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Optoelectronics Instrumentation and Data Processing

Magnetooptical resonances in cells with residual buffer gas in elliptical polarization of radiation

K. A. Nasyrov, S. Cartaleva

Published Jul 1, 2012• 5 citations
Publisher

Abstract

The dependence of the fluorescence intensity of Cs vapor irradiated by elliptically polarized light resonant with the F = 4 → F = 5 transition on the scanned magnetic field has a complex shape — a narrow dip against the background of a broad peak. It is found experimentally that the dip is due to the small component of the magnetic field orthogonal to the direction of the laser beam. A two-level atom model with degenerate angular-momentum states predicts an order of magnitude lower contrast of this dip. The experimentally observed contrast of the dip can be explained by assuming that due to the low buffer-gas pressure in the optical cell, part of the resonant atoms is able to return to the laser beam without thermalized collisions with the wall of the cell, increasing the contrast. In addition, accounting for all optical transitions of the D 2-line and the optical pumping of the hyperfine-structure components of the ground state is a fundamental point for the interpretation of the experimental results.

DOI
10.3103/s8756699012040097

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The dependence of the fluorescence intensity of Cs vapor irradiated by elliptically polarized light resonant with the F = 4 → F = 5 transition on the scanned magnetic field has a complex shape — a narrow dip against the background of a broad peak. It is found experimentally that the dip is due to the small component of the magnetic field orthogonal to the direction of the laser beam. A two-level atom model with degenerate angular-momentum states predicts an order of magnitude lower contrast of this dip. The experimentally observed contrast of the dip can be explained by assuming that due to the low buffer-gas pressure in the optical cell, part of the resonant atoms is able to return to the laser beam without thermalized collisions with the wall of the cell, increasing the contrast. In addition, accounting for all optical transitions of the D 2-line and the optical pumping of the hyperfine-structure components of the ground state is a fundamental point for the interpretation of the experimental results.
In the present communication we report on the investigation of the effect of elliptically polarized laser light exciting the Fg=3 Cs D2 absorption line. Coherent resonances in Cs have been studied, obtained in Hanle configuration by scanning of magnetic field parallel to the light propagation direction. The resonances were investigated depending on the polarization of the irradiating light field. It has been observed that for linear polarization dark resonances in the fluorescence are registered, while for circularly polarized light, bright resonances are obtained, probably due to stray orthogonal magnetic field. For elliptical polarization a narrow peak appears superimposed on a broader dip. Theoretical description has been made, which shows that at elliptical polarization and orthogonal magnetic field in the case of weak light fields, a single peak in the excited state population is observed, while strong fields lead to the splitting of this peak into two peaks. The presence of both weak and strong light fields, probably due to reflection of the light beam on the cell walls, could lead to the experimentally observed fluorescence behaviour.
It is predicted theoretically that the combination of elliptically polarized radiation and an antirelaxation coating of the walls of a vacuum cell containing alkali metal vapor provides an asymmetric shape of the magneto-optical resonance. The cause of the asymmetry is the optical pumping of the ground-state hyperfine component which is not destroyed upon collision of the atom with the wall. The effect is most pronounced when the laser frequency is tuned to the resonance with the the F → F + 1 transition.
The ground-state Hanle effect (GSHE) in alkali-metal atomic vapor using a single circularly polarized light wave underlies one of the most reliable and simple techniques of modern atomic magnetometry. This effect causes a narrow (subnatural-width) resonance in the light wave intensity passing through the vapor cell. The GSHE-based sensors typically operate in the so-called spin-exchange relaxation-free (SERF) regime to reduce the resonance linewidth. However, this regime requires a relatively high temperature of vapors (approximately equal to $150{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$), leading to high heat release and power consumption of the sensor head. In addition, without applying special measures, the SERF regime significantly limits the dynamic range of measurements. Here we study a pump-probe scheme involving a single elliptically polarized light wave and a polarimetric detection technique. The wave is in resonance with two adjacent optical transitions in the cesium D1{D}_{1}D1​ line ($\ensuremath{\lambda}\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}894.5$ nm) due to their overlap in the presence of a buffer gas (130 Torr neon). Using a small ($V\ensuremath{\approx}0.1\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{3}$) glass vapor cell, we demonstrate the possibility to observe subnatural-width resonances with a high contrast-to-width ratio (up to 45%/mG) in low-temperature ($60{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$) operation due to strong light-induced circular dichroism in the medium. Based on the $\mathrm{\ensuremath{\Lambda}}$ scheme of atomic energy levels, we obtain explicit analytical expressions for the shape of the resonance line. The model reveals a linewidth narrowing effect due to openness of the level scheme. The observed bright features are unusual for magneto-optical atomic spectroscopy as openness is commonly considered to be an undesirable effect that degrades resonance characteristics. Measuring the noise voltage, we estimate the sensitivity of the magnetic-field measurements to be 1.8 pT/Hz\mathrm{pT}/\sqrt{\mathrm{Hz}}pT/Hz​ with a sensitivity of 60 fT/Hz\mathrm{fT}/\sqrt{\mathrm{Hz}}fT/Hz​ in the photon-shot-noise limit. In general, the results contribute to the theory of GSHE resonances and also can be applied to development of a low-temperature high-sensitivity miniaturized magnetic-field sensor with an extended dynamic range.
We have measured changes in the ground-state populations of Cs vapor induced by optical pumping at high magnetic field. The 2.72.72.7-T field of our experiments is strong enough to decouple the nuclear and electronic spins, allowing us to independently measure each population. The spatial dependence of the Cs populations in small amounts of buffer gas obeys a simple coupled diffusion model and the relative populations reveal the details of relaxation within the vapor cell. Optical pumping can produce high nuclear polarization in the Cs vapor due to perturbations of the hyperfine interaction during collisions with buffer-gas particles and depending on the pumping transition, radiation trapping can strongly influence the electronic and nuclear polarizations in the vapor.
Coherent spectroscopy of degenerate two-level systems of the Doppler-broadened D2{D}_{2}D2​ cesium line has been performed in vacuum and in buffer gas cells. Subnatural-width (SNW) electromagnetically induced transparency and electromagnetically induced absorption (EIA) resonances have been observed with single-frequency laser excitation for different light polarizations and collision regimes. SNW-EIA resonances with linearly polarized excitation of the Fg=4{F}_{g}=4Fg​=4 level are observed, and they increase their contrast with power density increase and reach a contrast of 20% at $200 \mathrm{mW}{\mathrm{cm}}^{\ensuremath{-}2}.$ A theoretical model has been elaborated, taking into account the Doppler broadening of the transitions, the linewidth of the laser light, and the experimental conditions for observation of the fluorescence. Here the theoretical results are in agreement with those of the experiment. In a buffer gas cell, the SNW resonances in the fluorescence, in degenerate two-level systems are observed for the first time and they are only obtained with circularly polarized light. A theoretical explanation of this result is proposed. At the expense of contrast, in the presence of a buffer gas the coherent resonances are significantly narrower than in a vacuum cell for the same power density. The observation of SNW resonances in buffer gas is significantly less sensitive to the laser frequency drift than in a vacuum cell, which could be advantageous for applications in metrology and in the measurement of weak magnetic fields.
In a weak magnetic field coherent dark resonances in cesium vapor are split into up to 15 resolved components, depending on field direction and laser polarizations. We find that the selection rules are different for vapor cells with and without buffer gas due to a change in multipolarity of the two-photon coupling. At low laser intensities or sufficiently high buffer-gas pressure optical pumping between different dark resonances can be neglected so that a simple model allows one to calculate the relative line strengths, giving complete agreement with the experimental spectra.