Unidirectional current excitation in tunneling ionization of asymmetric molecules

Abstract

Asymmetric photocurrent produced by ionizing laser pulses in a gas is known to be a highly efficient source of terahertz radiation. We examine the possibility of exploiting the asymmetry of the medium itself, rather than the properties of the laser field acting on it, to facilitate the generation of directional photocurrents. We show that the magnitude of directional current and the efficiency of its excitation in tunneling ionization of asymmetric molecules can be significantly enhanced compared to the case of symmetric systems. The results obtained both in a simple classical model and in quantum-mechanical numerical simulations favor the subcycle asymmetry of the ionization process in combination with the effect of the Coulomb potential on the escaping electron as a mechanism responsible for a high-efficiency generation of residual current in tunneling ionization of oriented asymmetric molecules.

Generation of terahertz radiation at optical breakdown of air: The dependence of the optimal phase shift between the components of a two-color laser pulse on their intensity

“…We measured the positions of the BBO crystal corresponding to a local minimum of the energy of terahertz radiation ("nonoptimal" phase shift), because they can be measured more accurately than the positions corresponding to the maximum. The nonoptimal phase shift is displaced from the opti mal phase shift by π/2 ± πN, where N is an integer [9,10,[19][20][21][22][23][24]. Constraints on the energies of laser pulses in the reported measurements are determined by the sensitivity of the system of detection of terahertz radi ation (at low energies) and by the available energy of laser pulses in this experiment (at high energies).…”

“…DOI: 10.1134/S0021364015020095 different values of the optimal phase delay between laser radiation at frequencies ω and 2ω [15,16,[19][20][21][22] were obtained. Advanced approaches to the descrip tion of generation of directional currents with allow ance for the Coulomb interaction between a separated electron and the core of an atom or a molecule were proposed in [21][22][23][24]. These approaches make it possi ble to go beyond the strong field approximation and to perform simulation in a wide range of intensities of optical pulses.…”

Analytical description of generation of the residual current density in the plasma produced by a few-cycle laser pulse

“…[1][2][3][4][5][6][7][8][9] The essence of this phenomenon can be explained as follows. In the process of ionization of atoms and molecules by an intense laser field, the newly born free electrons acquire, along with oscillatory velocity, the constant drift velocity.…”

“…5,6 These approaches were used to calculate RCD dependences on CEP, intensity, and duration of the laser pulse. [2][3][4][5][6]8,9 It was shown that the efficiency of RCD excitation depends on the peak intensity of the pulse, and there exists an optimal intensity, whose value depends on the duration of the laser pulse. The RCD value, which corresponds to this optimal intensity, increases exponentially as the duration of the laser pulse decreases, which can be used for generation of high-power THz radiation employing very short laser pulses.…”

“…The value of this RCD depends on the carrier-envelope phase (CEP) of the fewcycle laser pulse and can be rather high if the laser pulse is very short. [1][2][3][4][5][6]9 The interest in this phenomenon is connected with the possibility of its application for generation of broadband radiation in the terahertz (THz) range, which is currently a topical problem. [10][11][12][13][14][15][16][17] The residual current is an initial impetus to plasma polarization and excitation of radiating plasma oscillations with THz frequencies in a wide range of gas pressures and laser pulse intensities.…”