Direct measurement of dispersive nonlinearities in GaAs.

Nonlinear refractive index changes in 299angstroms multiple- quantum-well (MQW) GaAs are obtained directly by measuring the Fabry-Perot transmission peak shift. These changes crosscheck nonlinear refractive index changes obtained by Kramers-Kronig transformations of the nonlinear absorption under identical pumping conditions. The nonlinear refractive index changes become dominated by nonresonant contributions with an increase of the pump power. Thermal refractive index changes were also measured directly.