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Physical Review A

Power broadening and Doppler effects of coherent dark resonances in Rb

Michael Erhard, S. Nußmann, Hanspeter Helm

Published Nov 3, 2000• 27 citations
Publisher

Abstract

Using a phase-locked laser pair we have observed dark resonances with linewidths below 30 Hz in a rubidium cell filled with neon as buffer gas. A model allowing for pressure broadening correctly reproduces the dependence of the width on the laser intensity. Consideration of velocity changing collisions reveals the absence of Doppler effects in the position and width of the dark resonance at high buffer-gas pressure.

DOI
10.1103/physreva.62.061802

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Using a phase-locked laser pair we have observed dark resonances with linewidths below 30 Hz in a rubidium cell filled with neon as buffer gas. A model allowing for pressure broadening correctly reproduces the dependence of the width on the laser intensity. Consideration of velocity changing collisions reveals the absence of Doppler effects in the position and width of the dark resonance at high buffer-gas pressure.
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.
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 measure the signal amplitude and linewidth of a dark line in coherent population trapping in the Rb vapour cell filled with mixed buffer gas N 2 and Ar as a function of cell temperature. We find that the dark line signal amplitude increases with temperature up to a maximum at 49°C and then drops at higher temperatures due to quenching effects of N 2 . The linewidth of the dark line remains basically constant, at 1080 Hz. We also measure the linewidth of the dark line as a function of laser intensity. The linewidth increases linearly with laser intensity. An intrinsic linewidth (FWHM=896 Hz at 3.4 GHz) of the Rb cell is obtained.
The coherent dark resonance between the hyperfine levels F=1, m F =0 and F=2, m F =0 of the rubidium ground state has been observed experimentally with the light of a pulsed mode-locked diode laser operating at the D1 transition frequency. The resonance occurs whenever the pulse repetition frequency matches an integer fraction of the rubidium 87 ground state hyperfine splitting of 6.8 GHz. Spectra have been taken by varying the pulse repetition frequency. Using cells with argon as a buffer gas a linewidth as narrow as 149 Hz was obtained. The rubidium ground state decoherence cross section for collisions with xenon atoms has been measured for the first time with this method using a pure isotope rubidium vapor cell and xenon as a buffer gas.
We present measurements of dark-line resonances excited in cesium atoms confined in submillimeter cells with a buffer gas. The width and contrast of the resonances were measured for cell lengths as low as 100 microm. The measured atomic Q factors are reduced in small cells because of frequent collisions of atoms with the cell walls. However, the contrast of coherent population trapping resonances measured in the small cells is similar in magnitude to that obtained in centimeter-sized cells, but substantially more laser intensity is needed to excite the resonance fully when increased buffer-gas pressure is used. The effect of the higher intensity on the linewidth is reduced because the intensity broadening rate decreases with buffer-gas pressure.