Strongly nonresonant four-wave mixing in semiconductors

When semiconductors are optically excited above or slightly below the band gap, the linear and nonlinear responses originate predominantly from the interband polarization. Here, we demonstrate that intraband excitations, i.e., rapidly oscillating currents that originate from the electric-field-induced acceleration of electrons and holes, contribute strongly to transient four-wave mixing when it is performed with center frequencies near half the band gap frequency. Within a two-band model we show that the presence of several pathways arising from different combinations of inter- and intraband excitations and their interference give rise to characteristic signatures in time- and spectrally resolved signals. Our approach is based on the semiconductor Bloch equations and includes the dynamics of off-resonant electron-hole excitations on a microscopic level. The predicted significant broadening and structure appearing in the four-wave-mixing spectra are in good qualitative agreement with experimental results.