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Chinese Physics B

Intrinsic transverse relaxation mechanisms of polarized alkali atoms enclosed in radio-frequency magnetometer cell*

Yang-Ying Fu, Jie Yuan

Published Jul 24, 2019• 8 citations
Publisher

Abstract

The intrinsic transverse relaxation mechanisms of polarized alkali atoms enclosed in the radio-frequency magnetometer cell are investigated. The intrinsic transverse relaxation rate of cesium atoms as a function of cell temperature is obtained. The absorption of alkali atoms by the glass wall and the reservoir effect are the main error factors which contribute to the disagreements between theory and experiments. A modified relaxation model is presented, in which both the absorption of alkali atoms by the glass wall and the reservoir effect are included. This study provides a more accurate description of the intrinsic transverse relaxation mechanisms of polarized alkali atoms, and enlightens the optimization of the cell design.

DOI
10.1088/1674-1056/ab3442

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The intrinsic transverse relaxation mechanisms of polarized alkali atoms enclosed in the radio-frequency magnetometer cell are investigated. The intrinsic transverse relaxation rate of cesium atoms as a function of cell temperature is obtained. The absorption of alkali atoms by the glass wall and the reservoir effect are the main error factors which contribute to the disagreements between theory and experiments. A modified relaxation model is presented, in which both the absorption of alkali atoms by the glass wall and the reservoir effect are included. This study provides a more accurate description of the intrinsic transverse relaxation mechanisms of polarized alkali atoms, and enlightens the optimization of the cell design.
In order to improve the sensitivity of radio-frequency atomic magnetometer, two methods to extend the transverse relaxation time of polarized alkali atoms are investigated. One way is the application of a non-relaxing coating, another way is the introduction of buffer gas into cell. A theory relaxation model is built, and the dependences of transverse relaxation time on buffer gas pressure, laser power, cell temperature, and magnetic-field gradient are obtained. The measurements of transverse relaxation time are carried out in a cesium atomic radio-frequency magnetometer. The optical experiment conditions which yield the highest sensitivity are determined in experiments. Experiments results indicate that the optimal pressure for neon is 200 Torr. The experimental results are in agreement with the theoretical ones. An ultimate sensitivity of 100 fT/Hz1/2 of the cesium atomic radio-frequency magnetometer has been achieved. This study helps further optimization of the cell performance and the sensitivity of radio-frequency atomic magnetometer.
Reducing the transverse relaxation rate of alkali atoms is crucial to improve the sensitivity of the magnetometers. In a perturbative treatment of the effects of the magnetic-field gradients, the transverse relaxation rates of alkali atoms are predicted to be proportional to the square of the longitudinal magnetic-field gradient. The theoretical predictions are confirmed in our experiments. The magnetic-field gradients in the vapor cell are measured by experiments. The magnetic-field gradient in the cell is suppressed drastically through compensating it with the compensation coils. Experimental results show that suppressing the magnetic-field gradient can reduce the transverse relaxation rate of alkali atoms significantly. This study contributes to the development of ultra-high sensitivity alkali atomic magnetometers.
The magnetic-resonance lines of alkali atoms are broadened considerably by the spin-exchange collisions for a radio-frequency magnetometer operating at high temperature. The resonance linewidths of cesium atoms are derived by solving the relaxation equations. When spin-exchange relaxation dominates, a light narrowing effect is predicted. For the cesium atomic radio-frequency magnetometer operates at high temperature, a remarkable narrowing of the cesium magnetic-resonance lines are observed by increasing the pump power. The Cs-Cs spin-exchange relaxation is partially suppressed by light narrowing. This study helps to expand the applications of radio-frequency magnetometers in the high-sensitivity radio-frequency magnetic-field detection.
The interaction of spin polarized cesium atoms on coated surfaces is examined. Coated surfaces find extensive applications in atomic devices-hydrogen masers and advanced rubidium frequency standards utilize wall coatings to obtain very narrow hyperfine resonances. A Cs atomic beam is hyperfine or spin polarized by optical pumping techniques using a single mode AlGaAs diode laser. The ground state distribution is probed before and after scattering from coated surfaces using laser induced fluorescence spectroscopy. The coatings under investigations are paraffin and organosilanes and these are known to be very weakly relaxing surfaces for polarized alkali atoms. The spin relaxation properties of paraffin and organosilane coatings have been studied in glass cell-vapor environment. In our experiments using polarized atomic beams, our goal is to examine in detail the hyperfine and spin relaxation properties of these coatings in a carefully controlled environment which is not possible in the gas cell environment. We measure the spin relaxation probability per wall collision as a function of temperature of the surface. Results of our studies will be presented.
Traditional practice of suppressing low-frequency noise in atomic magnetometers is mainly through the application of magnetic field modulation, while magnetic fields of this kind introduce extra transverse relaxation, which results in a decrease of sensitivity. In this study, a novel method of measuring relaxation rate induced by spin-destruction, wall collisions and modulation magnetic fields is proposed. The optical pumping model is studied both in optical-thin and optical-thick vapor cell through the analysis of total relaxation, thus the optical pumping rate can be separated from total relaxation by analyzing the optical depth. Furthermore, based on the analysis of optical pumping rate and relaxation rate, the polarization of the electron ensemble was evaluated. A compact magnetometer with single-beam configuration is developed for the proof of concept, and the experimental results show good agreements with theoretical calculation. Our method provides a simple and feasible approach to in-situ measurement of transverse relaxation rate and optical pumping rate with physical structure of the device untouched, which is especially advantageous for designing and optimizing compact atomic magnetometers.
The impact of radio-frequency magnetic field on the cesium atomic magnetometer is investigated. A theoretical model based on Bloch equation is provided, which predicts the properties of magnetometer. A cesium atomic magnetometer is designed for the experimental measurements. The magnetometer linewidth and sensitivity, as a function of radio-frequency magnetic field size are obtained. The experimental results are in agreement with what predicted by the theoretical model. The results, both theoretical and experimental included, indicate that there is an optimal radio-frequency magnetic field size, at which value the magnetometer sensitivity is highest. This work enlightens the parameter optimization of atomic magnetometer, and helps for the enhancement of magnetometer sensitivity.