| The dynamics of optically generated, coherent electronic excitations in Indium Phosphide at a temperature of 5 K are investigated over a 140 meV energy range near the band gap, using the two-pulse, self-diffracted, degenerate four-wave mixing technique with 30-50 fs near-infra-red pulses and spectrally resolved detection. The coherent dynamics of excitons associated with the fundamental energy gap are investigated under conditions of simultaneous excitation of continuum transitions. Exciton-carrier scattering is shown to produce the dominant source of diffraction for excitons due to excitation-induced dephasing. The dephasing rate of continuum excitations is found to increase abruptly at the threshold energy for LO phonon emission by electrons. Below this threshold energy the dephasing rate is determined to be at least 8 ps{dollar}sp{lcub}-1{rcub}{dollar}, whereas above the threshold energy, the dephasing rate is at least 13 ps{dollar}sp{lcub}-1{rcub}{dollar}.; Near the spin-orbit split-off valence to conduction band transition energy the diffracted spectrum is strongly affected by an interaction between split-off excitons and continuum excitations of heavy and light hole bands. A Fano-like spectral profile is observed in the coherent emission spectrum at the split-off transition and the coherent dynamics is discussed in terms of coupled exciton-continuum excitations. The Fano coupling parameter q, which characterizes the spectral profile, is found to decrease as excitation density increases. The interaction matrix element between split-off excitons and continuum excitations is determined to be approximately 5 meV, which corresponds to a 130 fs lifetime, over the density range investigated. Observation of the Fano interference phenomenon by four-wave mixing is found to depend on the relative orientation of the electric field polarization vectors. The Fano-like spectral profile is much less prominent for cross-polarized pulses than for co-polarized pulses. Evidence for Fano interference is also observed by spectrally integrated four-wave mixing; there is no evidence of the interference in the cross-polarized case. At present, the polarization dependence is not understood; therefore, the interaction mechanism is not identified but possible interaction mechanisms that could give rise to Fano interference are discussed. |
