Abstract

Using high repitition rate femtosecond pump-probe techniques we examine the thermalization of carriers injected a few LO phonon energies above the band in a CdS_0.24Se_0.76 microcrystallite doped glass. The ouput of a balanced CPM ring dye laser is amplified with a copper vapor laser at 7.3 kHz to 2-μJ energy and 200-fs duration. A fraction of the amplified pulse at 6200 Å is used as a pump pulse while the remainder is focused to 10¹³ W cm^-2 on an ethylene glycol jet producing a broad-bandwidth probe pulse. For various pump-probe delays the transmission of a 750-μm thick sample is measured. As shown in fig 1a., a red shift of the absorption edge is observed in < 100 fs at wavelengths in the vicinity of the band gap. This is due to the combined effects of band gap renormalization and tail state broadening. After +300 fs a prominent shift of the absorption edge toward higher energies is observed. This suggests that carriers have relaxed to the bottom of the band in a few hundred femtoseconds and initiated bandfilling. The blue shift of the absorption spectrum is in agreement with a steady state measurement reported previously.¹ Finally, after a few picoseconds the band-edge shift begins to recover via electron-hole recombination with nearly complete recovery after +40 ps. We observe a small residual shift which persists for > 500 ps, attributed to carriers confined to traps.

© 1986 Optical Society of America