Femtosecond filamentation of optical vortex in a medium with anomalous group velocity dispersion

Abstract

Abstract. Within the framework of the slowly varying wave approximation method, the pictures of spatio-temporal dynamics of femtosecond pulse in a ring beam with phase singularity were obtained during propagation in CaF2 crystal with anomalous group velocity dispersion. We show formation of tubular structure of the beam and explain reasons of its appearance. Fluence and linear plasma concentration dependencies on propagation distance are analyzed.Filamentation of laser radiation is a phenomenon of formation of a long spatiotemporal structure with high power density [1]. Femtosecond filamentation has been widely studied for Gaussian and other beams with a smooth phase [2,3]. Filamentation of circular beams with phase dislocations can be promising for such applications as tubular refractive index micromodifications, electrons accelerations, etc [4]. We have numerically investigated the filamentation of a femtosecond laser pulse in fused silica for the case of an annular beam with a phase singularity at a wavelength of 800 nm [5]. In this paper, we study propagation of a pulse in mid-IR in circular beam with phase dislocation in presence of anomalous group velocity dispersion typical for calcium fluorides.A numerical simulation of the problem was performed based on the system of differential equations for the slowly varying complex envelope of laser field ( , , ) and free-carrier concentration ( , , ):where ̂ is operator of wave nonstationarity [1], describing self-steepening of wave front, ̂ -dispersion operator, which is calculated in spectral domain according to Sellmeyer's formula. The model takes into account beam diffraction and pulse dispersion, Kerr's and plasma nonlinearities, Bremsstrahlung effect, nonlinear absorption and extinction, avalanche ionization and electron recombination.The initial condition represents a circular beam with located in the center phase dislocation in transform-limited Gaussian pulse.

Annular light bullets of a femtosecond optical vortex in a medium with anomalous group velocity dispersion

“…It is shown that optical vortex in a medium with normal group velocity dispersion forms a filament of tubular shape with a radius of several microns, whereas in an annular beam without phase dislocation, the filament is formed on the axis. Analysis of the formation of the tubular structure of the optical vortex, spatial distribution of the fluence and plasma concentration under the conditions of AGVD was performed in [15].…”