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Physical Review A

Suppression of the line broadening by fast Rabi flopping effects

G. S. Agarwal, Sunita Singh

Published Feb 1, 1989• 7 citations
Publisher

Abstract

We report on a new mechanism that can lead to line narrowing. We show that the line-broadening effects associated with a broadband source can be suppressed by irradiating the system with a coherent pump. We give explicit results for this suppression in spontaneous emission from a two-level system.
DOI
10.1103/physreva.39.2239

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The line shape of a Doppler-broadened transition is dramatically altered by the presence of a laser field resonating with a second Doppler-broadened transition sharing a common level: Two narrow resonances of different widths appear superimposed upon the broad background signal at frequencies symmetrically located about the corresponding line center. The effect has already found application in a number of seemingly different though intimately related studies, including high-resolution hfs and isotope-shift determinations. The theory of the effect is developed with reference to these applications. The treatment is formulated in terms of transition rates induced by two classical fields resonantly interacting with a pair of coupled Doppler-broadened transitions of arbitrary frequencies. The perturbation approach adopted is valid for one field fully saturating its transition; the resulting line-shape expression exhibits important power-broadening effects. This approach is equivalent to the familiar density-matrix formulation, which is also presented. Various features of the resulting expression are discussed in detail as they apply to two precision spectroscopic applications, mode crossing and spontaneous emission line narrowing. The connection with previous work is also discussed.
We show that it is possible at least in principle to have unbounded line narrowing in the spontaneous emission spectrum simultaneously with control of the corresponding intensity. For this purpose we employ a laser driven doublet of excited atomic, molecular, or semiconductor states with almost parallel dipole orientation, which is decaying to a ground state. Incoherent pumping is applied to control the population and thus the spontaneous emission intensity of the dressed state associated with the ultranarrow spectral line.
Interference effects between decay processes from two upper levels, which are coupled by the same vacuum modes to a lower level and are driven by a strong coherent field, on the spontaneous emission spectrum have been studied. The interference could lead to population trapping in the upper levels, two dark lines in the spectrum and spectral narrowing. The spectral narrowing depends on the ratio of the two upper levels' decay rates and the Rabi frequency of the driving field. One can change the spectrum by varying the Rabi frequency, an experimentally controllable parameter.
We report a mechanism that leads to the suppression of line broadening effects. We show that a strong competition between the effects of coherent and incoherent pumps can lead to suppression of spontaneous emission or to narrowing of the spectral profiles. Consider the interaction of a system with a broadband incoherent pump, the effects of which can be accounted for by changing the relaxation times T1, T2 in the Bloch equations. For example, for radiative relaxation (rate 2γ) we have where | x | is the Rabi frequency associated with the broadband pump, and Γ is its bandwidth. The spontaneous emission spectrum from such a system in the presence of a coherent drive is the Mollow spectrum with the broadband pump contributing an amount β to the widths of the peaks. We demonstrate that the effects of β can be suppressed if the strength of the coherent pump is such that many Rabi oscillations are possible within the coherence time of the incoherent pump. We show this explicitly by obtaining exact results for spontaneous emission from a model system.