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arXiv:2205.04925 · Physical Review A

Level-crossing resonances on open atomic transitions in a buffered Cs vapor cell: Linewidth narrowing, high contrast, and atomic magnetometry applications

Denis V. Brazhnikov, V. I. Vishnyakov, Andrei Goncharov, Emilia A. Alipieva, C. Andreeva, Elena T. Taskova

Published Jul 26, 2022• 12 citations
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Abstract

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 ${D}_{1}$ 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 $\mathrm{pT}/\sqrt{\mathrm{Hz}}$ with a sensitivity of 60 $\mathrm{fT}/\sqrt{\mathrm{Hz}}$ 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.

DOI
10.1103/physreva.106.013113

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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\ensuremath∘C150{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}150\ensuremath∘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 (\ensuremathλ\ensuremath≈894.5\ensuremath{\lambda}\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}894.5\ensuremathλ\ensuremath≈894.5 nm) due to their overlap in the presence of a buffer gas (130 Torr neon). Using a small (V\ensuremath≈0.1cm3V\ensuremath{\approx}0.1\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{3}V\ensuremath≈0.1cm3) 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\ensuremath∘C60{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}60\ensuremath∘C) operation due to strong light-induced circular dichroism in the medium. Based on the \ensuremathΛ\mathrm{\ensuremath{\Lambda}}\ensuremathΛ 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.
Level-crossing (LC) resonances in alkali-metal vapors are widely used for atomic magnetometry. One of the most simple and robust techniques involves a single circularly polarized light wave, while a transverse magnetic field (Bx⊥k) is scanned around zero to observe the subnatural-linewidth resonance of electromagnetically induced transparency (EIT) in the light wave transmission. This technique allows miniaturization of the magnetic field sensor to a great extent, maintaining high sensitivity of measurements. To obtain a high quality factor of the LC resonance and, therefore, high performance of the sensor, either a high temperature (>120 °C) or an extended volume of the vapor cell (V≫1 cm3) is usually required. Here, we propose a slight modification to the commonly used configuration, which can provide high-quality LC resonances in small (≪1 cm3) vapor cells at a relatively low temperature of 60 °C or less. The modification consists in adding the second (pump) counterpropagating light wave with opposite circular polarization (σ+σ− configuration). In our experiments, the waves excite the D1-line ground-state level Fg=4 in cesium atoms in the presence of a buffer gas (Ne, 130 Torr). In the proposed scheme, a subnatural-linewidth electromagnetically induced absorption (EIA) resonance is observed. We compare parameters of the EIA resonance with those obtained in the single-wave scheme to show benefits of using the proposed σ+σ− configuration. The results have good prospects for developing a low-power miniaturized atomic magnetometer with a wide operating range.
Abstract Zero-field level-crossing resonances (LCRs) underlay one of the most robust and simplest techniques in atomic magnetometry. LCR-based miniaturized magnetic field sensors have already found relevant applications in biology and medicine. Such sensors utilize a single circularly polarized light wave to interact with alkali-metal atoms (usually Rb or Cs) and to observe the LCR in a vapor-cell transmission when a transverse magnetic field is scanned around zero value. A high-temperature regime (T ≈ 120 − 160°C) is required to achieve a desirable sensitivity of measurements. It can be a problem for some applications, especially in a multi-channel mode of operation. Here, we consider two spectroscopy techniques that can provide high-contrast and narrow LCRs under a relatively low temperature of the cell (≤60°C). These techniques imply using two light waves: the pump wave to polarize the atoms and the probe one to register the resonance. A cubic glass cell of 5×5×5 mm3 size is used in the experiments. It is filled with cesium vapors and neon as a buffer gas. The results can be used for developing a miniaturized low-power high-sensitivity magnetic field sensor for biomedical applications.
We investigate the ground-state Hanle effect in alkali-metal vapor irradiating by a resonant elliptically polarized light wave. The magneto-optical resonances are observed as a change in the ellipticity parameter of the light wave polarization when scanning the transverse magnetic field near zero. We use a miniature (≈ \approx\,≈0.1250.1250.125 cm3^33) glass cesium vapor cell heated to a relatively low temperature of ≈ \approx\,≈85∘85^\circ85∘C. Under the current experimental conditions, the sensitivity of magnetic field measurements is limited by a technical noise, reaching 180180180 fT/√\surd√Hz in a 200200200 Hz bandwidth. The ultimate photon-shot-noise-limited sensitivity of the method is estimated to be ≈ \approx\,≈555 fT/√\surd√Hz. The proposed scheme is promising for the development of a zero-field atomic magnetometer with reduced heat dissipation of the sensor head and relaxed requirements for magnetic shielding compared to counterparts operating in the spin-exchange relaxation-free regime. These features are of particular value for biomedical applications.
Level-crossing (LC) resonances in a buffer-gas-filled cesium vapor cell are studied under counterpropagating pump and probe light waves with opposite circular polarizations. The waves excite the D1_11​-line ground-state level FgF_gFg​= =\,=444, while a transverse magnetic field (Bx{\rm B}_xBx​⊥\perp⊥k{\rm k}k) is scanned around zero to observe the resonance of electromagnetically induced absorption (EIA). It is shown that adding the pump light wave significantly improves the properties of the resonances in comparison with the commonly used scheme with a single light wave. As far as a small vapor cell ($\approx\,0.1cm0.1 cm0.1cm^3)atrelativelylowtemperature() at relatively low temperature ()atrelativelylowtemperature(\approx\,45−6045-6045−60\,^\circ$C) is utilized, the results have good prospects for developing a low-power miniaturized atomic magnetometer.
We describe a Cs atomic magnetometer operating in the spin-exchange-relaxation-free (SERF) regime. With a vapor cell temperature of $103\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ we achieve intrinsic magnetic resonance widths $\ensuremath{\Delta}B=17\text{ }\ensuremath{\mu}\text{G}$ corresponding to an electron spin-relaxation rate of $300\text{ }{\text{s}}^{\ensuremath{-}1}$ when the spin-exchange rate is ${\ensuremath{\Gamma}}_{\text{SE}}=14\text{ }000\text{ }{\text{s}}^{\ensuremath{-}1}$. We also observe an interesting narrowing effect due to diffusion. Signal-to-noise measurements yield a sensitivity of about 400 pG/Hz400\text{ }\text{pG}/\sqrt{\text{Hz}}400 pG/Hz​. Based on photon shot noise, we project a sensitivity of 40 pG/Hz40\text{ }\text{pG}/\sqrt{\text{Hz}}40 pG/Hz​. A theoretical optimization of the magnetometer indicates sensitivities on the order of 2 pG/Hz2\text{ }\text{pG}/\sqrt{\text{Hz}}2 pG/Hz​ should be achievable in a 1 cm31\text{ }{\text{cm}}^{3}1 cm3 volume. Because Cs has a higher saturated vapor pressure than other alkali metals, SERF magnetometers using Cs atoms are particularly attractive in applications requiring lower temperatures.
A new configuration for observation of magneto-optical subnatural-linewidth resonances of electromagnetically induced absorption (EIA) in alkali vapor has been verified experimentally. The configuration includes using two counter-propagating pump and probe light waves with mutually orthogonal linear polarizations, exciting an open optical transition of an alkali atom in the presence of a buffer gas. The main advantage of the novel observation scheme consists in the possibility of obtaining simultaneously high-contrast and quite narrow nonlinear signals. Here a 2.5-cm long rubidium-87 vapor cell filled with Ar buffer gas is used, and the excited optical transition is the Fg_gg​=2 →\to→ Fe_ee​=1 of the D1_11​ line. The signals registered reach a contrast of 57.7% with a FWHM of 7.2 mG. The contrast with respect to a wide Doppler pedestal well exceeds 100%. To our knowledge, to date this is the best result for EIA resonances in terms of contrast-to-width ratio. In general, the results demonstrate that the new magneto-optical scheme has very good prospects for various applications in quantum metrology, nonlinear optics and photonics.