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Physical Review A

Observation of subnatural-linewidth spectra in cold Li6 atoms

Rui Li, Yuelong Wu, Yang Rui, Haibin Wu

Published Mar 18, 2021• 5 citations
Publisher

Abstract

We report an observation of the subnatural-linewidth spectroscopy in cold $^{6}\mathrm{Li}$ atoms by combining the interference contrast method and the time-delayed coincidence technique. The measured linewidth of ${D}_{1}$ lines is as narrow as 3.6 MHz, which is well below its natural linewidth of 5.8 MHz. The hyperfine splitting of the $2{P}_{1/2}$ state is measured as 26.112(20) MHz with this subnatural-linewidth spectroscopy. The transitions of the ${D}_{2}$ lines are partially resolved by using the developed technique. The time-delayed coincidence technique based on the interference contrast method in cold atoms provides a route for robust fluorescence measurements and determination of unresolved transitions.

DOI
10.1103/physreva.103.032823

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We report an observation of the subnatural-linewidth spectroscopy in cold 6Li^{6}\mathrm{Li}6Li atoms by combining the interference contrast method and the time-delayed coincidence technique. The measured linewidth of D1{D}_{1}D1​ lines is as narrow as 3.6 MHz, which is well below its natural linewidth of 5.8 MHz. The hyperfine splitting of the 2P1/22{P}_{1/2}2P1/2​ state is measured as 26.112(20) MHz with this subnatural-linewidth spectroscopy. The transitions of the D2{D}_{2}D2​ lines are partially resolved by using the developed technique. The time-delayed coincidence technique based on the interference contrast method in cold atoms provides a route for robust fluorescence measurements and determination of unresolved transitions.
We investigate, theoretically and experimentally, absorption on an excited-state atomic transition in a thermal vapor where the lower state is coherently pumped. We find that the transition linewidth can be subnatural, that is, less than the combined linewidth of the lower and upper state. For the specific case of the $6{P}_{3/2}\ensuremath{\rightarrow}7{S}_{1/2}$ transition in room temperature cesium vapor, we measure a minimum linewidth of 6.6 MHz compared with the natural width of 8.5 MHz. Using perturbation techniques, an expression for the complex susceptibility is obtained which provides excellent agreement with the measured spectra.
Subnatural linewidth in the optical transitions of Na D2{D}_{2}D2​ lines was directly observed with use of phase switching of the optical field and gating of the photon counter. A linewidth as narrow as 4.5 MHz compared with the natural width of 10 MHz was obtained.
A method to obtain subnatural resolution in laser-induced fluorescence spectroscopy by phase switching is described. The subnatural linewidth is obtained by extracting a coherent transient response induced by the switching of the laser. The linewidth in this type of spectroscopy is determined by the time interval between the switching and observation, instead of the spontaneous lifetime. The result is compared with the experiment on the Na D1{D}_{1}D1​ line.
Spectra that are narrower than the natural width of decaying states can be achieved by beginning the observation of signals at a fixed delay time after excitation rather than immediately afterward. These signals are weaker (and noisier) than the full, time-unresolved signals. Applications of this line-narrowing technique to precision spectroscopy are discussed, and the properties of the resulting signals are studied. Numerical simulations demonstrate that time-resolved line narrowing is highly desirable in a large number of cases.
The resolution of a quantum-beat experiment, which is basically limited by the atomic natural linewidth, has been improved by observing the fluorescence only from those atoms which have survived in excited states after some time delay. The resulting narrowing enables one to resolve previously overlapping lines in beam-foil quantum-beats experiments using $\mathrm{H}\ensuremath{\alpha}$ and 4He^{4}\mathrm{He}4He ii (4\ensuremath{\rightarrow}3) deexcitation.
Abstract. We report sub-Doppler and sub-natural linewidth spectroscopy of 6Li on the D1 and D2 lines in both a vapour cell and an atomic beam. Electromagnetically induced transparency is created using co-propagating mutually coherent laser beams with a frequency difference equal to the hyperfine ground state splitting of 228.2 MHz. The effects of various optical polarization configurations and applied magnetic fields are investigated. In addition, we present an optical Ramsey spectroscopy technique which further reduces the observed resonance width.