Abstract

Intense (10¹² - 10¹⁴ W/cm²) femtosecond laser pulses create deep earth pressure-temperature conditions at a solid surface before hydrodynamic surface expansion occurs¹. Photoexcitation of carbon, silicon, and germanium from melting fluence F_m up to approximately 30 F_m by femtosecond pulses induces a phase transition from their crystalline states to metallic liquid states within 1 ps followed by hydrodynamic surface expansion, with scale lengths much less than the incident probe wavelengths. Quantitative optical properties of the well-characterized fluid surfaces are determined by the time-resolved measurement of s- and p-polarized reflectivity at variable probe incident angles and excitation fluences, and at two different probe wavelengths. The surface expanding dynamics in 20 ps as well as the subpicosecond risetimes of reflectivity responses are important for accurate determination of optical properties of liquid states of materials.

© 1994 Optical Society of America