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Journal of Physics B Atomic Molecular and Optical Physics

Observation of narrow Ramsey-type resonances in a caesium beam due to Zeeman coherences

G. Théobald, V. Giordano, N. Dimarcq, P. Cérez

Published Jul 14, 1991• 7 citations
Publisher

Abstract

The authors report the observation of narrow Ramsey-type fringes in a caesium beam by applying two spatially separated resonant laser fields. Zeeman coherences created by the first laser evolve in a constant magnetic field and are probed by the second laser. The fluorescence of the atoms in the probe light is recorded as a function of the magnetic field, showing a Ramsey-type interference pattern. Linewidths of the central valley of 0.5 mG are obtained when the distance between the two lasers is 34.5 cm. The influence of the thermal beam velocity distribution is discussed.

DOI
10.1088/0953-4075/24/13/011

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The authors report the observation of narrow Ramsey-type fringes in a caesium beam by applying two spatially separated resonant laser fields. Zeeman coherences created by the first laser evolve in a constant magnetic field and are probed by the second laser. The fluorescence of the atoms in the probe light is recorded as a function of the magnetic field, showing a Ramsey-type interference pattern. Linewidths of the central valley of 0.5 mG are obtained when the distance between the two lasers is 34.5 cm. The influence of the thermal beam velocity distribution is discussed.
We investigated effects of spatially separated laser fields on amplitudes and widths of electromagnetically induced transparency in M-type atomic system.We have shown narrowing of the electromagnetically induced transmission resonance and therefore enhancement of medium non-linearities that can be explained by diffusion-induced Ramsey narrowing.Depending on the polarization of two laser beams, narrow probe Hanle transmission resonance changes its sign in respect of the profile obtained when the pump laser beam is off.
Evidence of coherence effects has been observed in the Zeeman sublevels by means of nonlinear level crossing in a metastable Ca beam with resonance width 3 orders of magnitude less than the radiative linewidth of the investigated transitions. By using two separate traveling waves, Ramsey-like fringes have been observed. The final measured widths are determined by the transit time, are weakly dependent on the magnetic-field inhomogeneities, and are independent of the natural width of the optical transitions and the laser bandwidth.
We investigate the evolution of the atomic coherence between Zeeman sublevels with $\ensuremath{\Delta}{m}_{F}=2$ in the region between pump and probe laser beams, in a Rb vapor cell using the Hanle configuration. The transmission of the probe beam, placed inside the hollow pump beam, shows the narrowing of the Hanle resonance (at external magnetic field B=0B=0B=0) by Ramsey fringes whose separation from the central peak depends on the distance between the beams. The temporal evolution of the pump induced alignment is evident from the changes of the probe transmission spectra with increasing the angle between linear polarizations of each beam. Experimental results are in agreement with the results of the model which calculates probe fluorescence by solving time-dependent optical Bloch equations, from the time when atom enters the pump beam to the time when it leaves the probe beam. The results for total excited state populations were averaged over atomic velocities distribution.
We experimentally studied the temporal evolution of Zeeman electromagnetically induced transparency (EIT) resonances induced by the laser resonant to hyperfine transition of 87Rb in a rubidium buffer gas cell. We simultaneously modulated the laser beam intensity and polarization to achieve the repeated interaction of the laser beam with coherently prepared atoms. Our cell was placed in a homogenous magnetic field to obtain the Larmor precession of the phase of coherences. The weak laser beam was used to probe the atoms at the end of the Ramsey sequence. We measured the transparency of the probe pulse at different magnetic fields for a given excitation pulse and the period of free evolution of Zeeman coherences in the dark. From these data, we reconstructed the temporal evolution of EIT resonances. The Ramsey fringes that appeared on the EIT curves at the beginning of the second probing pulse disappeared at later moments due to various decay processes.
Experimental and theoretical investigations show the strong effect of the pump beam, spatially separated from the probe beam, on the probe's electromagnetically induced absorption (EIA) and nonlinear magneto-optical rotation (NMOR). Linearly polarized pump and probe laser beams are locked to the Fg = 2 → Fe = 3 transition of the 87Rb D2 line and pass a vacuum Rb gas cell coaxially. We show that the observed narrowing of EIA and NMOR resonances is due to the Ramsey effect. Linewidths of the resonances decrease when the size of the dark region between pump and probe lasers increases. Variation of the angle between pump and probe linear polarizations strongly influences the phases of atomic coherences generated by the pump beam and consequently the line-shapes of the probe EIA and NMOR resonances. Complete change of the resonance sign is possible if the phases of the ground state coherences, Δmg = 2, are altered by π. The central EIA fringe becomes less pronounced if the probe intensity increases, due to the larger probe contribution to atomic evolution. Ramsey-like interference is a manifestation of the evolution of ground state Zeeman coherences, required for EIA, in the dark region in the presence of a small magnetic field.
We report the observation of narrow resonances in the optical region by using the method of separated oscillatory fields in an atomic beam. Because of optical pumping, two nearly degenerate optical fields create a time-dependent modulation of the ground-state population, which interferes with the optical fields in a second region, resulting in a fringe pattern. The width and shape of the observed fringes are the same as those observed with the usual Ramsey method, although no coherence between the two levels under investigation is needed.