Ionization-Induced Multiwave Mixing: Terahertz Generation with Two-Color Laser Pulses of Various Frequency Ratios

Abstract

Ultrafast strong-field ionization is shown to be accompanied by atypical multiwave mixing with the number of mixed waves defined by the dependence of the ionization rate on the field strength. For two-color laser pulses of various frequency ratios, this results in the excitation of a free-electron current at laser combination frequencies and possibly in the excitation of the zero-frequency (residual) current responsible for terahertz (THz) generation in a formed plasma. The high-order nature of ionization-induced wave mixing may cause THz generation with uncommon laser frequency ratios (such as 2:3 and 3:4) to be virtually as effective as that with the commonly used frequency ratio of 1:2.

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

“…Previously, such analytic expression was obtained for the laser-pulse parameters corresponding to tunneling regime of ionization [8], where the absolute coincidence with the results of semiclassical approach is achieved [5]. In this paper, we also find an analytic expression for the low-frequency current density for multiphoton ionization regime.…”

“…The process of gas ionization by two-color laser pulses, containing the components with the frequency ratio close to two, is accompanied by excitation of the low-frequency current density (with frequencies much smaller than the frequency of the optical wave) [1][2][3][4][5]. The frequency detuning from the exact "synchronism" determines the frequency of the low-frequency current, which can be located in mid-infrared band [3][4][5].…”

“…This can be used to generate short pulses of mid-infrared radiation [4]. The low-frequency current density excited during gas ionization by a two-color laser pulse has been previously calculated analytically and numerically with the help of semiclassical and quantum-mechanical approaches [1][2][3][4][5][6][7][8][9]. 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 frequency detuning from the exact "synchronism" determines the frequency of the low-frequency current, which can be located in mid-infrared band [3][4][5]. This can be used to generate short pulses of mid-infrared radiation [4].…”

“…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.…”