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Physical Review Letters

Quantum Interference in Probe Absorption: Narrow Resonances, Transparency, and Gain without Population Inversion

Peng Zhou, S. Swain

Published Feb 3, 1997• 222 citations
Publisher

Abstract

We examine the absorption of a weak probe beam for a V-type atom with a closely spaced doublet and demonstrate that quantum interference between the two excitation pathways can result in very narrow resonances, transparency, and even gain without population inversion. The origin of these effects is discussed.
DOI
10.1103/physrevlett.78.832

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We discuss the effects of the incoherent pumping in both probe and coupling channels on the absorption and dispersion of the probe laser with quantum interference. The incoherent population pumping in the coupling channel decreases the probe transparency. When decay rate in the coupling channel is greater than that in the probe channel, the incoherent population pumping in the probe channel can create the gain without inversion. The probe absorption pattern is upsidedown if the interference is present. When the decay rates in the probe and channels are equal, the incoherent pumping in the probe channel doesn't create the gain until the existence of the interference.
In this paper we analyze the steady-state populations and gain lineshape of a V-type three-level atom with a closely spaced excited doublet. The atom is driven by a strong coherent field, a weak probe, and a single broadband squeezed vacuum. We focus our attention in the interplay between the quantum interference and the squeezed field on the probe gain. It is shown that the relative phases between the two coherent fields and the squeezed field play an important role in the optical properties of the atom. Specifically, we find that the probe can experience gain without population inversion for proper values of the parameters characterizing the squeezed field and in the absence of incoherent pumping. The system can be tailored to exhibit multiple dispersion regimes accompanied by negligible gain or absorption over a large bandwidth, a desirable feature for obtaining propagation of pulses with negligible distortion.
The absorption of a weak probe beam for a V-type atom with a closely spaced doublet is studied under conditions such that vacuum-induced interference among decay channels is important. It is shown how the probe polarization as well as the vacuum-induced interference influence the absorption amplitude and spectral structure. Comparing with similar models studied earlier, where probe polarization direction has not been taken into account, the present study shows that probe polarization has significant effects on the absorption spectra.
For a four-level atomic system with a doublet of closely spaced levels, we find that, owing to the coherence that results from the vacuum of the radiation field, population trapping at excited levels and probe gain with population inversion can be achieved with weak incoherent pumping. This gain is different from both the conventional lasing gain and gain without inversion in that there exists population inversion on probe transitions but the inversion is achieved by the vacuum-induced coherence.
We investigate the effects of collisional dephasing and Doppler broadening on the quantum beats, narrow resonances, and probe transparency induced by quantum interference in a Vee-type atom consisting of an excited doublet coupled to a single ground level by the vacuum. We show that the amplitudes of the quantum-beat oscillations are greatly reduced, and the narrow resonances are substantially suppressed and broadened, for even very small dephasing rates and Doppler broadening. Fluorescence quenching and the previously reported probe transparency can never occur in the presence of dephasing collisions and Doppler broadening.