Ultrashort filaments of light in weakly ionized, optically transparent media

Abstract

Modern laser sources nowadays deliver ultrashort light pulses reaching few cycles in duration, high energies beyond the Joule level and peak powers exceeding several terawatt (TW). When such pulses propagate through optically-transparent media, they first self-focus in space and grow in intensity, until they generate a tenuous plasma by photo-ionization. For free electron densities and beam intensities below their breakdown limits, these pulses evolve as self-guided objects, resulting from successive equilibria between the Kerr focusing process, the chromatic dispersion of the medium, and the defocusing action of the electron plasma. Discovered one decade ago, this selfchanneling mechanism reveals a new physics, widely extending the frontiers of nonlinear optics. Implications include long-distance propagation of TW beams in the atmosphere, supercontinuum emission, pulse shortening as well as high-order harmonic generation. This review presents the landmarks of the 10-odd-year progress in this field. Particular emphasis is laid to the theoretical modeling of the propagation equations, whose physical ingredients are discussed from numerical simulations. The dynamics of single filaments created over laboratory scales in various materials such as noble gases, liquids and dielectrics reveal new perspectives in pulse shortening techniques. Differences between femtosecond pulses propagating in gaseous or condensed materials are underlined. Attention is also paid to the multifilamentation instability of broad, powerful beams, breaking up the energy distribution into small-scale cells along the optical path. The robustness of the resulting filaments in adverse weathers, their large conical emission exploited for multipollutant remote sensing, nonlinear spectroscopy, and the possibility to guide electric discharges in air are finally addressed on the basis of experimental results.

Ultrafast supercontinuum generation in bulk condensed media

“…[1]), the SC generation in bulk media appears to be a more complex process that involves an intricate coupling between spatial and temporal effects. The physical picture of SC generation in transparent bulk media could be understood in the framework of femtosecond filamentation, which provides a universal scenario of nonlinear propagation and spectral broadening of intense femtosecond laser pulses in solids, liquids and gases [53][54][55][56].…”

Generation of Phase-Stable Sub-Cycle Mid-Infrared Pulses from Filamentation in Nitrogen

“…However, the nonlinearity of gas media is much smaller than that of bulk media. To overcome the low efficiency, filamentation [9][10][11] in gases can be used. The balance between self-focusing and plasma self-defocusing makes the pulse propagate much longer than the Rayleigh range with a very high intensity.…”

Ultrafast gaseous “half-wave plate”

“…The propagation of laser filaments in gases is now well understood as a dynamic balance between Kerr self-focusing and defocusing on laser-generated plasma [1][2][3][4][5]. Filaments have been characterized in detail in many regards, in particular from the spectral, temporal and spatial point of view.…”