Intrapulse impact processes in dense-gas femtosecond laser filamentation

Abstract

The processes of energy gain and redistribution in a dense gas subject to an intense ultrashort laser pulse are investigated theoretically for the case of high-pressure argon. The electrons released via strong-field ionization and driven by oscillating laser field collide with neutral neighbor atoms, thus effecting the energy gain in the emerging electron gas via a short-range inverse Bremsstrahlung interaction. These collisions also cause excitation and impact ionization of the atoms thus reducing the electron-gas energy. A kinetic model of these competing processes is developed which predicts the prevalence of excited atoms over ionized atoms by the end of the laser pulse. The creation of a significant number of excited atoms during the pulse in highpressure gases is consistent with the delayed ionization dynamics in the pulse wake, recently discovered by Gao et al 1 . This energy redistribution mechanism offers an approach to manage effectively the excitation vs. ionization patterns in dense gases interacting with intense laser pulses and thus opens new avenues for diagnostics and control in these settings. In the wake of a filamenting laser pulse, the system of energetic free electrons, ions, and

Delayed ionization and excitation dynamics in a filament wake channel in a dense-gas medium

“…The combination of partial-derivative terms, finite-difference terms, and an integral term in the right-hand side of this equation requires specially tailored ad hoc approach to numerical solution of this equation. 40 We have implemented an approach similar to that developed previously for delayed differential equations. 53 To mimic typical experimental conditions, 16 the dense-gas medium is assumed to be argon at the pressure of 60 atm and room temperature.…”

“…Although not reaching an avalanche scale, this impact ionization plays a significant role, overriding the effect of intensity clamping. 16,40 In this paper, we address the case of high-density gases. As in this situation collisional processes play a decisive role both during the laser pulse and during the wakechannel evolution, it becomes possible to devise a unified description of these two distinct stages of the medium transformation effected by the filamenting pulse.…”

“…In this situation, collisional excitation of the constituent atoms or molecules is as important as their impact ionization and may often lead to a preponderance of excited atoms over ionized atoms at the end of the filamenting pulse. 40 After the laser pulse, the combination of the large concentration of excited atoms and high gas density makes an additional phenomenon actively engaged in the wake channel evolution: Penning ionization. 16 This ionization mechanism involves interaction of two excited atoms, which results in de-excitation of one of them and ionization of the other, with the emerging free electron receiving considerable kinetic energy.…”

Pump-Probe Study of Plasma Dynamics in Gas-Filled Photonic Crystal Fiber Using Counterpropagating Solitons

“…Studies of photoionization by strong optical fields have led to a number of interesting developments in physics, ranging from fundamental studies of ionization and subsequent plasma evolution [1][2][3] to applications such as high harmonic generation (HHG) [1] and the formation of plasma channels that can be used in particle acceleration and for the generation of coherent X-rays [4]. While these initial studies focused on low-density gases, recent research has explored the full nonlinear polarization of gases at atmospheric and higher pressures [5], as well as more exotic effects such as Kramers-Henneberger quasibound states [6], and avalanche ionisation by collisions of excited atoms in the absence of a laser pulse [7][8][9]. Understanding the dynamics of ionised gases at high pressures is also of great importance for HHG in the water window, a spectral region where phase-matching generally requires helium pressures of several atmospheres [10,11].…”