Dynamic Picosecond Reflectivity Studies Of Highly Optically-Excited Crystalline Silicon

Abstract
Recent interest in picosecond Si optical limiters that can operate above the fluence required to melt the Si surface has lead us to the first detailed investigation of the nonlinear reflectivity of this semiconductor at 1-micron for fluences near and just above the melting threshold. Conventional reflectivity measurements at this wavelength are inconsistent with similar measurements at 532 nm and apparently indicate that the Si surface melts hundreds of picoseconds after the passage of the 48 ps (FWHM) pulse. Spatial correlation and time-resolved surface imaging shows that melting actually occurs during the pulse and the apparent delayed melting results from material evaporation. Discrepancies between measurements at 1-micron and those in the visible are resolved in terms of material evaporation, the weak wavelength dependence of the liquid Si absorption, and the dramatically different melting thresholds for these two wavelengths.

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