Residual-Current Excitation in Plasmas Produced by Few-Cycle Laser Pulses

Abstract

Along with the generation of extreme-ultraviolet and soft x-ray radiation, gas ionization by an intense few-cycle laser pulse can also induce the generation of low-frequency terahertz waves. The latter is caused by the excitation of a residual quasi-dc current in the produced plasma by the electric field of the laser pulse. We describe this phenomenon using the quantum-mechanical approach based on solving the 3D time-dependent Schrödinger equation. We calculate the dependences of the residual-current density on the carrier-envelope phase, duration, and intensity of the few-cycle laser pulse, and find optimal conditions for high-efficiency realization of the studied phenomenon.

Analysis of THz generation through the asymmetry of photoelectron angular distributions

“…So far, the evidence supports the photocurrent model, in that ionization of the medium is indispensable [13,14], yet the mechanism behind the generation of THz remains unclear. A lot of theoretical calculations have been carried out in the past ten years by solving the time-dependent Schrödinger equation (TDSE) [8,[15][16][17][18] to study the THz signals versus the phase between the two colors of the laser, the intensity of the fundamental and of the second harmonic [17][18][19]. These TDSE calculations have shown consistent results among the studies, but the underlying mechanism remains unclear.…”

“…However, unlike the better-known high-order harmonic generation, the mechanism of THz emission is still under debate. Two popular mechanisms are generally accepted: one is the photocurrent (PC) model [7,8], and the other is a third-order nonlinear four-wave mixing (FWM) model [5,9]. The PC model relies on residual currents in the laser-atom interaction, while the FWM relies on the polarization of the medium.…”

Tailoring terahertz radiation by controlling tunnel photoionization events in gases

“…In this mechanism, tunneling ionization and subsequent electron motion produce a quasi-dc photoinduced current, which in turn emits THz radiation. The quasi-dc plasma current can be efficiently produced in a gas irradiated with two-color laser pulses [5][6][7][8][9][10][11][12][13][14], in multicolor pump schemes [16] and in chirped [17] or few-cycle pulses [18,19]. There are other, generally less efficient mechanisms for THz generation in gases that employ single-color pumps.…”

Quantum-Mechanical Description of Ionization-Induced Generation of Tunable Mid-Infrared Pulses

“…The semiclassical approach is based on the solution of the hydrodynamic equation for the electron current density and the equation for the density of free electrons with a quasi-static probability of tunneling ionization per unit time [10]. The quantum-mechanical approach is based on the solution of the threedimensional time-dependent Schrödinger equation for the electron wave function [9,11]. The range of applicability of the semiclassical approach is limited by the laser pulse parameters corresponding to the tunneling ionization regime, at which the Keldysh parameter γ = I p /2U p [12] is much less than unity (here I p is the ionization potential of an atom, and U p is the ponderomotive energy of an electron in the laser field) [5,11,13].…”

“…The quantum-mechanical approach is based on the solution of the threedimensional time-dependent Schrödinger equation for the electron wave function [9,11]. The range of applicability of the semiclassical approach is limited by the laser pulse parameters corresponding to the tunneling ionization regime, at which the Keldysh parameter γ = I p /2U p [12] is much less than unity (here I p is the ionization potential of an atom, and U p is the ponderomotive energy of an electron in the laser field) [5,11,13]. For γ ≫ 1, the electron release occurs at a time of the order of the field period and greater, and to adequately calculate the low-frequency current density it is necessary to apply the quantum-mechanical approach.…”