Sudhir B. Trivedi

Photorefractivity at 1.5 μm in CdTe:V

We have for the first time demonstrated two‐beam coupling energy transfer at a wavelength of 1.5 μm. Beam coupling gain coefficients of 0.6 cm−1 have been obtained in vanadium ‐doped CdTe with only 5 mW/cm2 incident intensity. These gain coefficients exceed typical gain coefficients in GaAs at 1.06 μm wavelength by 50%. In preliminary measurements using the moving grating technique, we have measured a gain coefficient of 2.4 cm−1. Through adjustment of the doping level, CdTe:V can be used as a sensitive photorefractive material through the 0.9–1.5 μm spectral range.

Mid-Infrared Emission from Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a powerful analytical technique for detecting and identifying trace elemental contaminants by monitoring the visible atomic emission from small plasmas. However, mid-infrared (MIR), generally referring to the wavelength range between 2.5 to 25 μm, molecular vibrational and rotational emissions generated by a sample during a LIBS event has not been reported. The LIBS investigations reported in the literature largely involve spectral analysis in the ultraviolet–visible–near-infrared (UV-VIS-NIR) region (less than 1 μm) to probe elemental composition and profiles. Measurements were made to probe the MIR emission from a LIBS event between 3 and 5.75 μm. Oxidation of the sputtered carbon atoms and/or carbon-containing fragments from the sample and atmospheric oxygen produced CO2 and CO vibrational emission features from 4.2 to 4.8 μm. The LIBS MIR emission has the potential to augment the conventional UV-VIS electronic emission information with that in the MIR region.

Recent Progress in the Development of Neodymium-Doped Ceramic Yttria

Solid-state lasers play a significant role in providing the technology necessary for active remote sensing of the atmosphere. Neodymium-doped yttria (Nd:Y2O3) is considered to be an attractive material due to its possible lasing wavelengths of ~914 and ~946 nm for ozone profiling. These wavelengths, when frequency tripled, can generate ultraviolet (UV) light at ~305 and ~315 nm, which is particularly useful for ozone sensing using differential absorption light detection and ranging (LIDAR) technique. For practical realization of space-based UV transmitter technology, ceramic Nd:Y2O3 material is considered to possess a great potential. A plasma melting and quenching method has been developed to produce Nd3+-doped powders for consolidation into Nd: Y2O3 ceramic laser materials. This far-from-equilibrium processing methodology allows higher levels of rare earth doping than can be achieved by equilibrium methods. The method comprises two main steps: 1) plasma melting and quenching to generate dense, and homogeneous doped metastable powders and 2) pressure-assisted consolidation of these powders by hot isostatic pressing to make dense nanocomposite ceramics. Using this process, several in 1times1 ceramic cylinders have been produced. The infrared transmission of a 2-mm-thick undoped Y2O3 ceramic was as high as ~75% without antireflection coating. In the case of Nd:Y2O3, ceramics infrared transmission values of ~50% were achieved for a similar sample thickness. Furthermore, Nd:Y2O3 samples with dopant concentrations of up to ~2 at.% were prepared without significant emission quenching.

Long-wave, infrared laser-induced breakdown (LIBS) spectroscopy emissions from energetic materials.

Laser-induced breakdown spectroscopy (LIBS) has shown great promise for applications in chemical, biological, and explosives sensing and has significant potential for real-time standoff detection and analysis. In this study, LIBS emissions were obtained in the mid-infrared (MIR) and long-wave infrared (LWIR) spectral regions for potential applications in explosive material sensing. The IR spectroscopy region revealed vibrational and rotational signatures of functional groups in molecules and fragments thereof. The silicon-based detector for conventional ultraviolet–visible LIBS operations was replaced with a mercury–cadmium–telluride detector for MIR–LWIR spectral detection. The IR spectral signature region between 4 and 12 μm was mined for the appearance of MIR and LWIR–LIBS emissions directly indicative of oxygenated breakdown products as well as dissociated, and/or recombined sample molecular fragments. Distinct LWIR–LIBS emission signatures from dissociated-recombination sample molecular fragments between 4 and 12 μm are observed for the first time.

Mid-Infrared Laser-Induced Breakdown Spectroscopy Emissions from Alkali Metal Halides

CLAYTON S.-C. YANG,* E. BROWN, UWE HOMMERICH, SUDHIR B. TRIVEDI, ALAN C. SAMUELS, and A. PETER SNYDER Battelle Eastern Science and Technology Center, Aberdeen, Maryland 21001 (C.S.-C.Y.); Department of Physics, Hampton University, Hampton, Virginia 23668 (E.B., U.H.); Brimrose Corporation of America, Baltimore, Maryland 21152 (S.B.T.); and Edgewood Chemical Biological Center, Aberdeen Proving Ground, Maryland 21010-5424 (A.C.S., A.P.S.)

Enhanced detection bandwidth for optical doppler frequency measurements using moving space charge field effects in GaAs multiple quantum wells

Moving space charge field (photo-emf) effects are observed in standard and low temperature grown GaAs multiple quantum well (MQW) samples in which dc photocurrents are generated by constant speed moving optical interference patterns. Experiments reveal an enhanced detection bandwidth for optical doppler frequency measurements compared to previously studied bulk II-VI and III-V materials due to the higher quantum efficiency and optical absorption of GaAs MQWs, in spite of their reduced carrier mobility-lifetime product values resulting mainly from the significantly shorter free carrier lifetimes. The high defect density of low temperature grown MQWs can also yield large field-of-view and, for optical ultrasound detections, high cutoff frequency.

Signal strength enhancement and bandwidth tuning in moving space charge field photodetectors using alternating bias field

Enhancement of the photocurrent density generated by internal moving space charge electric fields in semi-insulating GaAs multiple quantum wells is observed when the sample is biased with a high speed, zero-mean square wave electric field which increases the internal space charge electric field strength. Tunability of the space charge field formation time is also demonstrated by adjusting the amplitude of the bias signal. This technique can be used to enhance the sensitivity of optical doppler frequency measurements based on moving space charge field effects in photoconductive semiconductors. Electronic tunability of the space charge field formation time also adds versatility in optical power spectrum analysis applications.

Mid-Infrared, Long Wave Infrared (4–12 μm) Molecular Emission Signatures from Pharmaceuticals Using Laser-Induced Breakdown Spectroscopy (LIBS)

In an effort to augment the atomic emission spectra of conventional laser-induced breakdown spectroscopy (LIBS) and to provide an increase in selectivity, mid-wave to long-wave infrared (IR), LIBS studies were performed on several organic pharmaceuticals. Laser-induced breakdown spectroscopy signature molecular emissions of target organic compounds are observed for the first time in the IR fingerprint spectral region between 4–12 μm. The IR emission spectra of select organic pharmaceuticals closely correlate with their respective standard Fourier transform infrared spectra. Intact and/or fragment sample molecular species evidently survive the LIBS event. The combination of atomic emission signatures derived from conventional ultraviolet–visible-near-infrared LIBS with fingerprints of intact molecular entities determined from IR LIBS promises to be a powerful tool for chemical detection.

Development of mercurous halide crystals for acousto-optic devices

Large crystals of Hg2Cl2 and Hg2Br2 (48 mm in diameter, up to 600g in weight) have been grown by self-seeded contactless Physical Vapor Transport (PVT) technique in closed ampoules in vertical configuration. Seed selection was accomplished in the small diameter tubing at the ampoule tip. Material purification was done by resublimation processing. The crystals show high transparency, with good transmittance and good crystallographic quality. An acousto-optic modulator built from the Hg2Cl2 crystal showed good performance consistent with predicted device parameters.

Synthesis and 1.5μm emission properties of Nd3+ activated lead bromide and lead iodide crystals

We report on the purification, synthesis, and infrared emission properties of Nd doped PbBr2 and PbI2 crystals. Under optical excitation at ∼0.8μm, characteristic Nd3+ emission lines arising from the F3∕24 level were observed. In addition, near infrared emission lines were also observed at ∼0.97, ∼1.2, and ∼1.5μm arising from the F5∕24,H9∕22 states of Nd3+. The emission efficiency at 1.5μm was determined to be 74% for Nd:PbBr2 and 64% for Nd:PbI2 with peak emission cross sections in the range of (0.2–0.5)×10−20cm2. These cross sections are comparable to those of Er3+ doped materials employed in optical communications and eye-safe lasers.