Abstract

Semiconductor-laser melting studies have shown the validity of a thermal model down to picosecond time scales. When excited with an intense femtosecond pulse at 620 nm, however, GaAs undergoes a phase transition because of partial energy transfer from the hot electron plasma to the ionic motion. We have performed pump-probe experiments on crystalline GaAs in which we measure both the linear reflectivity and the reflected second-harmonic light. The second-harmonic radiation is generated in the upper ~100 atomic layers of the crystal and provides a sensitive probe of the symmetry of the material to this depth. The data at high fluence are consistent with an ultrarapid transformation of the excited material to a high-reflectivity molten phase that produces negligible second harmonic within 200 fs of the excitation. Below melting fluence, a plasma-induced dip in reflectivity is observed. This dip, which lasts approximately 2 ps, is followed by a slight rise that can be attributed to lattice heating. We will present measurements of the second-harmonic and reflectivity from GaAs d and n surfaces probed under various polarizations during femtosecond melting.

© 1990 Optical Society of America