The propagation of powerful ultrashort laser pulses in transparent media: Self-focusing, continuum generation and conical emission.

摘要

Recent developments in the technology of powerful ultrafast lasers have created a need to revisit some fundamental aspects of laser pulse propagation. This thesis investigates phenomena arising in the propagation of powerful near-infrared femtosecond laser pulses in transparent condensed media and in air. Experiments were performed using a titanium-sapphire laser chain.; Propagation in transparent condensed media can lead to self-focusing of the beam and a transformation of the pulse spectrum into a continuum covering the entire visible spectral range (white light). A new technique is presented that allows monitoring of the beam profile, pulse spectrum and pulse energy at various stages of propagation during self-focusing and continuum generation. The observations show that continuum generation is triggered by self-focusing and reveal a strong dependence on the band-gap of the medium. A band-gap threshold is found below which there is no continuum generation and above which the width of the continuum increases with the band-gap. This is the first report of a parameter predicting the width of the continuum in condensed media. Self-phase modulation enhanced by free-electron generation due to multiphoton excitation is proposed as the primary mechanism of continuum generation. The anomalous beam divergence and the conical emission associated with the continuum are also investigated.; Propagation in air can give rise to a filamentary propagation mode initiated by self-focusing and sustained for several tens of meters. The filament is usually accompanied by rainbow-like conical emission. Experimental measurements of the filament energy reveal that the filament ends at a distance equal to the diffraction length of the beam ({dollar}sim{dollar}100 m in our experiment), independently of the initial peak power. An explanation for the filament is provided in terms of the moving-focus model of self-focusing modified by ionization of the air. Along the same lines a new mechanism is proposed to explain the conical emission in terms of laser-plasma interactions. The measured filament and conical emission are reproduced in numerical simulations of the nonlinear Schrodinger equation involving ionization of the air.