Thermal and nonthermal melting of gallium arsenide after femtosecond laser excitation

Thermal- and nonthermal melting in gallium arsenide after femtosecond laser excitation has been investigated by means of time resolved microscopy. Electronic melting within a few hundred femtoseconds is observed for rather strong excitation and the data reveal a distinct threshold fluence of $150{\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^2$ for this nonthermal process. Below that threshold melting occurs on a 100 ps time scale and is of thermal nature. Using a simple numerical model we describe this type of the phase transition as heterogeneous melting under strongly overheated conditions.