No abstract

The method of interferometric cross-correlation is used to obtain precise information about the phase and amplitude of the laser pulse induced interband polarization in semiconductors. With this technique it is possible to study the linear as well as nonlinear response of a sample. The light transmitted through a thin GaAs crystal after excitation with femtosecond pulses near the band gap is measured. The phase shift between the driving laser pulses and the induced polarization is measured in the time domain. The influence of additional pump pulses on the decay of the polarization is studied. The instantaneous frequency of the polarization depends on the laser center frequency, on the pump pulse delay time, and on the relative polarization of the pump pulse with respect to the test pulse.

We report on amplitude and phase precision measurements of 40 fs pulses propagating through an ultrapure, 3.8 mm thick GaAs platelet at 2 K. Additionally to the propagation beats in the transmitted intensity, see Fro È hlich et al., we found a characteristic behavior of the phase of the transmitted field. While for low excitation densities (`10 13 pairs/cm 3 ) phase shifts ascend at the individual beat minima, with increasing excitation the jumps of the phase shifts flip from p to Àp. A simple one-oscillator model for the dielectric function fails to describe this effect, however, our more sophisticated analysis of the exciton line shape based on the solution of the Bethe-Salpeter equation for the susceptibility shows, that the flipping is caused by an asymmetry of the 1s-exciton line caused by scattering of excited carriers with each other and with the laser-induced polarization.

We report amplitude and phase resolved measurements of polariton propagation excited with 45 fs laser pulses in a pure bulk gallium arsenide sample of thickness 4.2 μm at 1.8 K. The time development of the envelope E(τ) and the phase φ(τ) of the transmitted pulses was measured with interferometric cross correlation. The amplitude shows aperiodic beating behavior that is well described with polariton propagation in the framework of a local dielectric response. However, the instantaneous frequency of the transmitted pulse is not constant as predicted by the model but gradually approaches the exciton resonance within roughly 10 ps from 2 meV below.

We present a ultrasensitive method for measuring the ultrafast optical response of a sample with respect to its amplitude and phase. Combining interferometric crosscorrelation, a wavemeter and a numerical lock-in technique, the method is distinguished by its dynamic range (7 orders of magnitude in the intensity) and phase resolution ( A @ /~T = 1/50). Time resolution depends on residual dispersion ofthe set-up but is limited to -40 fs by the width of the input pulses. We demonstrate, as an example, high precision measurements of nonlinear Is-exciton-polariton propagation through GaAs.Most experiments in ultrafast spectroscopy involve two pulses, an input and an output pulse. If the input pulse can be characterized in a separate set-up, linear correlation techniques can be used to determine the optical response function &(t,t'). The advantages of linear techniques are their potential high sensitivity regarding minimum average power and dynamic range. Our method consist of two parts, the optical set-up and a numerical evaluation algorithm. Three signals are measured in the same Mach-Zehnder interferometer simultaneously ( Fig. 1): i) a linear crosscorrelation signal using both exit interference patterns that is proportional to the optical response function of the sample, ii) an autocorrelation of the pulses to determine the power spectrum of the pulses used, and ii) the autocorrelation of a HeNe laser that represents a reference with which mechanical or thermal drifts of the interferometer can be removed. The amplitude and phase of all signals are determined by a numerical lock-in technique.To demonstrate the performance of the I &h".