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Journal of the Optical Society of America B

Gain spectrum of a laser-driven Λ-type atom with vacuum-induced coherence

Weihua Xu, Jin-Yue Gao

Published Nov 1, 2005• 3 citations
Publisher

Abstract

We examine the gain spectrum of a coherently driven Λ-type atom with a pair of closely lying ground levels, where a single laser field drives both transitions from the upper state to the lower states simultaneously. The effects of vacuum-induced coherence among decay channels are taken into account. Gain spectra under various conditions are presented and analyzed. We show that the spectral structure and the gain amplitude of the probe are strongly influenced by the vacuum-induced coherence as well as the frequency separation between the two ground levels. We present an explanation to our numerical results by introducing dipole moments in the dressed-state representation.

DOI
10.1364/josab.22.002385

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We examine the gain spectrum of a coherently driven Λ-type atom with a pair of closely lying ground levels, where a single laser field drives both transitions from the upper state to the lower states simultaneously. The effects of vacuum-induced coherence among decay channels are taken into account. Gain spectra under various conditions are presented and analyzed. We show that the spectral structure and the gain amplitude of the probe are strongly influenced by the vacuum-induced coherence as well as the frequency separation between the two ground levels. We present an explanation to our numerical results by introducing dipole moments in the dressed-state representation.
We examine the absorption of a weak probe beam in a laser-driven tripod-type atom with three closely lying ground levels, where both the driving and probe lasers interact simultaneously with the three transitions. The effects of spontaneously generated coherence (SGC) are taken into account. We introduce dipole moments in the dressed-state picture and the Hamiltonian in terms of the dressed states describing the interaction between the probe and the atom. Gain spectrum under various conditions are presented and analyzed. We show that the spectral structure and the gain amplitude of the probe are strongly influenced by the effect of SGC and the frequency separation of the three closely lying ground levels. The tripod-type atomic system with quantum interference in spontaneous emission can be simulated in the dressed-state picture of a coherently driven four-level N-type system where no SGC effect exists.
We examine the absorption of a weak probe beam by a laser driven V-type atom with a pair of closely lying excited levels, where both the driving and probe lasers interact simultaneously with the two transitions. The effects of quantum interference among decay channels on the absorption spectra are also investigated. We introduce dipole moments in the dressed-state representation and the Hamiltonian in terms of the dressed states describing the interaction between the probe and the atom. In the degenerate case, features similar to that of a driven two-level atomic system are found due to some dark transitions in the spontaneous emission and the fact that the probe beam only detects certain transitions. In the nondegenerate case, the absorption spectrum is strongly influenced by the degree of quantum interference, resulting in different line shapes for emission peaks, absorption peaks, and dispersionlike profiles. The effect of probe polarization on the absorption spectrum is also investigated.
When a four-level Y\mathsf{Y}Y-type atom with two highest nearly degenerate lying levels interacts with the same vacuum radiation field, its excited levels have vacuum-induced coherence due to the quantum interference between the spontaneous decay channels. We consider the effects of the vacuum-induced coherence on the electromagnetically induced transparency against single- and two-photon absorption in the atomic system. We find that the vacuum-induced coherence can lead to probe gain without incoherent pumping. On the other hand, the coherence can suppress the two-photon transparency, even in some cases where it can enhance the two-photon absorption. Another important result is the finding of the crucial role played by the relative phase between the probe and coupling fields: the two-photon transparency and the probe gain spectra can be modulated by this phase.
Abstract The coherence phenomenon of atomic population trapping in an excited state of a medium is interesting regarding the basic physics of laser-matter interaction and applications to the generation of pulsed lasers. Here, we propose N numbers of five-level atoms as a gain medium in a weak magnetic field with three excited closely-spaced Zeeman-hyperfine energy levels and two lower energy levels. The excited levels are strongly coupled with a lower level via a coherent-field and to the ground energy level via vacuum field modes. The spontaneous emission spectrum, in general, produces four spectral lines associated with the decaying four dressed states of the atom. Based on parallel dipole moments and radiative decay rates, we find the condition of double coherent population trapping (DCPT) originating from the non-decaying states of the excited dressed states. The decay from the states generates coherence among themselves to eliminate two distinct spectral lines from the spectrum using the trapping condition. In each of the two non-decaying dressed states, 23% of the total atom population traps.
In this paper we investigate the resonance fluorescence, the squeezing, and the absorption spectra of a laser-driven four-level atom consisting of three closely spaced upper levels decaying to a common lower level. The three upper levels are coupled by the same vacuum modes to the lower level leading to spontaneously generated coherence effects. High population inversion and extremely narrow emission lines are obtained with moderate Rabi frequencies, which are a direct consequence of decay-induced interferences. The fluorescent field is shown to exhibit two-mode squeezing in the in-phase quadrature. Squeezing in both quadratures simultaneously at different frequencies is greatly enhanced when quantum interference is accounted for. We also examine the absorption spectrum of a weak field and demonstrate that quantum interference can induce two prominent and nearly transparent holes where the slope of the refractive index is very steep. This special situation could allow the simultaneous propagation of two weak pulses with different frequencies, which could be feasibly exploited towards the realization of the entanglement of two photons.
We consider a four-level atomic system with a degenerate or quasidegenerate ground-state doublet and two excited states. One of the upper levels is coupled to the ground doublet by a coherent field of variable strength, which we refer to as the Raman field, whose role is to create atomic coherence between the driven atomic states. The other higher lying state is coupled to the ground states when laser action develops. This model includes an incoherent pump mechanism to create some population in the top lasing state. In appropriate conditions, for the incoherent pump rate and the Raman field strength, the absorption profile of the laser transition develops gain features. We discuss the physical origin of this gain which we find to be the consequence of a population inversion between dressed atomic states. It is interesting to compare the results of this analysis with a similar study of a three-level lambda system which was previously recognized to yield lasing without inversion.1 In this case the appearance of gain can be attributed to the induced atomic coherence.