2 DISTINCT TRANSITIONS IN ULTRAFAST SOLID LIQUID-PHASE TRANSFORMATIONS OF GAAS

Femtosecond-laser-induced changes of the optical reflectivity and the reflected second harmonic are measured over a wide range of times and laser fluences. Changes of the linear and nonlinear optical properties suggestive of a, transition to a metallic state have been observed. For relatively low fluences these changes take tens of picoseconds to develop. For higher fluences the transition takes place in just a few hundred femtoseconds. Our data suggest that, depending on the laser excitation conditions, two distinctly different types of phase transformations are observed.