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Chinese Physics B

Influence of pump intensity on atomic spin relaxation in a vapor cell*

Yang Chen, Guan-Hua Zuo, Zhuangzhuang Tian, Yuchi Zhang, Tiancai Zhang

Published Oct 11, 2019• 2 citations
Publisher

Abstract

Atomic spin relaxation in a vapor cell, which can be characterized by the magnetic resonance linewidth (MRL), is an important parameter that eventually determines the sensitivity of an atomic magnetometer. In this paper, we have extensively studied how the pump intensity affects the spin relaxation. The experiment is performed with a cesium vapor cell, and the influence of the pump intensity on MRL is measured at room temperature at zero-field resonance. A simple model with five atomic levels of a Λ-like configuration is discussed theoretically, which can be used to represent the experimental process approximately, and the experimental results can be explained to some extent. Both the experimental and the theoretical results show a nonlinear broadening of the MRL when the pump intensity is increasing. The work helps to understand the mechanism of pump induced atomic spin relaxation in the atomic magnetometers.

DOI
10.1088/1674-1056/ab4cde

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Atomic spin relaxation in a vapor cell, which can be characterized by the magnetic resonance linewidth (MRL), is an important parameter that eventually determines the sensitivity of an atomic magnetometer. In this paper, we have extensively studied how the pump intensity affects the spin relaxation. The experiment is performed with a cesium vapor cell, and the influence of the pump intensity on MRL is measured at room temperature at zero-field resonance. A simple model with five atomic levels of a Λ-like configuration is discussed theoretically, which can be used to represent the experimental process approximately, and the experimental results can be explained to some extent. Both the experimental and the theoretical results show a nonlinear broadening of the MRL when the pump intensity is increasing. The work helps to understand the mechanism of pump induced atomic spin relaxation in the atomic magnetometers.
One of the peculiar phenomenons in non-zero magnetic resonance magnetometer is that, with the increase of the temperature, the magnetic resonance linewidth is narrowed at first instead of broadened due to the increasing collision rate. The magnetometer usually operates at the narrowest linewidth temperature to obtain the best sensitivity. Here, we explain this phenomenon quantitatively considering the nonlinear of the optical pumping in the cell and did experiments to verify this explanation. The magnetic resonance linewidth is measured using one amplitude-modulated pump laser and one continuous probe laser. The field is along the direction orthogonal to the plane of pump and probe beams. We change the temperature from 53 °C to 93 °C and the pumping light from 0.1 mW to 2 mW. The experimental results agree well with the theoretical calculations.
Nowadays, quantum sensor research is a very interesting and popular topic. High-sensitivity magnetic field measurements using quantum sensors have evolved into the most sensitive magnetic field sensor category today with the invention of atomic magnetometers operating in the spin-exchange relaxation-free (SERF) regime. This letter was prepared to investigate the magnetic field sensitivity changes depending on the buffer gas, temperature, and laser intensity using indigenously produced atomic vapor cells. In addition, the aim was to determine the optimum operating conditions of the atomic magnetometer operating in the SERF regime. As a result, the resonance linewidth and signal-to-noise ratio were measured, and the minimum detectable magnetic field was calculated. According to the obtained results, the lowest magnetic field for Cells I–III was measured as 5.12, 4.21, and 4.81 pT, respectively. The lowest magnetic field of all conditions was obtained in vapor cell (Cell-II) at 100 °C with 5 mW laser intensity. As a result, it has been experimentally proven that under the same conditions, increasing the buffer gas pressure in vapor cells can narrow the resonance linewidth, and the magnetic field sensitivity can be improved by increasing the laser intensity applied to the vapor cells.
Field-inhomogeneity-induced relaxation of atomic spins confined in vapor cells with depolarizing walls is studied. In contrast to nuclear spins, such as noble-gas spins, which experience minimal polarization loss at cell walls, atomic spins in uncoated cells undergo randomization at the boundaries. This distinct boundary condition results in a varied dependence of the relaxation rate on the field gradient. By solving the Bloch-Torrey equation under fully depolarizing boundary conditions, we illustrate that the relaxation rate induced by field inhomogeneity is more pronounced for spins with a smaller original relaxation rate (in the absence of the inhomogeneous field). We establish an upper limit for the relaxation rate through calculations in the perturbation regime. Moreover, we connect it to the spin-exchange-relaxation-free magnetometers, demonstrating that its linewidth is most sensitive to inhomogeneous fields along the magnetometer's sensitive axis. Our theoretical result agrees with the experimental data for cells subjected to small pump power. A deviation in high input-power scenarios arises from pump field attenuation, resulting in a nonuniformly distributed light shift that behaves like an inhomogeneous magnetic field.
A new magnetometer with Cs vapor in the spin exchange relaxation free(SERF) regime is described.Cs has a higher saturated vapor pressure than other alkali metals;so cesium SERF magnetometer is particularly attractive in applications that require lower power dissipation.Four factors affect atomic transverse relaxations in the SERF magnetometer,including collisions and diffusions among Cs,N2 and He,laser pumping and detecting processes;the buffer gas pressure of He that affects the relaxation is calculated and discussed emphatically in theory at 105 ℃,the result shows that the optimum buffer gas pressure is 350×133.3 Pa.Under the optimum pressure,the polarized spectrum linewidth as the function of the cell temperature is discussed briefly.It is found the least linewidth is only 3.7 Hz corresponding to the electric spin relaxation rate of 278 s-1 at 105 ℃.The result provides a help to increase the sensitivity of the magnetometer with Cs vapor.
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.
In this work, we experimentally measure the broadened linewidth of magneto-optical resonance signal due to magnetic-field gradient with an alkali buffer-gas-free anti-relaxation-coated cell. We also study how the temperature of the atomic vapor cell affects the magneto-optical resonance signal linewidth under different magnetic-field gradients. Theoretical calculations show that the relaxation caused by magnetic-field gradient could be suppressed with increased temperature, which could be useful to optimize the working parameters for unshielded atomic magnetometer based on anti-relaxation-coated cells.