Effects of parametric scattering, energy-gap narrowing, and state filling on the picosecond optical response of germanium

Recently the nonlinear, nonequilibrium optical properties of germanium at a wavelength of 1.06 {μ}m have been studied on a picosecond time scale by employing the excite and probe technique. In addition to structure in the probe transmission versus delay lasting many tens of picoseconds, investigators have observed a narrow spike in the probe transmission occuring near zero delay and having a width less than the optical pulse width of 11 psec. This spike has been attributed by some to a parametric scattering of the intense excitation pulse into the probe beam path by a grating created on the sample surface by the interference of the two beams and by others to the combined effects of state filling and band-gap narrowing. Although it is likely that both effects occur to some degree, we present the results of measurements that indicate that parametric scattering fully accounts for the observed spike. No contribution from state filling and band-gap narrowing is observed.