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Laser & Infrared

Nonlinear Effects of the Spontaneous Emission in Four-level Atomic System Coupled by Weak Fields

Weijian Tian

Published Jan 1, 2002• 0 citations
OpenAlex

Abstract

The quantum interference is studied in a spontaneous emission of a four-level atom driven by three coherent fields. Deriving the equation of motion in the interaction picture we obtain the susceptibility. It is shown the susceptibility is different from the conventional nonlinear optics. The atom exhibits strong nonlinear effects when the spontaneous is at resonance, though it is weak. We can apply this new phenomenon to many fields such as making quantum device, detecting weak signal and processing quantum information.

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The quantum interference is studied in a spontaneous emission of a four-level atom driven by three coherent fields. Deriving the equation of motion in the interaction picture we obtain the susceptibility. It is shown the susceptibility is different from the conventional nonlinear optics. The atom exhibits strong nonlinear effects when the spontaneous is at resonance, though it is weak. We can apply this new phenomenon to many fields such as making quantum device, detecting weak signal and processing quantum information.
We have studied nonlinear quantum optical effects in a doubly driven four-level atomic system in a homogeneously broadened regime. On application of two strong fields the system can be made to display double electromagnetically induced transparency (EIT) because of constructive quantum interference giving rise to nonlinear absorption at three-photon resonance condition. This fact may be used to obtain other phenomena like the effect of photon switching in a pulsed regime and large cross phase modulation (XPM) effect. We also show that the behaviour of spatial modulation of transparency of the weak probe beam in a three-level system interacted with a strong standing wave pump field can be dramatically changed in our four-level ladder type system. Here, strong coupling field acting on the uppermost transition is represented by a standing wave whereas strong pump and weak probe fields acting successively on the lower transitions are represented by travelling waves. The effect of double EIT on nonlinear light generation based on the four-wave-mixing (FWM) technique is also investigated. We have examined that the four wave mixing efficiency can be effectively controlled by varying the strength of the pump field.
We investigate the spontaneous emission spectrum of a three-level atom coherently driven by a microwave field. The microwave excitation establishes a coherent link between the bottom two levels, and as a consequence induces the quantum interference of previously independent decay pathways. As the decay pathways differ from one another by an odd number of stimulated emission processes, the spontaneous emission spectrum acquires a dependence on the phase of the excitation field, a feature that is absent in other resonance fluorescence phenomena.
We discuss quantum interference effects in the resonance fluorescence spectrum of a \ensuremath{\Lambda} three-level atom when the lower-level doublet is driven by a coherent field. The interfering pathways that lead to the same final state involve both spontaneous decays and stimulated transitions, and differ from one another by an odd number of stimulated processes induced by the driving field. As a consequence, the interference structures depend upon the phase of the coherent field, an effect that is absent in other resonance fluorescence phenomena. The phase dependence of the quantum interference contribution is especially significant when the level splitting of the driven doublet is comparable to the spontaneous decay rates of the competing optical paths.
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.
Abstract We describe the linear and nonlinear optical properties of a four-level atomic system, after reducing it to an effective two-level atomic model under the condition of adiabatic shifting of resonances driven by two coherent off-resonant fields. The reduced form of the Hamiltonian corresponding to the two-level system is obtained by employing an adiabatic elimination procedure in the rate equations of the probability amplitudes for the proposed four-level model. For a weak probe field operating in the system, the nonlinear dependence of complex susceptibility on the Rabi frequencies and the detuning parameters of the off-resonant driving fields makes it possible to exhibit coherent control of single-photon and two-photon absorption and transparency, the evolution of enhanced Self-Kerr nonlinearity and noticeable dispersive switching. We have shown how the quantum interference results in the generic four-level model at the adiabatic limit. The present scheme describes the appearance of single-photon transparency without invoking any exact two-photon resonance.