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arXiv:1210.5817

Amplitude and phase control of gain without inversion in a four-level atomic system using loop-transitio

Zou, Jinhua, Dahai Xu, Huafeng Zhang

Published Oct 22, 2012• 0 citations
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Abstract

Amplitude and phase control of gain without inversion is investigated in a four level loop-structure atomic system. Two features are presented. One is that gain without inversion can be obtained through the amplitude control of the applied fields. The other is that gain without inversion show a phase-dependence on the relative phase between the fields applied on the two two-photon transitions. Gain and phase-dependence originate from interference induced by such a loop-transition structure.

DOI
10.48550/arxiv.1210.5817

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Amplitude and phase control of gain without inversion is investigated in a four level loop-structure atomic system. Two features are presented. One is that gain without inversion can be obtained through the amplitude control of the applied fields. The other is that gain without inversion show a phase-dependence on the relative phase between the fields applied on the two two-photon transitions. Gain and phase-dependence originate from interference induced by such a loop-transition structure.
We study a four-level atomic system controlled by a three-level closed loop subsystem in order to obtain the phase-sensitive probe gain without population inversion. In the case of three-photon resonance, a large steady-state inversionless probe gain can be obtained. Due to the quantum interference effect, the inversionless gain is sensitive to the relative phase of the three-level closed loop subsystem. Further, the field intensity of the three-level closed loop subsystem is also an important parameter that can affect gain characteristic of the probe field.
In order to achieve the phase-sensitive probe gain without population inversion, we investigate a four-level atomic system, where the probe field is controlled by a three-level closed loop subsystem. In the case of three-photon resonance, large steady-state inversionless probe gain can be obtained. Owing to the quantum interference, the probe gain is sensitive to the relative phase of the three-level closed loop subsystem. The intensities of the fields of the three-level closed loop subsystem are also important factors that can result in different gain behaviors of the probe field.
We propose a new scheme for achieving inversionless gain that depends on both an internal interference effect and external coherent fields. We have found that, owing to the quantum interference effect that originates from spontaneous emissions from two closely lying levels, large inversionless gains can be achieved under appropriate conditions. In particular, in addition to the amplitudes, the phases of the applied fields also affect inversionless gain remarkably. That is to say, we can use the relative phase between two intense fields to control probe gain without inversion.
A four-level atomic system with a closed interaction loop connected by two coherent driving fields and a microwave field is investigated. The results show that inversionless gain can be achieved on a higher frequency transition outside the closed interaction loop, and the gain behaviour can be modulated by the phase of the closed loop as well as the amplitude of the microwave field. The phase sensitivity property in such a scheme is similar to that in an analogous configuration with spontaneously generated coherence, but it is beyond the rigorous condition of near-degenerate levels with non-orthogonal dipole moments. Therefore this scheme is much more convenient in experimental realization.
We examine the gain property of a probe field interacting with two different three-level Lambda-type atomic systems with an open loop or a close loop. In the atomic system with an open loop, there exists quantum interference resulting from spontaneous emissions to two near-degenerate lower levels from a common upper level, and the weak probe field can be greatly amplified due to the spontaneously generated coherence. Moreover, the probe gain becomes sensitive to the relative phase between the probe and coupling fields, so we can realize phase control of the probe gain in principle. In the atomic system with a close loop, we use a microwave field to couple the two well-spaced lower levels so that quantum coherence similar to SGC can be generated. In this atomic system, we also can achieve the phase-sensitive probe gain due to the quantum interference between two different absorption channels for the probe field. Note, only in the case of three-photon resonance, this close-loop atomic system can reach a steady state. While in the case of three-photon off-resonance, the probe gain without inversion always oscillates periodically versus time, thus no steady-state probe gain can be achieved.
We consider the one- and two-photon absorption spectra of a four-level Y-type atom with the two highest lying levels driven by a microwave field. We found that in the one-photon absorption case, the microwave field can lead to the probe gain, and the absorption and gain spectral structures depend strongly on the microwave field amplitude. For the two-photon absorption case, the strong microwave field can enhance the absorption. When the microwave field amplitude is reduced to a certain value, the single absorption peak in the two-photon spectrum changes into a structure of two-peak structure with different magnitudes. Moreover, the one- and two-photon absorption spectra can be modulated by the phase of the microwave field which produces a closed-loop configuration. Finally, we use the analytic solutions in terms of dressed-state basis to explain the results from our numerical calculation.