Dynamics of phase transitions induced by femtosecond laser pulse irradiation of indium phosphide

The structural transformation dynamics of single-crystalline indium phosphide (InP) irradiated with 150 fs laser pulses at 800 nm has been investigated by means of time-resolved reflectivity measurements covering a time window from 150 fs up to 500 ns. The results obtained show that for fluences above a threshold of 0.16 J/cm(2) thermal melting of the material occurs on the timescale of 1-2 ps. The evolution of the reflectivity on a longer timescale reveals the reflectivity of the liquid phase and shows resolidification times typically around 10-30 ns after which an amorphous layer several tens of nanometers thick is formed on the surface. This amorphous layer significantly alters the optical properties of the surface and finally leads to a reduced ablation threshold for subsequent laser pulses. Single-pulse ablation at higher fluences (> 0.23 J/cm(2)) is preceded by an ultrafast phase transition (non-thermal melting) occurring within 400 fs after the arrival of the pulse to the surface.