Early phase laser induced plasma diagnostics and mass removal during single-pulse laser ablation of silicon

The electron number density and temperature during the early phase (<300 ns) of laser-induced plasmas from silicon using a 266-nm, 3-ns Nd:YAG laser were deduced via spectroscopic methods. These parameters were measured as a function of delay time vs. irradiance in the range of 2–80 GW/cm², and compared with crater volume measurements. A dramatic change in plasma characteristics (electron number density, temperature, and degree of ionization) as well as a sharp increase of mass removal was observed when the irradiance was increased beyond a threshold of 20 GW/cm². Possible mechanisms such as inverse bremsstrahlung and self-regulation were used to describe these data in the low irradiance region. Laser self-focusing and critical temperature are discussed to explain the dramatic changes after the irradiance reaches the threshold.