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Physical Review A

Buffer-gas effects on dark resonances: Theory and experiment

Michael Erhard, Hanspeter Helm

Published Mar 15, 2001• 125 citations
Publisher

Abstract

Dark resonances with widths below 30 Hz have been observed in a rubidium cell filled with neon as buffer gas at room temperature. We compare an approximate analytic solution of a \ensuremath{\Lambda} system to our data and show that under our experimental conditions the presence of the buffer gas reduces the power broadening of the dark resonances by two orders of magnitude. We also present numerical calculations that take into account the thermal motion and velocity-changing collisions with the buffer-gas atoms. The resulting dark-resonance features exhibit strong Dicke-type narrowing effects and thereby explain the elimination of Doppler shifts and Doppler broadening, leading to observation of a single ultranarrow dark line.

DOI
10.1103/physreva.63.043813

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Dark resonances with widths below 30 Hz have been observed in a rubidium cell filled with neon as buffer gas at room temperature. We compare an approximate analytic solution of a \ensuremath{\Lambda} system to our data and show that under our experimental conditions the presence of the buffer gas reduces the power broadening of the dark resonances by two orders of magnitude. We also present numerical calculations that take into account the thermal motion and velocity-changing collisions with the buffer-gas atoms. The resulting dark-resonance features exhibit strong Dicke-type narrowing effects and thereby explain the elimination of Doppler shifts and Doppler broadening, leading to observation of a single ultranarrow dark line.
We have studied dark resonances in a thermal gas of rubidium atoms using a variable angle between the two driving optical beams. The inclined angle was tuned between a Doppler-free situation (i.e., collinear beams) and near ${10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\mathrm{rad}$, yielding an effective Doppler sensitivity of a far-infrared photon. For the case of buffer gas cells, Dicke narrowing occurs. We can carefully control the transition between the Dicke regime, in which the Doppler broadening is suppressed, and the Doppler-broadened regime, in which the thermal atomic motion strongly couples to the three-level atoms.
When neon is introduced as a buffer gas the interaction time of cesium atoms in a vapor cell with resonant laser beams is drastically increased. Using a pair of phase-locked lasers we have observed linewidths as narrow as $42 $Hz for coherent dark resonances in a cesium vapor cell. We study the influence of power and pressure broadening and systematic shifts of the resonance frequency. Our experiments demonstrate that coherent dark resonances could rival direct radio-frequency precision measurements, which have a wide range of applications in physics.
Comparison of the resonant absorption and fluorescence in the extremely thin cell (ETC) filled with pure Rb with another one filled with Rb and Neon gas with the pressures 6 Torr and 20 Torr is provided. The effect of the collapse and revival of Dicke-type narrowing is still observable for the thickness L=λ/2, and L=λ, where λ is a resonant laser wavelength 794 nm (D1 line of the Rb), i.e. the spectra of the resonant absorption and fluorescence demonstrate the sub-Doppler narrowing for the thickness L=λ/2 and broadening for the thickness L=λ. It is important to note, that in an ordinary Rb cell with the thickness L in the range of 1mm- 10 cm filled with buffer gas, with a pressure higher than 1 Torr in the Saturation Absorption scheme, any sub-Doppler features are absent. For L= λ the so called velocity selective optical pumping (VSOP) resonance located at the atomic transition is still observable when Ne pressure is 6 Torr , while the linewidth is increased by 4 times in comparison with that obtained in the pure Rb ETC. This VSOP resonance is wash out when Ne pressure is 20 Torr. It is demonstrated that use of the fluorescence spectra at L=λ/2 allows one to use the ETC for the buffer gas pressure measurement. Comparison with the theory is presented.
We observe a narrow, isolated, two-photon absorption resonance in 87Rb for large one-photon detuning in the presence of a buffer gas. In the absence of a buffer gas, a standard Λ configuration of two laser frequencies gives rise to electromagnetically induced transparency (EIT) for all values of one-photon detuning throughout the inhomogeneously (Doppler) broadened line. However, when a buffer gas is added and the one-photon detuning is comparable to or greater than the Doppler width, an absorption resonance appears instead of the usual EIT resonance. We also observe a large negative group delay (≈−300 μs for a Gaussian pulse that propagates through the media with respect to a reference pulse not affected by the media), corresponding to a superluminal group velocity vg=−c/(3.6×106)=−84 m/s.