Abstract

Time-resolved four-wave mixing (FWM) is a very powerful method to study the nature and dephasing of elementary excitations in condensed matter. In FWM experiments, two delayed ultrashort laser pulses with wavevectors k₁ and k₂ interfere in a sample to produce a diffracted signal (DS) in the direction k_s = 2 k₂ - k₁. In the case of a homogeneously broadened system, the DS is emitted immediately after the second pulse (“free-induction decay”), while in a strongly inhomogeneously broadened system the DS appears delayed by an amount equal to the time delay Δt = t₂ - t₁ between the input pulses (“photon echo”)^[1]. However, because of the short dephasing times T₂ in condensed matter, the DS is usually not time-resolved, but is measured using a slow detector that integrates the emitted light, so that valuable information contained in the temporal profile of the DS is lost. Here, we report what we believe to be the first measurement of the temporal evolution of the DS on a fs time scale for the case of selective excitation of roomtemperature excitons in semiconductors.

© 1992 IQEC