported, indicating a 37% decay of the grating strength.

We investigate the dynamics of photoinduced index changes in chalcogenide As(2)S(3) fibers. Using a novel phase sensitive technique for measuring the photoinduced index change, we find that the index evolution is a two-stage process: it consists of a fast reduction and a subsequent slow increase in the refractive index. We show that the index change depends strongly on the beam intensity with both positive and negative changes possible. These findings can have application in design and fabrication of photoinduced devices such as Bragg gratings and photonic cavities.

An octave spanning spectrum is generated in an As₂S₃ taper via 77 pJ pulses from an ultrafast fiber laser. Using a previously developed tapering method, we construct a 1.3 μm taper that has a zero-dispersion wavelength around 1.4 μm. The low two-photon absorption of sulfide-based chalcogenide fiber allows for higher input powers than previous efforts in selenium-based chalcogenide tapered fibers. This higher power handling capability combined with input pulse chirp compensation allows an octave spanning spectrum to be generated directly from the taper using the unamplified laser output.

We present simulation results for supercontinuum generation using As(2)S(3) chalcogenide photonic crystal fibers. We found that more than 25% of input power can be shifted into the region between 3 microm and 5 microm using a pump wavelength of 2 microm with a peak power of 1 kW and an FWHM of 500 fs. The broad dispersion profile and high nonlinearity in As(2)S(3) chalcogenide glass are essential for this application.

We describe in detail a procedure for maximizing the bandwidth of supercontinuum generation in As(2)Se(3) chalcogenide fibers and the physics behind this procedure. First, we determine the key parameters that govern the design. Second, we find the conditions for the fiber to be endlessly single-mode; the fiber should be endlessly single-mode to maintain high nonlinearity and low coupling loss. We find that supercontinuum generation in As(2)Se(3) fibers proceeds in two stages--an initial stage that is dominated by four-wave mixing and a later stage that is dominated by the Raman-induced soliton self-frequency shift. Third, we determine the conditions to maximize the Stokes wavelength that is generated by four-wave mixing in the initial stage. Finally, we put all these pieces together to maximize the bandwidth. We show that it is possible to generate an optical bandwidth of more than 4 microm with an input pump wavelength of 2.5 microm using an As(2)Se(3) fiber with an air-hole-diameter-to-pitch ratio of 0.4 and a pitch of 3 microm. Obtaining this bandwidth requires a careful choice of the fiber's waveguide parameters and the pulse's peak power and duration, which determine respectively the fiber's dispersion and nonlinearity.

We characterize the nonlinear propagation of picosecond pulses in chalcogenide As(2)S(3) single-mode fiber using a pump-probe technique. The cross-phase modulation (XPM)-induced sideband broadening and stimulated Raman scattering (SRS)-induced sideband amplification are measured in order to map out the Raman gain spectrum of this glass across the C-band. We extract the Raman response function from the Raman gain spectrum and determine the power and polarization dependence of the SRS. In contrast to previous work using As(2)Se(3) fiber, we find that the As(2)S(3) fiber does not suffer from large two-photon absorption (TPA) in the wavelength range of the telecommunications band. We achieved a 20 dB peak Raman gain at a Stokes shift of 350 cm(-1) in a 205 mm length of As(2)S(3) single-mode fiber. The Raman gain coefficient is estimated to be 4.3x10(-12) m/W and the threshold pump peak power is estimated to be 16.2 W for the 205 mm As(2)S(3) fiber. We also demonstrate that we can infer the dispersion of the As(2)S(3) fiber and justify the Raman response function by comparing simulation and experimental results.

We report the first demonstration of high bit rate signal processing by a fiber-based photonic wire. We achieve 160 Gb/s demultiplexing via four wave mixing in a 1.9 microm diameter photonic wire tapered from As(2)S(3) chalcogenide glass single mode fibre, with very low pump power requirements ( < 20 mW average power, 0.45 W peak power), enabled by a very high nonlinearity (gamma approximately 7850 W(-1) km (-1) ) and greatly reduced dispersion.

Human HaCaT cells, exposed for 24 h to a 1 mT (rms) 50 Hz sinusoidal magnetic field in a temperature-regulated solenoid, suffer detectable changes in their biochemical properties and shapes. By using infrared wavelength-selective scanning near-field optical microscopy, we observed changes in the distribution of the inner chemical functional groups and in the cell morphology with a resolution of 80-100 nm.

Due to its surface sensitivity and high spatial resolution, scanning near-field optical microscopy (SNOM) has a significant potential to study the lateral organization of membrane domains and clusters. Compared to other techniques, infrared near-field microscopy in the spectroscopic mode has the advantage to be sensitive to specific chemical bonds. In fact, spectroscopic SNOM in the infrared spectral range (IR-SNOM) reveals the chemical content of the sample with a lateral resolution around 100 nm (Cricenti et al., 1998a, 1998b, 2003). Model lipid membranes were studied by IR-SNOM at several wavelengths. Topographical micrographs reveal the presence of islands at the surface and the optical images indicate the formation of locally ordered multiple bilayers - both critically important features for biotechnology and medical applications.

We present a technique for measuring the modal filtering ability of single mode fibers. The ideal modal filter rejects all input field components that have no overlap with the fundamental mode of the filter and does not attenuate the fundamental mode. We define the quality of a nonideal modal filter Q(f) as the ratio of transmittance for the fundamental mode to the transmittance for an input field that has no overlap with the fundamental mode. We demonstrate the technique on a 20 cm long mid-infrared fiber that was produced by the U.S. Naval Research Laboratory. The filter quality Q(f) for this fiber at 10.5 microm wavelength is 1000+/-300. The absorption and scattering losses in the fundamental mode are approximately 8 dB/m. The total transmittance for the fundamental mode, including Fresnel reflections, is 0.428+/-0.002. The application of interest is the search for extrasolar Earthlike planets using nulling interferometry. It requires high rejection ratios to suppress the light of a bright star, so that the faint planet becomes visible. The use of modal filters increases the rejection ratio (or, equivalently, relaxes requirements on the wavefront quality) by reducing the sensitivity to small wavefront errors. We show theoretically that, exclusive of coupling losses, the use of a modal filter leads to the improvement of the rejection ratio in a two-beam interferometer by a factor of Q(f).

Barium gallogermanate (BGG) glasses are currently being explored as a viable low cost material for numerous U.S. defense and commercial visible-infrared window applications. These glasses are transparent from 0.4 mum to beyond 5.0 mum and can be easily made in large optics and complex shapes with high index homogeneity. For high-energy laser (HEL) applications, knowledge of the thermo-optic coefficient (dn/dT) of the window material is important in determining the optical path distortion. The dn/dT measurements were made on BGG glass at 633 and 3390 nm and compared with the values for multispectral ZnS. The dn/dT for BGG glass was approximately 1/5 the value for multispectral ZnS, giving BGG glass a clear advantage for HEL applications.

A modified Barium Gallo-Germanate glass has been developed as an exit window for high energy lasers operating in the mid-infrared wavelength region. All the physical properties, for application as a window for high energy laser systems have been measured. Absorption loss and thermo-optic coefficient were identified as key in developing the Barium Gallo-Germanate glass for high energy laser applications. A purification method was developed to reduce the absorption loss of the glass from 6x10(-2) cm(-1) to 2x10(-3) cm(-1) at 3.8 mum. Manufacturability in large size windows has been demonstrated with the fabrication of an 18" diameter prototype window. Modified Barium Gallo-Germanate glasses have also been developed with lower thermo-optic coefficient resulting in lower optical path distortion.

Waveguide amplifiers fabricated in Er3+-doped gallium lanthanum sulfide (GLS) glass are demonstrated. GLS is deposited onto fused silica substrates by RF magnetron sputtering, and waveguides are patterned by use of the lift-off technique. The waveguides exhibit a total internal gain of 6.7 dB (2.8 dB/cm) for a signal with a wavelength of 1.55 mum. This experiment is, to the best of our knowledge, the first demonstration of gain in an Er3+-doped chalcogenide glass waveguide. The fabrication methods we apply, if used with other rare earth dopants, could potentially be employed to produce sources operating in the mid-IR.

Inhibitors of the G(2) DNA damage checkpoint can selectively sensitize cancer cells with mutated p53 to killing by DNA-damaging agents. Isogranulatimide is a G(2) checkpoint inhibitor containing a unique indole/maleimide/imidazole skeleton identified in a phenotypic cell-based screen; however, the mechanism of action of isogranulatimide is unknown. Using natural and synthetic isogranulatimide analogues, we show that the imide nitrogen and a basic nitrogen at position 14 or 15 in the imidazole ring are important for checkpoint inhibition. Isogranulatimide shows structural resemblance to the aglycon of UCN-01, a potent bisindolemaleimide inhibitor of protein kinase C beta (IC(50), 0.001 micromol/L) and of the checkpoint kinase Chk1 (IC(50), 0.007 micromol/L). In vitro kinase assays show that isogranulatimide inhibits Chk1 (IC(50), 0.1 micromol/L) but not protein kinase C beta. Of 13 additional protein kinases tested, isogranulatimide significantly inhibits only glycogen synthase kinase-3beta (IC(50), 0.5 micromol/L). We determined the crystal structure of the Chk1 catalytic domain complexed with isogranulatimide. Like UCN-01, isogranulatimide binds in the ATP-binding pocket of Chk1 and hydrogen bonds with the backbone carbonyl oxygen of Glu(85) and the amide nitrogen of Cys(87). Unlike UCN-01, the basic N15 of isogranulatimide interacts with Glu(17), causing a conformation change in the kinase glycine-rich loop that may contribute importantly to inhibition. The mechanism by which isogranulatimide inhibits Chk1 and its favorable kinase selectivity profile make it a promising candidate for modulating checkpoint responses in tumors for therapeutic benefit.

Ablation at wavelengths near lambda = 6.45 microm results in tissue ablation with minimal collateral damage (< 40 microm) yet yields a high ablation rate that is useful for human surgery. However, delivery of this wavelength has been limited to that in air and thus to applications in which the target tissue can be readily exposed. The goal of this study is to investigate the potential of a pulsed infrared laser at lambda = 6.45 microm for noncontact ablation in a liquid environment. To this end we investigated fiber delivery in combination with the use of infrared transparent liquids. Transmission characteristics and damage thresholds for two types of fiber materials (silver halide and arsenic sulfide), for high-power pulsed laser radiation were determined using the Mark III free electron laser. Both fibers had comparable bulk losses (0.54 dB/m and 0.62 dB/m, respectively) while the arsenic sulfide fibers showed more coupling losses (37 versus 27%). Damage thresholds were higher in arsenic sulfide fibers than in silver halide fibers (1.12 GW/cm2 versus 0.54 GW/cm2), but both fibers were sufficient to deliver radiant exposures well above the ablation threshold in tissue. Seven different perfluorocarbon liquids (PFCLs), known for their transparency at lambda = 2.94 microm, were investigated and their optical transmission was determined using Fourier transform infrared and direct Beer's law measurements. All of the PFCLs tested had similar values for an absorption coefficient mu(a) at a given wavelength (mu(a) = 0.05 mm(-1) at lambda = 2.94 microm and mu(a) is approximately 3 mm(-1) at lambda = 6.45 microm). Pump-probe imaging showed the ablation sequence (lambda = 6.45 microm) at the fiber tip in a water environment, which revealed a fast expanding and collapsing bubble. In contrast, the volatile PF-5060 showed no fast bubble expansion and collapse, but rather formation of nontransient gas bubbles. Perfluorodecalin did not show any bubble formation at the radiant exposures used. It was shown that using the lambda = 6.45 microm wavelength delivered via fiber optics in combination with perfluorodecalin allows a noncontact laser surgical procedure. Deeper structures, however, are effectively shielded because the radiant exposure of the beam will fall below the ablation threshold owing to the absorption by perfluorodecalin. This may optimize the efficacy and safety of laser-based vitreoretinal surgery.

The infrared (IR) absorption of a biological system can potentially report on fundamentally important microchemical properties. For example, molecular IR profiles are known to change during increases in metabolic flux, protein phosphorylation, or proteolytic cleavage. However, practical implementation of intracellular IR imaging has been problematic because the diffraction limit of conventional infrared microscopy results in low spatial resolution. We have overcome this limitation by using an IR spectroscopic version of scanning near-field optical microscopy (SNOM), in conjunction with a tunable free-electron laser source. The results presented here clearly reveal different chemical constituents in thin films and biological cells. The space distribution of specific chemical species was obtained by taking SNOM images at IR wavelengths (lambda) corresponding to stretch absorption bands of common biochemical bonds, such as the amide bond. In our SNOM implementation, this chemical sensitivity is combined with a lateral resolution of 0.1 micro m ( approximately lambda/70), well below the diffraction limit of standard infrared microscopy. The potential applications of this approach touch virtually every aspect of the life sciences and medical research, as well as problems in materials science, chemistry, physics, and environmental research.

We have demonstrated Raman small-core As-Se fiber. More than 20-dB of gain was observed in a 1.1-m length of fiber pumped by a nanosecond pulse of approximately 10.8-W peak power at 1.50 microm. The peak of the Raman gain occurred at a shift of approximately 240 cm(-1). The Raman gain coefficient is estimated to be approximately 2.3 x 10(-11) m/W, which is more than 300 times greater than that of silica. The large Raman gain coefficient coupled with the large IR transparency window of these fibers shows promise for development of As-Se Raman fiber lasers and amplifiers in the near-, mid-, and long-IR spectral regions.

Gallogermanate glasses are the subject of intense study as a result of their unique combination of physical and optical properties, including transmission from 0.4 to beyond 5.0 microm. These glasses can be easily made into large optics with high-index homogeneity for numerous U.S. Department of Defense and commercial visible-IR window applications such as reconnaissance, missile domes, IR countermeasures, avionics, and collision avoidance on automobiles. These applications require a knowledge of the refractive index of glass throughout the region of transmission. Consequently, we have measured the refractive index of BaO-Ga2O3-GeO2 glass from 0.4 to 5.0 microm and calculated the Sellmeier coefficients required for optical device design.

We have synthesized a series of chalcogenide glasses from the As-S-Se system that is designed to have strong nonlinearities. Measurements reveal that many of these glasses offer optical Kerr nonlinearities greater than 400 times that of fused silica at 1.25 and 1.55mum and figures of merit for all-optical switching greater than 5 at 1.55mum .

Hydrogen chemistry in thin films and biological systems is one of the most difficult experimental problems in today's science and technology. We successfully tested a novel solution, based on the spectroscopic version of scanning near-field optical microscopy (SNOM). The tunable infrared radiation of the Vanderbilt free electron laser enabled us to reveal clearly hydrogen-decorated grain boundaries on nominally hydrogen-free diamond films. The images were obtained by SNOM detection of reflected 3.5 microm photons, corresponding to the C-H stretch absorption, and reached a lateral resolution of 0.2 microm, well below the lambda/2 (lambda = wavelength) limit of classical microscopy.

Bacterial lipopolysaccharide (LPS) elicits responses by macrophages that help the body repel infections. Recent evidence indicates that phosphatidylinositol 3-kinase (PI 3-kinase) may mediate some of these responses. Here, we show that exposing macrophages to LPS rapidly increased membrane-associated PI 3-kinase activity and also elevated p70 S6 kinase activity. Inhibitors of PI 3-kinase or the mammalian target of rapamycin (mTOR) fully blocked p70 S6 kinase activation, implying that this kinase is controlled by PI 3-kinase and mTOR. These inhibitors also substantially reduced LPS-induced nitric oxide (NO) production. This inhibition was, in part, attributable to impaired LPS-stimulated secretion of interferon-beta, an autocrine co-factor for NO production. However, the addition of exogenous interferon-beta did not fully restore NO production, indicating that the NO response was being inhibited by another mechanism as well. Together, these data suggest that PI 3-kinase, mTOR, and possibly p70 S6 kinase mediate LPS-induced NO production by regulating the secretion of interferon-beta and by a second undefined mechanism.