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Chinese Physics B

Sub-natural linewidth of the probe absorption spectrum in a cold Cs atom–molecule system

Zhifang Feng, Weidong Li, Liantuan Xiao, Jia Suo-Tang

Published Nov 1, 2009• 2 citations
Publisher

Abstract

We theoretically analysed the linewidth of the probe absorption spectrum in a cold Cs atom–molecule system. The tunnelling coupling between the two excited molecular states, especially for the Cs atom–molecule system, plays an important role in obtaining the sub-natural linewidth of the probe absorption spectrum. For example, when the tunnelling couple strength fulfils σ 12 = 10 γ ab 1 , the linewidth is only about 0.66 MHz. Moreover, since the linewidth of interest is dominated by the tunnelling coupling, the absorption peak becomes very narrow even in the case of large pump laser intensities.

DOI
10.1088/1674-1056/18/11/047

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We theoretically analysed the linewidth of the probe absorption spectrum in a cold Cs atom–molecule system. The tunnelling coupling between the two excited molecular states, especially for the Cs atom–molecule system, plays an important role in obtaining the sub-natural linewidth of the probe absorption spectrum. For example, when the tunnelling couple strength fulfils σ 12 = 10 γ ab 1 , the linewidth is only about 0.66 MHz. Moreover, since the linewidth of interest is dominated by the tunnelling coupling, the absorption peak becomes very narrow even in the case of large pump laser intensities.
We have studied the absorption of a weak probe beam through cold rubidium atoms in a magneto-optic trap. The absorption spectrum shows two peaks, with the smaller peak having linewidth as small as 28% of the natural linewidth. The modification happens because the laser beams used for trapping also drive the atoms coherently between the ground and excited states. This creates "dressed" states whose energies are shifted depending on the strength of the drive. Linewidth narrowing occurs due to quantum coherence between the dressed states. The separation of the states and the linewidth depends on the trapping-laser intensity and detuning as predicted by this model.
We theoretically investigated the properties of the effective four-level stimulated Raman adiabatic passage scheme in a cold gas of Cs atoms and molecules, where exists the tunnelling coupling between two excited molecular states due to the 0(g)- (6S,6P(3/2)) double well structure. The double dark resonance is predicted in the absorption spectrum when the tunnelling coupling strength is large enough. The double dark resonance not only reveals the formation of the ultra-cold molecules, but also provides further evidence for the tunnelling as one effective coupling mechanism between the two excited molecular states. The effect of the various experimental conditions on this phenomena has been discussed.
A new nano-cell with smoothly varying longitudinal thickness of the atomic vapour layer L in the range of 350 - 5100 nm allowing to study the resonant absorption of D1 and D2 lines of Cs atoms for thicknesses changing from L = Lambda/2 to L = 6 Lambda with the step of Lambda/2 (Lambda = 852 nm or 894 nm are the resonant laser wavelengths) and for different intensities is developed. It is revealed that for low laser intensities there is narrowing of the resonant absorption spectrum for the thicknesses L = (2n + 1)Lambda/2 (where n is an integer) up to L = 7Lambda /2 and broadening of the spectrum for L = n Lambda. For relatively high laser intensity (>1 mW/cm2), velocity selective optical pumping/saturated resonances of a reduced absorption (with the line-width close to the natural one), and centred on the hyperfine transitions occur when L = n Lambda. The possible application of these resonance peaks is given. The developed theoretical model describes well the experiment.
Absorption spectra of cold Cs atoms in a magneto-optical trap under the influence of a strong external pump laser field with arbitrary detuning and intensity have been systematically investigated. The spectra featured the Rabi sidebands of each transition, and their behavior was nicely fitted by the dressed-atom theory. A pronounced dispersion-type profile with sub-natural linewidth at the pump frequency may be attributable to the stimulated Raman process by sublevels that are lightshifted and optically pumped by the pump laser.
Depending on a four-level inverted-Y atomic system, we demonstrate the limitation of linewidth-narrowing for the probe absorption spectrum in the electromagnetic induced absorption platform. Thanks to the use of an auxiliary control field which couples one hyperfine ground state and one middle-excited state we show that the linewidth limitation can be constrained by a coherence decay rate between two hyperfine ground states, rather than by the decay rate between the ground and the excited states as in previous Ladder schemes. That fact makes the theoretically-predicted absorption linewidth at least two orders of magnitude narrower. By using a suitable adjustment for the control-field amplitude and the detuning we numerically show that an extremely-narrowed probe absorption spectrum accompanied by a higher spectra contrast can be obtained, which confirms well with our theoretical predictions. We study the transient time response to the absorption spectrum and show that a relatively longer response time arises due to the small coherence decay rate between two hyperfine ground states. Furthermore, we reduce the influence on linewidth-narrowing from the Doppler effect via an optimized design of lasers, and reveal that no Doppler-free effect exists due to the lack of three-photon process. Our results may pave a route to the development of high-resolution spectroscopy in current experiments.
We measure sub-MHz AC Stark shifts, also known as light shifts, in an undergraduate laboratory setting using Raman pump-probe spectroscopy to observe sub-natural linewidth spectral features in the transmission spectrum of a weak probe beam passing through a sample of cold 85Rb atoms confined in a magneto-optical trap. To make this observation a pair of inexpensive fast photodiodes and acousto-optic modulators is needed, in addition to equipment commonly found in advanced undergraduate optics labs with laser cooling and atom trapping setups. A theoretical description of light shifts accessible to junior and senior-level physics majors is provided.