Picosecond study of near-band-gap nonlinearities in GaInAsP

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Abstract
From picosecond pump-probe and forward degenerate four-wave mixing (DFWM) experiments, we obtain the near-band-gap nonlinear absorption and refraction properties of GaInAsP for λ∼1.5 μm. Using a mode-locked color center laser, the nonlinear signals are studied in room-temperature samples as a function of time and wavelength for different pump energies and for materials with different band-gap energies. Nonlinear absorption cross sections σeh as large as −5.7×10−15 cm2 are obtained from the pump-probe results, while effective nonlinear cross sections σeff as large as 7.8×10−16 cm2 (corresponding to a steady state ‖χ(3)‖∼3.8×10−3 esu for a 20-ns relaxation time) are measured in the DFWM experiments. The spectral behavior of the data shows that above the band gap, the nonlinearity is due both to band filling and screening of excitonic effects. However, the effectiveness of the screening diminishes within one or two plasma frequencies of the band edge.