Picosecond Time-Resolved Detection of Plasma Formation and Phase Transition in Silicon

Abstract

The interaction of laser pulses with strongly absorbing media has received a great deal of attention during the past five years, especially in silicon [1–3]. An important issue has been the time scale of energy transfer between a dense electron-hole plasma and the lattice. If this energy transfer were insignificant during a picosecond pulse, it is clear that the plasma temperature would attain much higher values. We have presented ample evidence [4,5] that melting of a silicon surface layer can take place during a 20 ps laser pulse. There is a sharply defined fluence threshold (0.2 J/cm² for a 20 ps pulse at λ = 532 nm on silicon) above which the reflectivity changes to a value characteristic of liquid phase, the evaporation of silicon atoms becomes appreciable, and an amorphous phase may be created by rapid resolidification, with cooling rates exceeding 10¹³ °C/sec, of a thin (10 nm) molten layer on top of a cool crystalline substrate.