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Physical Review A

Dark resonances with variable Doppler sensitivity

Christian Bolkart, Danijela Rostohar, Martin Weitz

Published Apr 19, 2005• 9 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.71.043816

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Optics and Spectroscopy•Oct 1, 2020•2 citations
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−3rad{10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\mathrm{rad}10\ensuremath−3rad, 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.
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.
A gas of three-level atoms with Λ configuration of energy levels was taken as an example to demonstrate that the influence of particle motion on the two-photon resonances extends further than the residual Doppler shift ( k 1 − k 2 ) v . In particular, a narrow dip in the absorption spectrum (“dark” resonance) undergoes substantial narrowing, as compared to the atoms at rest. The width of this resonance is studied nonperturbatively as a function of the intensities of probe and strong fields.
The rubidium $5S\text{\ensuremath{-}}5D$ Doppler-free two-photon transition has been observed in a thermal vapor cell by detecting the infrared radiation emitted during the decay from the 5D5D5D excited level to the 5P5P5P intermediate level. Different to the usual detection scheme based on blue light emission this approach does not suffer from reabsorption and still works at high densities. This not only allows for a dramatically enhanced detectable emission rate but also for investigating collisional effects such as collisional broadening and collisional energy transfer. Furthermore, we observe a yet unknown temperature dependence of the resonance linewidth for transitions into the states of the 5D5/25{D}_{\text{5/2}}5D5/2​ level. Consequences for the design of an optical Rb-frequency standard are discussed.
As a rule the thermal motion removes the dark resonance from the spectrum of a three-level system driven by two strong fields. An example has been found where, conversely, the dark resonance manifests itself even after averaging over the velocities. The four-level system is studied with a closed cycle of four resonant fields, when two relatively strong fields interact with opposite transitions. If Rabi frequencies exceed relaxation constants, then the frequency dependence of the nonlinear susceptibility consists of several narrow peaks. Each peak corresponds to a ‘turning point’, the resonant frequency with zero derivative with respect to the velocity. It is shown that two peaks disappear when non-absorbing states coincide with the turning points. The coincidence occurs for equal Rabi frequencies of relatively weak fields.
Abstract The effect of electromagnetically induced transparency (EIT) in a three-level ladder system 5S1/2‒5P3/2 –5D5/2 of Rb atoms is investigated. The effect of spectral narrowing of a dark resonance (DR) as a function of atomic vapor density and thickness L of a spectroscopic cell containing Rb vapor is demonstrated. Thickness L was varied in the range of 390 nm to 4 mm, while atomic density N was increased up to ~1016 cm–3; in the process, high-intensity coupling radiation and weak probe radiation were used. The strongest effect, namely, a 22-fold spectral narrowing of the DR, was achieved in a cell of thickness L = 4 mm. The effect of spectral narrowing was decreasing with decrease in thickness L: a 2.4-fold spectral narrowing of the DR was observed at L = 2 μm. Spectral narrowing was nearly absent at L = 0.8 μm, and spectral broadening of the DR was observed with further decrease in the cell thickness down to L = 0.4 μm. A close to 100% DR contrast was achieved in nearly all cases at moderate atomic densities and high intensities of the coupling radiation. An interpretation of the effect of spectral narrowing and broadening of the DR is proposed.