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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Statistical simulation of many-level system pulse pumping by high-intensity laser radiation

Vladimir I. Denisov, Alex Kourbatov

Published Aug 16, 1994• 0 citations
Publisher

Abstract

In this work the method of statistical simulation allowing someone to calculate an interaction of laser pulse with nonlinear absorbing many-level medium when the time duration of pulse is equal to longitudinal characteristic relaxation time is suggested and realized. Two problems were solved by this method. The first problem was the one of laser energy storage on the metastable level of medium atoms. The space and time characteristics of this inverted medium of plane geometry were calculated. The amplification coefficient of forbidden transition versus different parameters of this problem was obtained. The second problem was the calculation of many-steps selective photoionization of the atomic beam. Under specific conditions a superradiance had an effect on output of ions during decay of resonant exciting level into metastable level.

DOI
10.1117/12.184589

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In this work the method of statistical simulation allowing someone to calculate an interaction of laser pulse with nonlinear absorbing many-level medium when the time duration of pulse is equal to longitudinal characteristic relaxation time is suggested and realized. Two problems were solved by this method. The first problem was the one of laser energy storage on the metastable level of medium atoms. The space and time characteristics of this inverted medium of plane geometry were calculated. The amplification coefficient of forbidden transition versus different parameters of this problem was obtained. The second problem was the calculation of many-steps selective photoionization of the atomic beam. Under specific conditions a superradiance had an effect on output of ions during decay of resonant exciting level into metastable level.
In this work the method of statistical simulation allowing to calculate an interaction of a laser pulse with nonlinear absorbing many-level medium when the time duration of the pulse is equal to longitudinal characteristic relaxation time has been suggested and realized. The two problems were solved by this method. The first problem was the one of laser energy storage on the metastable level of the medium atoms. The space and time characteristics of this inverted medium of plane geometry were calculated. The amplification coefficient of forbidden transition versus different parameters of this problem was obtained. Using Asl as an example the mechanism of two-step photoionization of atoms and mechanism of formation of inversion of the population between a metastable level and a ground state (mechanism of accumulation of laser radiation energy in volume) are investigated. It is shown that due to the "imprisonment" (trapping) of reemited photons leads to the sharp decrease both of the length of interaction between laser radiation and medium, and the length of accumulation of laser radiation energy in volume. In this case the gain factor of inverted medium is one order of magnitude less than that obtained by theoretical estimates (neglecting this effect). The algorithm suggested allows solving these problems within multi-pulse mode taking into account the collision and diffusion processes.
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The propagation of short light pulses in a three-level medium with reverse saturable absorption taking into account relaxation is considered. It is shown that in such a medium soliton-like pulses of constant duration can propagate. The areas in the five-dimensional parameter space in which such pulses can exist are determined by numerical methods. The parameters of the problem are the characteristics of the medium – the relaxation time, the ratio of the absorption cross sections of the first excited and ground states and the initial transmittance, as well as the characteristics of the pulse – the duration and intensity.
Numerical integration of a system of equations describing the coherent interaction between radiation and a two-level medium (e.g., a semiconductor) is employed to analyze the effect of relaxation processes on the energy characteristics and dynamics of radiation pulses of duration τpΣ T '2 (T '2 is the irreversible polarization relaxation time). It is shown that the interaction of these pulses with the medium is strongly dependent on their area on entry to the medium and the attenuation of the pulses as a result of relaxation processes decreases with increasing pulse input power. This may be utilized for probing the medium in the resonant absorption range for short polarization relaxation times, in solution 42.65.Gv.
The interaction of intense laser fields with matter is accompanied by a multitude of phenomena, ranging from the generation of coherent radiation across various spectral ranges (from terahertz to soft X-rays) to the creation of beams of accelerated charged particles and micromodification of the volume and surface of various samples.To describe this diversity of phenomena, several theoretical approaches have been developed to date.These approaches utilize different approximations to address this complex nonlinear problem, allowing for the explanation of specific aspects of such interactions.The developed theoretical frameworks consider the interaction at both microscopic and macroscopic levels; at the microscopic level, they investigate the characteristics of single atoms interacting with laser fields, while at the macroscopic level, they analyze nonlinear susceptibilities that describe the nonlinear polarization of the medium during the propagation of laser radiation through it.This work presents an overview of methods describing some of the most commonly employed theoretical approaches to model the response of a medium at a microscopic (quantum-mechanical) level, as well as various methods for controlling the properties of generated coherent radiation.A model for the gas medium is described that accounts for both the response characteristics of individual atoms and allows for consideration of phase and quasiphase matching effects during radiation generation.The influence of medium parameters on the efficiency of generation and polarization properties of generated radiation is analyzed under both phase and quasi-phase matching conditions.A detailed description of an analytical method for calculating the nonlinear susceptibility of a gas medium is provided.The obtained values of nonlinear susceptibility for arbitrary orders may be useful in numerical calculations concerning the propagation of laser radiation in matter.Various channels for radiation generation are analyzed as parameters of the laser field are varied.