Nonlinear absorption of intense femtosecond laser radiation in air

Abstract

A new mechanism of nonlinear absorption of intense femtosecond laser radiation in air in the intensity range I = 10(11)-10(12) W/cm(2) when the ionization is not important yet is experimentally observed and investigated. This absorption is much greater than for nanosecond pulses. A model of the nonlinear absorption based on the rotational excitation of molecules by linearly polarized ultrashort pulses through the interaction of an induced dipole moment with an electric field is developed. The observed nonlinear absorption of intense femtosecond laser radiation can play an important role in the process of propagation of such radiation in the atmosphere.

Energy deposition from focused terawatt laser pulses in air undergoing multifilamentation

“…Plasma generation itself, together with plasma absorption, form the main channel for energy deposition. Stimulated rotational Raman scattering on air molecules can also account for energy depletion of the laser pulse [2][3][4]. This energy, initially stored as potential and kinetic energy of plasma free electrons and as rotational energy of air molecules, is eventually converted into gas thermal energy after plasma recombination and rotational thermalization, occurring over ∼ 1 ns [5] and ∼ 100 ps timescales [6], respectively.…”

Sensitivity of propagation and energy deposition in femtosecond filamentation to the nonlinear refractive index

“…Acoustic measurements of optical filaments have been used in a number of prior contexts [24][25][26][27]. Other possible filament diagnostics are plasma conductivity [28], fluorescence [29], and direct [30] and indirect [29,[31][32][33] measurements of filament plasma density, none of which are directly proportional to absorbed laser energy, and all of which require a combination of nontrivial optical setups and data retrieval, and complex auxiliary modeling for interpretation.…”

Sound emission from the gas of molecular superrotors

“…Acoustic waves associated with this heating mechanism have been observed and studied [3,5,8]. In the case of weaker non-ionizing laser pulses, rotational excitation of molecules, rather than their ionization, has been suggested as the primary mechanism of depositing energy to the molecular media [4,6,9]. Recent experiments employing time-resolved double-pulse detection schemes have provided convincing evidence of energy conversion from molecular rotation to sound [11,15].…”