Using interdisciplinary fields relevant to a highly excited semiconductor - nonequilibrium phenomena in high density plasma, light-induced changes of optical properties, and dynamic holography, we developed time-resolved four-wave mixing technique for monitoring the spatial and temporal carrier dynamics in wide band-gap semiconductors. This opened a new possibility to analyse fast electronic processes in a non-destructive "all-optical" way, i.e. without any electrical contacts. This technique allowed evaluation of recombination and transport processes and the determination of important carrier parameters which directly reveal the material quality: carrier lifetime, bipolar diffusion coefficients, surface recombination rate, nonlinear recombination rate, diffusion length, threshold of stimulated recombination. The recent experimental studies of differently grown group III-nitrides (heterostructures and free standing films) as well silicon carbide epilayers by nondegenerate picosecond four-wave mixing are presented.
We present experimental data on degenerate four-wave mixing as well as simulation results of fast optical nonlinearities in highly-excited semi-insulating InP under applied dc-field. Hot-electron transport governed enhancement of optical nonlinearity is obtained by applying a dc-field of 10-14 kV/cm at full-modulation depth of a light-interference pattern. The hydrodynamic model, which incorporates both free-carrier and photorefractive nonlinearities is used to explain the experimentally observed features. We show that the enhancement of optical nonlinearity is due to the quadratic electrooptic effect.
Time-resolved photoluminescence and four-wave mixing techniques have been combined for studies of carrier relaxation dynamics in a highly photoexcited GaN epilayer. For a moderate excitation density below 1 mJ/cm^2, carrier recombination was due to free carrier capture by deep traps. The characteristic time of carrier capture,τ_e=550 ps, was measured under deep trap saturation regime. The ambipolar diffusion coefficient for free carriers, D=1.7 cm^2/s, was estimated from the analysis of the transients of the light-induced gratings of various periods. A complete saturation of the four-wave mixing efficiency was observed for the excitation energy density exceeding 1.5 mJ/cm^2. The latter saturation effect was shown to be related to electron-hole plasma degeneration, which results in a significant enhancement of carrier recombination rate due to onset of stimulated emission.
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