Few cycle pulse laser ablation study of single layer TiO2 thin films using time resolved surface microscopy.

The few cycle optical pulse induced strong field laser solid interaction is a rich area of fundamental and applied research that spans from the study of extreme non-linearities in solids and next generation ultra-broad band high damage threshold optics design and fabrication to peta-Hertz optoelectronics of the future. Our understanding of the extremely non-pertubative phenomena of few cycle pulse (FCP) laser damage and ablation of bulk solids and thin films is still limited. In this work, we present a systematic study of the dynamics of the FCP laser ablation process of single layer TiO2 thin films from 1 ps to 10 ns after a single 9 fs pulse with nominal wavelength of 760 nm interacts with the surface using time-resolved surface microscopy (TRSM) technique. It is observed that FCP ablation craters for certain films exhibit markedly different features when compared to those created by 50 - 150 fs pulses with similar fluences. TRSM measurements also reveal that FCP ablation dynamics strongly depend on the thickness-dependent E-field distribution inside the films (nominally λ/2 vs λ/4), in which the dynamics of free carrier generation due to strong field ionization may play an important role as well. A one-dimensional finite-difference time-domain (FDTD) simulation that takes into account strong field ionization and free carrier absorption is used in conjunction with the TRSM measurements to estimate the excited free carrier density prior to ablation. We also propose a mechanism for the differences in ablation craters between the films based on the FDTD simulation results.