Intensity-dependent degenerate and non-degenerate nonlinear optical absorption of direct-gap semiconductors

摘要

The nonlinear optical response of direct-gap semiconductors is investigated with a focus on non-degenerate multiphotonabsorption processes. The theoretical approach is based on the semiconductor Bloch equations and yieldsthe absorption rate either perturbatively or non-perturbatively in the incident light intensities. We describe thesemiconductor by a two-band model and consider a pump-probe scheme where the weak probe pulse provides oneof the simultaneously absorbed photons. The perturbative response can be described analytically within someapproximations and we give simple expressions for two, three, and four-photon absorption coefficients. Theseare compared with numerical results for the absorption of pulses with a finite duration, where the inuence ofdephasing and relaxation as well as higher-order corrections are also investigated. For strong pump fields thatare treated non-perturbatively we demonstrate non-trivial dependencies of the absorption on the time delaybetween the pulses. In the non-perturbative response of a single light pulse characteristic modulations appearin the absorption dependence on the field strength that may be interpreted as multi-photon Rabi oscillations.Finally, we present measurements of the non-degenerate two-photon absorption coefficient of bulk GaAs via timedelayand polarization-dependent transmissivity changes in a pump probe setup. The observed strong increaseof the absorption coefficient with frequency ratios deviating from unity qualitatively agrees with theoreticalexpectations.