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Saratov Fall Meeting: Laser Physics, Photonic Technologies, and Molecular Modeling

The propagation of ultrashort laser pulses in phototropic media

Vladislav Yu. Gribkov, Rimma Sh. Zatrudina

Published Jun 3, 2019• 0 citations
Publisher

Abstract

In this paper, the propagation of nanosecond laser pulses in various media with reverse saturable absorption is studied by numerical methods. Numerical simulation of the propagation of nanosecond laser pulses in a three-level medium with reverse saturable absorption was carried out using the classical transport equation for the case of incoherent nonlinear interaction of radiation with the medium described by the proposed three-level model. The degree of pulse deformation in such media is estimated. It is shown that with an increase in intensity, the pulse begins to deform, the pulse front shortens, the trailing edge becomes longer, the asymmetry coefficient increases. An additional maximum begins to form during a trailing edge with a further increase in intensity. As the additional maximum increases, the asymmetry coefficient monotonously decreases to negative values.

DOI
10.1117/12.2523332

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In this paper, the propagation of nanosecond laser pulses in various media with reverse saturable absorption is studied by numerical methods. Numerical simulation of the propagation of nanosecond laser pulses in a three-level medium with reverse saturable absorption was carried out using the classical transport equation for the case of incoherent nonlinear interaction of radiation with the medium described by the proposed three-level model. The degree of pulse deformation in such media is estimated. It is shown that with an increase in intensity, the pulse begins to deform, the pulse front shortens, the trailing edge becomes longer, the asymmetry coefficient increases. An additional maximum begins to form during a trailing edge with a further increase in intensity. As the additional maximum increases, the asymmetry coefficient monotonously decreases to negative values.
The propagation of nanosecond laser pulses in solutions of large-scale carbon nanostructures has been theoretically and numerically investigated in this paper. The three-level ladder-type scheme was used to study the medium with RSA. Numerical simulation of the propagation of nanosecond laser pulses in RSA media was carried out using the classical transport equation for incoherent nonlinear interaction of radiation with the medium described by the proposed three-level model. It is shown that for sufficiently small relaxation times of the medium and high intensity of, a splitting of the transmitted pulse by two is possible.
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.
Significant deviation of light pulse group velocity from the speed of light с due to the anomalous dispersion of a medium (so-called “slow” and “fast” light phenomena) may be caused by several mechanisms. One of these mechanisms is reverse saturable absorption. This work presents experimental research of the propagation of high-power ultrashort laser pulses through the aqueous solution of the carbon nanostructures. Our experimental results demonstrate a “fast light” behavior of pulses transmitted through the aqueous solution of carbon nanostructures. Results of our numerical simulation are in good agreement with the experimental results and confirm that the observed phenomenon is due to reverse saturable absorption. The group velocity of nanosecond high-power laser pulses is negative and achieves value to –c/275.
We develop an analytical approach to the dynamics of band populations of reverse saturable absorbers modeled by the three-level approximation of the five-level rate equations. We find high-accuracy approximate solutions to these rate equations, taking into account the temporal shape of the incident laser pulse for different regimes of excitation. The results obtained are confirmed by direct numerical integration of the rate equations and are verified by solution of the full system of the rate and the propagation equations. The validity ranges of the approximations are determined. We also prove that for input pulses that are much longer than the lifetime of the first excited state the ground-state depletion obeys the same functional dependence on the input fluence as in the case of rectangular input pulses. The dynamics of the excited states, however, explicitly depends on the pulse shape. We quantitatively estimate the effect of various parameters on the nonlinear absorption coefficient and discuss implementation of the approach by a beam propagation method to reduce the computational time.
The Maxwell–Schrödinger equations describing the ultrashort laser pulse propagation in multilevel molecular media are shown to be analytically solvable and to reduce to nonlinear multisine-Gordon equations for certain models of the field–molecule system. Special cases of level schemes are shown to lead to soliton propagation. A numerical study is presented in order to examine the effect of nonresonant (virtual) transitions on pulse propagation and amplification in a three-level molecular system as a function of intensity and detuning.