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Bulletin of the Russian Academy of Sciences Physics

Energy exchange between laser pulses in an atomic medium with a closed excitation contour

К. А. Баранцев, A. N. Litvinov, Е. Н. Попов

Published Jul 1, 2016• 1 citation
Publisher

Abstract

The effect the phase difference between carrier frequencies of two laser pulses has on their propagation in an optically dense medium of three-level point scatterers is investigated. The medium is exposed to constant radiation, creating a closed excitation contour. Situations are considered for pulse durations shorter and longer than the lifetime of the excited state of scatterers.

DOI
10.3103/s1062873816070054

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The effect the phase difference between carrier frequencies of two laser pulses has on their propagation in an optically dense medium of three-level point scatterers is investigated. The medium is exposed to constant radiation, creating a closed excitation contour. Situations are considered for pulse durations shorter and longer than the lifetime of the excited state of scatterers.
In this work we investigate propagation of two laser pulses in the optically dense atomic medium (three-level Λ -system) in the case of a closed excitation contour. The closed excitation contour is created by additional microwave radiation which interacts with the low-frequency transition of the Λ -system. Two cases are considered: when the pulse duration is more and less then lifetime of the excited atomic level. It is shown that the propagation of the pulses depends on the relative phase of the fields. When the relative phase is zero, the pulses energy is absorbed linearly. When the relative phase is π / 2, the pulses exchange energy between each other.
A closed contour of excitation (Δ-scheme) of a three-level atom in an optically dense medium under the action of two optical fields and a microwave field has been studied. It is found that phase relations between exciting fields in this medium play a determining role. It is established that, under certain conditions, in an optically dense Δ-atom medium exhibits a quasi-periodic dependence of the refractive index on the coordinate. The conditions under which this dependence is most pronounced are determined.
We study the propagation of frequency-chirped laser pulses in optically thick media. We consider a medium of atoms with a $\ensuremath{\Lambda}$ level-scheme (Lambda atoms) and also, for comparison, a medium of two-level atoms. Frequency-chirped laser pulses that induce adiabatic population transfer between the atomic levels are considered. They induce transitions between the two lower (metastable) levels of the $\ensuremath{\Lambda}$-atoms and between the ground and excited states of the two-level atoms. We show that associated with this adiabatic population transfer in $\ensuremath{\Lambda}$-atoms, there is a regime of enhanced transparency of the medium---the pulses are distorted much less than in the medium of two-level atoms and retain their ability to transfer the atomic population much longer during propagation.
We investigate numerically the propagation behaviors of an ultrashort laser pulse in a dense Λ-type three-level atomic medium by using the full Maxwell-Bloch equations without the slowly varying envelope approximation and the rotating-wave approximation. We find that with propagation distance and time increasing, variations of the Rabbi frequency and the population in a dense medium are quite different from those in a dilute medium even though Lorentz local field correction (LFC) is not considered. In a dilute medium, the populations occupied initially at the ground state are completely transferred into the lower excited-state with the increase of the propagation distance; however, in the dense medium, about a half is still kept on the ground state and the others are transferred into the lower excited-state. The population oscillation occurs at the input surface of a dense three-level medium due to carrier-wave Rabi oscillation, and the oscillating time with LFC is much longer than that without LFC; the time derivative of the electric field has stronger effects on the time evolution of the pulse in the dense medium than that in a dilute medium, which is clearly shown from the time evolution of the Rabi frequency; especially the oscillating amplitude with LFC in the trailing edge of the pulse is larger than that without LFC at the input surface of the three-level medium.
Abstract In the framework of the consistent microscopic theory, universal analytic expressions describing absorption of laser pulses of an arbitrary duration in an optically dense medium have been derived, and their limiting cases have been considered. For the Gaussian and Lorentzian spectral profiles, simple expressions have been obtained for the absorption coefficient as a function of dimensionless parameters (optical thickness of the medium, the pulse duration and carrier frequency). Based on the performed calculations, the effect of the optical thickness of the medium on the dependences of the absorption coefficient on the pulse parameters has been revealed.