Effects of radiation on laser diodes.

The effects of ionizing and neutron radiation on the characteristics and performance of laser diodes are reviewed, and the formation mechanisms for nonradiative recombination centers, the primary type of radiation damage in laser diodes, are discussed. Additional topics include the detrimental effects of aluminum in the active (lasing) volume, the transient effects of high-dose-rate pulses of ionizing radiation, and a summary of ways to improve the radiation hardness of laser diodes. Radiation effects on laser diodes emitting in the wavelength region around 808 nm are emphasized

Studies of the laser-induced fluorescence of explosives and explosive compositions.

Continuing use of explosives by terrorists throughout the world has led to great interest in explosives detection technology, especially in technologies that have potential for standoff detection. This LDRD was undertaken in order to investigate the possible detection of explosive particulates at safe standoff distances in an attempt to identify vehicles that might contain large vehicle bombs (LVBs). The explosives investigated have included the common homogeneous or molecular explosives, 2,4,6-trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), cyclonite or hexogen (RDX), octogen (HMX), and the heterogeneous explosive, ammonium nitrate/fuel oil (ANFO), and its components. We have investigated standard excited/dispersed fluorescence, laser-excited prompt and delayed dispersed fluorescence using excitation wavelengths of 266 and 355 nm, the effects of polarization of the laser excitation light, and fluorescence imaging microscopy using 365- and 470-nm excitation. The four nitro-based, homogeneous explosives (TNT, PETN, RDX, and HMX) exhibit virtually no native fluorescence, but do exhibit quenching effects of varying magnitude when adsorbed on fluorescing surfaces. Ammonium nitrate and fuel oil mixtures fluoresce primarily due to the fuel oil, and, in some cases, due to the presence of hydrophobic coatings on ammonium nitrate prill or impurities in the ammonium nitrate itself. Pure ammonium nitrate shows no detectable fluorescence. These results are of scientific interest, but they provide little hope for the use of UV-excited fluorescence as a technique to perform safe standoff detection of adsorbed explosive particulates under real-world conditions with a useful degree of reliability

Structural and dynamical studies of fluorozirconate glass

Fluorozirconate glass structure was studied by comparison with fluorozirconate crystal data and by the molecular dynamics computer simulation technique. A detailed tabulation of the bond lengths and angles in crystalline fluorozirconates of known structure yielded general trends regarding the effects of bridging, countercation size, and countercation charge. These observations were applied to the interpretation of analogous solid-state effects on the vibrational frequencies of fluorozirconate glasses and crystals. Spectroscopic evidence for the occurrence of double fluoride bridges (edge-bridging) between Zr atoms in the glasses is presented, and a general treatment based on Badger\u27s rule is used to estimate the characteristic symmetric stretching frequencies, \nu\sb{\rm s}, of various ZrF\sb{\rm n} groups in the presence of divalent and monovalent countercations. New vibrational spectra of $\alpha$-ZrF\sb4, MgZrF\sb6, Ba\sb2ZrF\sb8, $\beta$-BaZr\sb2F\sb{10}, $\alpha$-BaZr\sb2F\sb{10}, and Ba\sb3ZrF\sb{10} are presented and incorporated into the method. The factors affecting \nu\sb{\rm s} are outlined and discussed in detail. Complications of estimated frequencies are used to deduce local structure around Zr ions in binary barium fluorozirconate glasses and in fluorozirconate melts containing lithium and sodium ions. It is concluded that the presence of 8-coordinate Zr in some barium fluorozirconate glasses is not inconsistent with the results of Raman spectroscopy. The viscosity and activation energy for viscous flow of fluorozirconate melts were calculated by molecular dynamics computer simulation and compared with experimental results for fluorozirconates, as well as with simulated results for silica melts. Vibrational spectra of fluorozirconate glasses and crystals were also simulated by this technique