Nonlinear laser propagation phenomena in transparent media

Abstract

This thesis is focused on investigations of unanswered questions in the filamentation field. Two techniques for measuring electron plasma densities in light filaments are presented in detail. Pump-probe in-line holographic microscopy technique as a powerful and accurate method for retrieving very small perturbations on the wavefront of the probe beam, caused by the filamentation of the pump beam is presented. Plasma conductivity technique, as a simple method for measuring plasma densities in gases and an easy approach to calibrate this method for quantitative measurements are presented. We present our measurement of the electron plasma densities in fused silica under the conditions, which lead to nanograting formation in the bulk of fused silica glass. Moreover, the dynamics of nonlinear propagation of femtosecond laser pulses in the bulk of PMMA, over a wide temporal range is investigated. Based on our measurements, it is shown that filamentation in PMMA is followed by fast excitation of electrons and excited states, which decay quickly and lead to material modification that in turn leads to a pressure wave formation and creation of voids at latter times. Third-harmonic generation in air is investigated and it is shown that by inserting a plasma string in the path of a light filament, the third-harmonic generation efficiency can be significantly enhanced (by two-orders of magnitude). This enhancement is attributed to enhanced third-order nonlinearity of plasma and a physical model for third-harmonic generation in plasmas is presented. Moreover, it is shown that two intersecting filaments can form a plasma grating in air and the attributes of such grating are characterized. It is shown that the filamentation tailoring can be achieved by introducing a periodic refractive index lattice in the medium or by modifying the input beam characteristics. The results of filamentation tailoring using periodic plasma lattice are presented. It is shown that by using non-diffracting Bessel beams and utilizing a linear technique, the filament attributes can be tuned. The linear propagation characteristics of Airy beams are studied and an easy approach to generate tunable Airy beams is presented. It is shown that by combining an Airy pulse in time with a 2D Airy beam in space, Airy3 light bullets can be generated, which are robust and resist spreading in all spatiotemporal dimensions. Moreover, propagation of Airy beams in different nonlinear regimes is studied.