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Physical Review A

Lasing without or with inversion in a closed four-level system

Yifu Zhu

Published Jan 1, 1993• 28 citations
Publisher

Abstract

I show that a closed four-level system pumped by a single laser exhibits the following features: lasing without population inversion in any state basis, lasing with population inversion in the dressed-state basis, and lasing with population inversion in the bare-state basis. The system can also exhibit a large refractive index with either zero or negative absorption.

DOI
10.1103/physreva.47.495

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Chinese Physics B•Dec 1, 2012•4 citations
I show that a closed four-level system pumped by a single laser exhibits the following features: lasing without population inversion in any state basis, lasing with population inversion in the dressed-state basis, and lasing with population inversion in the bare-state basis. The system can also exhibit a large refractive index with either zero or negative absorption.
We show that a closed four-level system can exhibit lasing without population inversion in any state basis. The same system can be also made to exhibit lasing without population inversion in the bare-state basis but with population inversion in the dressed-state basis. We derive the conditions of lasing without inversion and examine the dependence of the gain on various system parameters. The lasing threshold and the steady-state photon numbers are calculated. Comparison with a ssV-type, three-level model is presented.
We study the operational properties of the lasing field generated from strongly driven two-level atoms in a ring cavity by including the field saturation to all orders. Lasing without inversion between bare atomic states can occur for both off-resonant-- and resonant-pump fields. In the case of an off-resonant--pump field, lasing occurs at one of the Rabi sidebands, for which there exists population inversion between the relevant dressed-atom--field states. In the case of a resonant-pump field, lasing can occur near either of two Rabi sidebands, for which there is no population inversion between the dressed states; here the gain is due to the atomic coherence between the dressed states. While the operation of the off-resonantly--pumped dressed-state laser is similar to that of an ordinary laser, the operation of the resonantly pumped dressed-state laser exhibits some interesting features, such as (1) mode pushing and (2) larger laser intensity coming from smaller linear gain as the detuning is changed. In order to show such features, the calculations have to go beyond the secular approximation.