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arXiv:1209.4709 · Laser Physics

Collision induced coherence in plasma and branching ratio in multilevel system

Dong Sun, Victor V Kozlov, Lan Yuan, Yuri V. Rostovtsev

Published Jan 18, 2019• 0 citations
arXivHTMLPDF

Abstract

Abstract Our work is based on the collision-induced coherence of two decay channels along two optical transitions. The quantum interference of pumping processes creates the dark state and the more atoms are pumped in this collision-induced dark state, the stronger the suppression of the spontaneous emission. The efficiency of this suppression is quantified by putting it in comparison with the spontaneous emission on the ultraviolet transition, which proceeds in a regular fashion. The branching ratio of these two (visible and ultraviolet) transitions is introduced as the effective measure of the degree of the suppression of the spontaneous emission on the visible transition. Our preliminary calculations show that a significant decrease of the branching ratio with an increase of electron densities is reproduced in the theory.

DOI
10.1088/1555-6611/aaf98e

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Abstract Our work is based on the collision-induced coherence of two decay channels along two optical transitions. The quantum interference of pumping processes creates the dark state and the more atoms are pumped in this collision-induced dark state, the stronger the suppression of the spontaneous emission. The efficiency of this suppression is quantified by putting it in comparison with the spontaneous emission on the ultraviolet transition, which proceeds in a regular fashion. The branching ratio of these two (visible and ultraviolet) transitions is introduced as the effective measure of the degree of the suppression of the spontaneous emission on the visible transition. Our preliminary calculations show that a significant decrease of the branching ratio with an increase of electron densities is reproduced in the theory.
The spontaneous emission is investigated for an effective atomic two-level system in an intense coherent field with frequency lower than the vacuum-induced decay width. As this additional low-frequency field is assumed to be intense, multiphoton processes may be induced, which can be seen as alternative transition pathways in addition to the simple spontaneous decay. The interplay of the various interfering transition pathways influences the decay dynamics of the two-level system and may be used to slow down the spontaneous decay considerably. We derive from first principles an expression for the Hamiltonian including up to three-photon processes. This Hamiltonian is then applied to a quantum mechanical simulation of the decay dynamics of the two-level system. Finally, we discuss numerical results of this simulation based on a rubidium atom and show that the spontaneous emission in this system may be suppressed substantially.
The modification of the spontaneous emission spectrum has been studied in this paper in a four-level doubly driven N-type atomic system. In the presence of two coherent fields, emphasis has been given to studying the spectral distribution of spontaneous emission in the relaxation process of the initially prepared excited level. The phenomenon of quantum interference dynamically induced in the system by these fields can significantly change the spontaneous emission spectrum. We have shown the emergence of a two-peak spectrum where two widely separated symmetric peaks are associated with a sharp dark line at their mid-position. At different dynamic conditions, we have also shown that two symmetric peaks may appear in the spectrum with an extended dark zone between them. Depending on the values of the Rabi frequencies and the detunings of the external fields, one can achieve a large enhancement of very narrow spectral components within the spontaneous emission line shape under different conditions. The phenomenon of suppression of spectral line and the selective cancellation of spontaneous emission have also been discussed. The results shown in various cases are analysed by considering all possible dressed states corresponding to the bare state model. The influence of collision-induced effects on the spontaneous emission spectrum is investigated as a conditional case.
Quantum interference in a decaying three-level system of V type with degenerate upper levels driven by a single incoherent field is shown to lead to a coherent population-trapping state and more generally to a population-locked state. The latter is a state with half the population locked in two upper states regardless of the strength of incoherent pumping and decay rates. We reveal the mechanism by which half the population is pumped to the upper states no matter how weak is the incoherent pumping. Transient regimes of gain without inversion and inversion without gain are demonstrated. Quenching of spontaneous emission due to electron collisions is also discussed in support of the experiments and ideas of Suckewer and [Phys. Rev. Lett. 60, 1122 (1988)].
The quantum interference in the spontaneous-emission spectrum of a driven atom, when the upper level of the atomic transition is coupled to a third level via a classical driving field, results in a dark line in the absence of the atomic decays. The existence of the dark line is independent of the Rabi frequency associated with the driving field. There is no such dark line in the spontaneous-emission spectrum when, instead of the upper level, the lower level is coupled to a third level via a driving field. In this paper, we include the level decays, incoherent pumping, and the finite bandwidth of the driving field in our analysis and study their role in the spontaneous-emission spectrum in the two configurations.
It has been shown recently [see, for example, S.-Y. Zhu and M. O. Scully, Phys. Rev. Lett. 76, 388 (1996)] that a dynamical suppression of spontaneous emission can occur in a three-level system when an external field drives transitions between a metastable state and two decaying states. What is unusual in the decay scheme is that the decaying states are coupled directly by the vacuum radiation field. It is shown that the decay dynamics required for total suppression of spontaneous emission necessarily implies that the level scheme is isomorphic to a three-level \ensuremath{\Lambda} system, in which the lower two levels are both metastable and each is coupled to the decaying state. As such, the total suppression of spontaneous emission can be explained in terms of conventional dark states and coherent population trapping.
We show that radiative coupling between two multilevel atoms having near-degenerate states can produce new interference effects in spontaneous emission. We explicitly demonstrate this possibility by considering two identical V systems each having a pair of transition dipole matrix elements that are orthogonal to each other. We discuss in detail the origin of the new interference terms and their consequences. Such terms lead to the evolution of certain coherences and excitations that would not occur otherwise. The special choice of the orientation of the transition dipole matrix elements enables us to illustrate the significance of vacuum-induced coherence in multiatom multilevel systems. These coherences can be significant in energy transfer studies.