Investigation of New Geometries for High Duty Cycle Far-Infrared p-Type Germanium Lasers
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INTRODUCTION The germanium hole population inversion laser is a unique pulsed source of very farinfrared, coherent lighl. It is continuously tunable from 75 to 300 jrm,[l] and can emit powers of up to a few Watts.[2] The laser is unique in that it is the only semiconductor laser which is tunable over such a large frequency range. It is also the only semiconductor laser that emits in the very farinfrared. Although these are achievements, successful applications for the laser still require further device research and development. One example of an application for the germanium laser is its use as the local oscillator in heterodyne mixing spectroscopy for detection of molecular species in star-forming regions of outer space. Since the atmosphere absorbs wide portions of the far-infrared spectrum, for astronomical observation it is desirable to operate a far-infrared spectrometer in space instead of within the
earth's atmosphere. Use on satellite platforms requires a laser to be small, light, continuously tunable, continuous-wave, and rugged. The germanium laser fulfills all but one of these criteria: currently it is only operable in a pulsed mode. Spontaneous emission from inter-valence band transitions in p-type germanium was first observed by Komiyama in 1982 [3] and stimulated emission followed with the discovery by Andronov's group in 1984.[2] The laser operates near liquid helium temperatures and under perpendicularly crossed electric field (E) and magnetic induction (B). Under these conditions the maximum kinetic energy E,,.. of a charge carrier with effective mass m* is given by the equation: .etin s=2 m* (E/B)2
(1)
which is based on the Lorentz force. Equation (1) shows that the kinetic energy is proportional to the effective mass, i. e., heavy holes in the germanium valence band will have a higher maximum kinetic energy than light holes. For a certain range of E/B ratio heavy holes will emit optical phonons while light holes cannot. When the heavy holes emit optical phonons, there is a finite probability for them to enter the light hole band, becoming light holes. This process creates a population inversion between the heavy and light hole sub-bands within the valence band. Farinfrared stimulated emission originates from transitions of holes between light and heavy hole Landau levels. t Cunfent address: Physical Chemistry H,Ruhr-University Bochum, Universitaetsstrasse 150, Bochum, Germany. 285 Mat. Res. Soc. Symp. Proc. Vol. 607 © 2000 Materials Research Society
The hole current necessary for operation generates a large amount of resistive heating. This increases the acoustic phonon density, and at a temperature between 20 to 40 K, depending on the doping concentration, the population inversion necessary for lasing is destroyed.[4] In order to keep the laser crystal temperature sufficiently low, the lasers must be operated in a pulsed mode. To achieve continuous-wave operation, it is necessary to decrease the electrical power input into the laser crystal and increase the cooling power of the heat sinks a
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