OPTICAL REFLECTIVITY OF SI ABOVE THE MELTING-POINT

The variation of the optical reflectivity of a Si single crystal during irradiation with two successive Nd:YAG laser pulses is investigated with ns resolution. The first pulse melts the surface and therefore the reflection coefficient increases up to the value of the metallic liquid at the melting temperature (1685 K). Upon further heating the surface with the second, time-delayed pulse, a decrease of the reflection coefficient is observed, resulting from the temperature-dependent dielectric function of the molten Si. The largest decrease in the reflectivity that could be reached before damaging the surface amounted to 9% for a probe wavelength of 633 nm. The application of a simple Drude model for the optical constants above the melting point is discussed.