Ultrafast heating of silicon on sapphire by femtosecond optical pulses

We monitor the refractive index n+ik of an optically thin silicon layer following femtosecond photoexcitation below the threshold fluence ${\mathrm{E}}_{\mathrm{th}}$ for melting, in order to measure the rate of lattice temperature rise. We find that most of the heating is delayed with respect to the photoexcitation, but becomes faster as ${\mathrm{E}}_{\mathrm{th}}$ is approached. A model based on delayed Auger heating is presented.