Infrared Probing of Thermal and Spatial Properties of Laser Generated Carrier Distributions in Germanium.
Laser-generated high density electron-hole plasmas in germanium are investigated theoretically and experimentally in order to study the spatial and thermal distributions of the carriers and the role of various carrier scattering mechanisms. Theoretical carrier diffusion equations which determine the evolution in space and time of the plasma density and temperature are solved. The influence of the photogenerated free carriers on the mid-infrared optical absorption and dispersion is discussed theoretically in terms of the classical Drude model with the inclusion of intervalence band absorption. Experimentally, solid state plasmas with peak densities in excess of 10('19) cm('-3) are generated in germanium at 1.06 (mu)m with 100 ns pulses at intensities on the order of 1 MW/cm('2) with a Q-switched Nd('3+):glass laser. The optically injected plasmas are probed in reflection and transmission at 10.6 (mu)m with a pulsed CO(,2) laser. The transient free carrier plasma reflectivity minimum at 10.6 (mu)m is observed and studied as a function of laser excitation intensity, sample surface preparation and sample temperature. Analysis of the results is given in terms of the computed density and temperature profiles and the Drude model. The observation that the reflectivity minimum increases with both increasing excitation intensity and sample temperature is used to conclude that carrier-lattice scattering as opposed to carrier-carrier interactions determines the carrier momentum relaxation time in these plasmas. Infrared transmission and photoluminescence experiments provide information about carrier-lattice thermalization, diffusion and recombination which support the theoretical model. Additional confirmation of the high temperatures induced by the excitation laser is provided by reflectivity data at 0.633 (mu)m for very high laser intensities. The small differences between the experimental results and the theoretical model are accounted for in terms of plasma inhomogeneity effects and surface properties. Laser induced plasmas in other semiconductors, GaSb and InSb, are also studied. However due to the stronger thermal effects in these materials, it is not clear that the plasma reflectivity minimum is observed. Recommendations for further research into the properties of solid state hosted plasmas are presented.
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- Physics: Optics