Abstract

Femtosecond time-resolved excite-probe experiments have been performed in various II-VI semiconductors in transmission and reflection geometries. In particular, we have used the 100 fsec pulses from an amplified colliding-pulsed-modelocked (CPM) laser operating at a wavelength of 620 nm and with energies of 30 µJ to excite carriers in CdS, CdSe and CdS_xSe_1−x semiconductors. In some of these materials the bandgap energy exceeds the energy of the exciting photons and allows us to directly measure the two-photon absorption (2PA) coefficient at zero time delay. At longer delays, in all materials the excitation induced changes in the linear optical properties are measured at a variety of different wavelengths selected from a continuum. At high excitation fluences, some of these materials display modulation in the linear optical properties within the first 2 ps when probed with photons with energies which fall below the bandgap energy. We offer interpretation of our results in terms of various competing mechanisms including free-carrier absorption, interband transitions, bandgap renormalization and absorption saturation.

© 1990 Optical Society of America