A new class of semiconductor lasers that can potentially produce much more short pulse energy is presented. This new laser is not limited in volume or aspect ratio by the depth of a p-n junction and are created from current filaments in semi-insulating GaAs. A current filament semiconductor lasers (CFSL) that have produced 75 nJ of 890 nm radiation in 1.5 ns were tested. A filaments as long as 3.4 cm and several hundred microns in diameter in high gain GaAs photoconductive switches were observed. Their smallest dimension can be more than 100 times the carrier diffusion length in GaAs. The spectral narrowing, lasing thresholds, beam divergence, temporal narrowing and energies which imply lasing for several configurations of CFSL are reported.
This report provides a summary of the LDRD project titled: An Electromagnetic Imaging System for Environmental Site Reconnaissance. The major initial challenge of this LDRD was to develop a ground penetrating radar (GPR) whose peak and average radiated power surpassed that of any other in existence. Goals were set to use such a system to detect the following: (1) disrupted soil layers where there is potential for buried waste, (2) buried objects such as 55-gallon drums at depths up to 3 m, and (3) detecting contaminated soil. Initial modeling of the problem suggested that for soil conditions similar to Puerto Rican clay loam, moisture content 10 percent (conductivity = 0.01 mhos at 350 MHz), a buried 55-gallon drum could be detected in a straightforward manner by an UWB GPR system at a depth of 3 meters. From the simulations, the highest attenuation ({minus}50 dB) was the result of scattering from a 3-m deep vertically orientated drum. A system loss of {minus}100 dB is a typical limit for all kinds of radar systems (either direct time-domain or swept frequency). The modeling work also determined that the waveshape of the pulse scattered off the buried drum would be relatively insensitive to drum orientation, and thus easier to detect with the GPR system.
This paper reports on a recent comparison made between the Air Force Research Laboratory (AFRL) gallium arsenide, optically-triggered switch test configuration and the Sandia National Laboratories (SNL) gallium arsenide, optically-triggered switch test configuration. The purpose of these measurements was to compare the temporal switch jitter times. It is found that the optical trigger laser characteristics are dominant in determining the PCSS jitter.
The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) has been extended to over 100 million pulses at 23A, and over 100 pulses at 1kA. This is achieved by improving the ohmic contacts by doping the semi-insulating GaAs underneath the metal, and by achieving a more uniform distribution of contact wear across the entire switch by distributing the trigger light to form multiple filaments. This paper will compare various approaches to doping the contacts, including ion implantation, thermal diffusion, and epitaxial growth. The device characterization also includes examination of the filament behavior using open-shutter, infra-red imaging during high gain switching. These techniques provide information on the filament carrier densities as well as the influence that the different contact structures and trigger light distributions have on the distribution of the current in the devices. This information is guiding the continuing refinement of contact structures and geometries for further improvements in switch longevity.
A new class of semiconductor laser is presented that does not require p-n junctions. Spectral narrowing, lasing thresholds, beam divergence, temporal narrowing, and energies are shown for these lasers based on current filaments in bulk GaAs.