Direct measurement of dispersive nonlinearities in GaAs

Abstract

Nonlinear refractive-index changes in GaAs are obtained at room temperature by measuring shifts of Fabry-Perot (F.P.) transmission. The 299-Å multiple-quantum-well GaAs sample was sandwiched between two dielectric mirrors (90% reflectivity) to form a F.P. etalon (finesse =10). The pump beam was tuned to 821 nm and modulated to obtain 1-μs pulses at 10 kHz. The photoluminescence from a focused pump spot at the GaAs etalon was used as a broadband probe source. Since the photoluminescence is due to carriers generated by a pump beam, the pump and probe beams automatically align themselves. The fast electronic nonlinearity exhibited blue shifts (negative index changes) of the F.P. transmission peaks, while the slow thermal nonlinearity showed red shifts (positive index changes). Nonlinear index changes were calculated from the measured shifts of the F.P. peaks for different pump intensities. A self-consistent F.P. phase function for this calculation was obtained assuming lengths of 2.9 and 1.5 μm for the GaAs and air gap, respectively. Optical bistability was most easily observed for the region of large index change (873-878 nm). The slow nonlinear index changes below the band gap showed nonresonant characteristics rather than resonant, saturable behavior.