Analysis of Combination Frequencies Arising from the Ionization of Gases by Multicolor Femtosecond Pulses

Abstract

Ionization-induced multiwave mixing is attracting much interest nowadays due to the possibility of generating short pulses of secondary radiation over a very wide spectral range, from terahertz to far ultraviolet. This paper presents an analytical method for calculating the amplitudes of arbitrary spectral components of free electron currents arising under the action of multicolor ionizing laser pulses. We show that this method can be used to obtain the dependences of characteristics of a frequency-tunable third harmonic of the intense component of a three-color pulse obtained in an optical parametric generator. The obtained results are in good agreement with quantum mechanical calculations.

Dynamics of Gas Ionization by Laser Pulses with Different Envelope Shapes

“…The equations that we used for analytical derivations and numerical calculations in wide range of parameters are based on the semiclassical approach (photocurrent model) [5,8,9]. To calculate the current density of free electrons generated by the laser field, we used the following system of equations:…”

“…However, in general, we see the correct dependencies on the TDSE solutions, corresponding to the proposed theory. In addition to the three basic envelope shapes ( 11)-( 13), Figure 3 also shows TDSE calculations with a frequently used [9][10][11][12] shape f (t) = cos 2 (πt/T), saving computational time, which is a valuable resource for 3D TDSE. As can be seen, in the region of high intensities 10 15 W/cm 2 , this shape gives an increase in the RCD even greater than that of the Gaussian, since it decreases even faster.…”

“…Most often, the object of study is the characteristics of the pulse carrier, while the shape of its envelope is considered to be arbitrarily given. As a rule, Gaussian envelopes are used in semiclassical (photocurrent) calculations [5][6][7][8][9], while abruptly terminating envelopes can be used in quantum mechanical calculations to save computational resources [9][10][11][12], and for few-cycle pulses, additional requirements may be introduced [3,13].…”

“…In this case, the generation of free electrons shifts to the forefront of the pulse [7]. It is only during the active ionization that all the nonlinear (combinational/Brunel) components are generated in the plasma currents [8,9].…”