Effect of nonlinear scattering of radiation in air on material ablation by femtosecond laser pulses

Abstract

This work investigates the role of ambient atmosphere in material ablation by ultra-short intense laser pulses. It is shown, that ablative action of femtosecond pulses reveals limitations imposed by nonlinear optical response of gases resulting in significant modification or the incident laser beam. This phenomenon called conical emission (CE) manifests itself as strong scattering or emission of radiation in the forward direction developed at focusing of intense pulses of Ti:Sa laser (π=110÷1500 fs) in air. Transformation of the nearly Gaussian spatial profile into a wide angle cone is followed by spectral conversion of the fundamental laser frequency into a broad spectrum with relatively shorter wavelengths extending up to the visible range. Thresholds, converted energy, spectra and profiles of scattered radiation were measured at variable laser pulse duration and the ambient pressure. It was found, that more than 70% of the incident pulse energy can be scattered at conventional focusing of the beam by a long focal length lens. Effect of CE on material ablation in air was investigated, and the data obtained allowed to explain paradox morphology of steel channels drilled by high power femtosecond pulses.