Inhomogeneous Energy Deposition in Crystalline Silicon with Picosecond Pulses of One Micron Radiation
In recent years, the extensive use of ultrashort laser pulses to time resolve the transmission and reflectivity of Si has led to a much improved understanding of the kinetics of energy deposition and redistribution immediately preceding and following optically induced phase transitions . With few exceptions, visible sources of radiation with photon energies well above the indirect bandgap have been used for excitation in these studies. By contrast, because the interaction of near-bandgap picosecond 1 μm radiation with Si is more complicated, it has received little attention. We have recently presented preliminary results of the first studies of the pulsewidth dependence of the nonlinear absorption  and the various phase transitions and changes in surface morphology  for 1 μm excitation pulses in the 4–260 ps range. These measurements demonstrates that the absorption of 1 μm picosecond radiation below the melting point is strictly fluence dependent, with no observable intensity-dependent contributions. Moreover, the nonlinear behavior of the absorption in this regime can be completely accounted for by indirect and free-carrier absorption, when lattice-heating effects are included. In addition, the fluence required for single-shot melting of the Si at 1 μm is found to decrease significant with decreasing pulsewidth.
KeywordsAttenuation Recrystallization Coupler HeNe
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