Differential Transmission Signal Research Articles

Femtosecond degenerate and nondegenerate pump-probe experiments in bulk GaAs below the band gap

We perform degenerate and nondegenerate pump-probe experiments on bulk GaAs at 100 K below the band gap. We mostly observe a negative differential transmission signal both in the degenerate and nondegenerate experiments. We interpret our signal as due to two-photon absorption. This negative signal has a different origin from the normally considered band gap renormalization for resonant excitations.

Coherent signature of differential transmission signals in semiconductors: Theory and experiment

Transient pump-probe signals of a GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum well are investigated both theoretically and experimentally for different excitation conditions. The calculations are based on a microscopic density-matrix approach taking higher-order correlations into account. We present spectrally resolved transients which provide insights into the nature of the differential transmission change. Especially the complex phase signature of heavy-hole\char21{}light-hole quantum beats in dependence of the detection energy allows for an identification of different microscopic processes in the pump-probe signal. The theoretical results are verified experimentally by comparison with the coherent and incoherent parts of the time-resolved data. We find a dominant role of the Coulomb-induced correlations up to the ${\mathrm{\ensuremath{\chi}}}^{(3)}$ relevant four-point level in the coherent signature in comparison to the Pauli blocking contributions.

Measuring the ultrafast dynamics of Coulomb correlation effects: Dephasing in nonequilibrium Fermi seas

We measure the dynamics of the quantum phase coherence of the wave function describing interacting electrons and holes optically excited into the continuum of a GaAs/AlGaAs quantum well. We achieve this by spectrally resolving both the transient four-wave mixing signal and the differential transmission signal arising from such interacting carriers. We observe a dynamical blue shift of the coherent four-wave mixing emission spectrum with respect to the excitation laser pulse and interpret this as a direct manifestation of the dynamic Coulomb correlation effects in this many-body system.

Static and dynamic response of multiple-quantum-well voltage-controlled bistable laser diodes

We have simulated the static and dynamic characteristics of voltage-controlled multiple-quantum-well (MQW) bistable laser diodes. To investigate the time response of the saturable absorber under applied electric field, we performed pump-probe measurements with picosecond resolution. The obtained differential transmission signals indicate the reduction of the carrier escape time for the saturable absorber with increasing applied electric field. The field screening effect caused by spatial change of the carrier distribution is an important factor, as is phase space filling due to the photogenerated carriers. On the basis of the time response measurements, we have designed an MQW bistable laser by solving the modified rate equation including the recovery time response of the absorption saturation, A saturable absorption region narrower than 10 /spl mu/m is suitable for obtaining a low threshold device. To achieve low switching power and high switching speed, it is important to optimize the bias conditions and the MQW structures. We can expect a turn-off time of less than 10 ps, and a repetition rate of over 5 GHz from the calculations.

ULTRAFAST DYNAMICS OF COULOMB CORRELATION EFFECTS IN NON-EQUILIBRIUM FERMI SEAS

By spectrally resolving the transient four-wave mixing (FWM) signal and the differential transmission signal arising from electron-hole pairs optically excited into the continuum of a GaAs/AlGaAs quantum well structure, we directly measure the dynamics of the quantum coherence among such interacting carriers. We observe a dynamic blue-shift of the coherent FWM emission spectrum with respect to the excitation laser pulse. We interpret our results in terms of dynamic Coulomb correlation effects arising in non-equilibrium degenerate electron-hole systems.