Surface analysis correlated with the Raman measurements of a femtosecond laser irradiated Ca F₂

Abstract

Ultra fast electronic and thermal processes for energy deposition mechanism during femtosecond laser ablation of CaF2 have been identified by means of Atomic Force Microscopy (AFM) and Raman scattering technique. For this purpose, a single crystal CaF2 (1 1 1) was exposed with 800 nm, 25 fs Ti: Sapphire laser pulses at different laser fluences both in air and under UHV condition. Various nano and microstructures on the surface of the irradiated samples are revealed by a detailed surface topography analysis. Around the ablation threshold fluence, most significant features identified at the CaF2 surface are nanohillock like structures. These nanohillocks are typical features related to the fast electronic processes and are explainable on the basis of Coulomb explosion. At moderate irradiation fluence, bump formation is considered to be due to ultrafast melting. Further increase in the laser fluence resulted into strong enhancement of the thermal process with the appearance of larger humps and craters. These fast electronic and thermal processes are well correlated with the structural and crystallographic alterations inferred from Raman spectroscopy analysis. The nanohillocks appearing at a lower fluence are due to calcium colloid formation (aggregates of metal clusters). At higher fluences and dozes, the compressive as well as tensile stresses along with the presence of calcium carbonate are associated to diffusion, transformation and aggregation of defects which are typical features of thermal processes leading to the growth of larger humps.