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Physical Review A

Subnatural-linewidth laser spectroscopy by phase switching

Fujio Shimizu, Kazuko Shimizu, Hiroshi Takuma

Published Oct 1, 1983• 15 citations
Publisher

Abstract

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 ${D}_{1}$ line.

DOI
10.1103/physreva.28.2248

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Physical Review A•Nov 1, 1996•38 citations
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.
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.
We demonstrate a simple technique to lock the frequency of a laser to an atomic transition with a new technique of subnatural linewidth spectroscopy. The coupled rubidium system exhibits an enhanced absorption spectrum of the probe beam with a subnatural linewidth. Compared with other techniques with an additional laser, our scheme only utilizes a single laser, which will be locked. The observed spectroscopic linewidth is down to about 3.5 MHz. The laser frequency fluctuation is about 226 kHz after locking on. The demonstrated scheme offers a convenient and low-cost technique for laser frequency stabilization and related precision measurements.
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.
Optical resonances with subnatural linewidth as narrow as 2.6 MHz have been observed with excellent signal-to-noise ratio in the Na D1{D}_{1}D1​ line (natural width 10 MHz) in a polarization spectroscopy experiment. The new, subnatural resonances arise from Zeeman coherences which are produced by a strong probe beam and destroyed by a velocity-selective optical Hanle effect.
We show how control lasers lead to subnatural linewidths in spontaneous emission as well as to the quenching of the quantum noise. We trace this quenching to the dispersive contributions to the line shape. We obtain analytical expressions for linewidths that for $\ensuremath{\wedge}$ systems are half the natural linewidths with the possibility of much more reduction for detuned control laser. We further present a simple physical analysis in terms of a two-photon Fermi golden rule to understand cancellation of spontaneous emission. Finally, we relate our results to recent experiments.