Time-dependent absorption of TiO_2 optical thin films under pulsed and continuous wave 790 nm laser irradiation

摘要

The time-dependent absorption at 790 nm of TiO_2 films prepared by ion-beam sputtering (IBS) and electron-beam evaporation (EBE) was measured. The pump source was a Ti:sapphire oscillator that was operated in CW and pulsed (50 fs) modes. The absorption coefficient of the IBS film under CW illumination was 8 cm~(-1), independent of time and power. Under pulsed illumination, there was evidence of three-photon absorption, and the total absorption increased 10-fold over time at the highest measured irradiance. The absorption of the EBE film had higher initial absorption (～24 cm~(-1)) and increased under both CW and pulsed illumination with time. An electron state model based on band-to-band excitation and electron trapping is presented that explains the observed results. The implications for laser-induced damage of oxide coatings are discussed.