Alkali Vapor Cells Research Articles

Wafer-scale fabrication of evacuated alkali vapor cells.

We describe a process for fabricating a wafer-scale array of alkali metal vapor cells with low residual gas pressure. We show that by etching long, thin channels between the cells on the Si wafer surface, the residual gas pressure in the evacuated vapor cell can be reduced to below 0.5 kPa (4 Torr) with a yield above 50%. The low residual gas pressure in these mass-producible alkali vapor cells can enable a new generation of low-cost chip-scale atomic devices such as vapor cell optical clocks, wavelength references, and Rydberg sensors.

Identification and Manipulation of Atomic Polarization Moments for Nonlinear Magneto‐Optical Rotation Atomic Magnetometers

AbstractPolarization moments play a crucial role in measuring magnetic fields for nonlinear magneto‐optical rotation (NMOR) atomic magnetometers. However, it is challenging to distinguish between each polarization moment and evaluate its effect on the magnetic resonance response signal in an alkali vapor cell with buffer gas. To address this issue, a method is proposed to identify different polarization moments through the frequency shift of the magnetic resonance response signal. The proportion of each polarization moment is determined, and it is demonstrated that the magnetic resonance response signal is affected by the hexadecapole moment, resulting in a frequency shift and a decrease in signal amplitude. To mitigate this effect, an approach is investigated to manipulate the polarization moments by flipping the phase of the pump light. Ultimately, a 15.19% increase in response amplitude is achieved in the simulated geomagnetic environment within the magnetic shield barrel. The theory and method presented here provide strong support for the study of the polarization moments in an alkali vapor cell with buffer gas, which potentially enhance the performance of NMOR atomic magnetometers.

Comparative analysis of light storage in antirelaxation-coated and buffer-gas-filled alkali vapor cells

We explore light storage in antirelaxation-coated and buffer-gas-filled alkali vapor cells, employing electromagnetically induced transparency (EIT) in warm rubidium vapor. We conduct a comparative study of light storage performance under identical experimental conditions for these two cell types. Using a buffer-gas-filled cell resulted in approximately a tenfold improvement in memory efficiency and storage time compared to antirelaxation-coated cells. Moreover, we demonstrate that memory efficiency can be further enhanced by choosing a near-resonant EIT Λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Lambda$$\\end{document}-scheme over a resonant one. Our findings provide valuable insights for optimizing light storage, thereby contributing to the development of field-deployable quantum memories.

Method to quickly estimate T1 value by suppressing spin exchange relaxation and magnetic field gradient relaxation in atomic sensors

The accuracy of atomic sensor is determined by the transverse nuclear spin relaxation time (T2). As the longitudinal nuclear spin relaxation time (T1) is the upper limit of T2, it becomes an important metric for evaluating the merits of alkali vapor cells. However, the conventional method for measuring T1 of pulse delay takes dozens of minutes, which will increase the effect of random errors during the testing process. In order to solve this problem, a method to quickly estimate the value of T1 is proposed by suppressing the influence of spin exchange relaxation and magnetic field gradient relaxation terms on T2. Experimental results indicate that the accuracy is maintained at above 95%, and the testing time is reduced to less than 5%.

Integration of Passivated Gold Mirrors into Microfabricated Alkali Vapor Cells

Measurements of weak magnetic fields demand a small distance between the sensor and the to-be-measured object. Optically pumped magnetometers (OPMs) utilize laser light and the Zeeman effect in alkali vapor cells to measure those fields. OPMs can be used in transmission or reflection geometry. A minimization of the distance between active volume and magnetized source calls for reflection geometry with integrated mirrors. Unfortunately, cesium reacts chemically with most materials, especially high-performing materials, such as gold. Herein, we show the first functional OPM cell using a gold mirror inside the cell. We fabricated the gold mirrors with and without a passivation layer in order to evaluate the feasibility of expanding on the limited list of possible mirror materials. A comparison of this implementation revealed that mirrors without a passivation layer only reach a reflectivity of about 6% while mirrors with a passivation layer retain reflectivity values of about 90% in the visible light to near-infrared spectrum. This result and the proof of elemental cesium in the alkali vapor cell demonstrates the feasibility of passivated gold mirrors for applications in alkali vapor cells for OPMs.

Suppression of thermal coupling noise in the SERF atomic co-magnetometer

Heating the alkali metal vapor cell to obtain high atomic density is a prerequisite for realizing the high-precision measurement for spin-exchange relaxation-free (SERF) co-magnetometer. However, the thermo-magnetic and thermo-optical coupling noise caused by the temperature diffusion from the alkali vapor cell is one of the main factors restricting the co-magnetometer performance at present. Here the influence of heating on the SERF co-magnetometer was comprehensively studied, and the thermal error model of the system was established. A novel heat insulation structure for the co-magnetometer was designed, and the effectiveness was verified through monitoring of characteristic temperature points. Compared to the conventional oven support, the average maximum temperature difference within 12 h of the optical path and magnetic shielding systems (OPMSS) decreased by 52%, the bias instability of the co-magnetometer decreased by 24%, and the noise of the co-magnetometer at 1 Hz decreased by 33%. The experimental results demonstrate that this method reduces the temperature fluctuation of the OPMSS, effectively suppresses the thermal coupling noise, and thus improves the performance of the SERF co-magnetometer. Moreover, this method is also applicable to other quantum sensors based on alkali metal vapor cell.

Reduction of helium permeation in microfabricated cells using aluminosilicate glass substrates and Al2O3 coatings

The stability and accuracy of atomic devices can be degraded by the evolution of their cell inner atmosphere. Hence, the undesired entrance or leakage of background or buffer gas, respectively, that can permeate through the cell walls, should be slowed down. In this work, we investigate helium permeation in microfabricated alkali vapor cells filled with He and whose windows are made of borosilicate glass (BSG) or aluminosilicate glass (ASG). The permeation is then derived from routine measurements of the pressure-shifted hyperfine transition frequency of an atomic clock. We first confirm that ASG reduces the He permeation rate by more than two orders of magnitude, in comparison to BSG. In addition, we demonstrate that Al2O3 thin-film coatings, known to avoid alkali consumption in vapor cells, can also significantly reduce He permeation. The permeation through BSG is thereby reduced by a factor up to 130, whereas the one through ASG is decreased by a factor up to 5.0 compared to uncoated substrates. These results may contribute to the development of miniaturized atomic clocks and sensors with improved long-term stability or sensitivity.

High-sensitivity pump–probe atomic magnetometer based on single fiber-coupled

In this paper, we demonstrate an all-optical spin-exchange relaxation-free atomic magnetometer based on optical-fiber coupling, which simultaneously transmits orthogonally coupled pump and probe beam to an alkali vapor cell through a single polarization-maintaining fiber. The extinction ratios of the two pump–probe beams propagating along the fast- and slow-axis of the polarization-maintaining fiber were measured experimentally. To achieve the best sensitivity of the magnetometer, we theoretically analyze and experimentally optimize the triaxial magnetic compensation and optical parameters. Based on this scheme, it is proved that the compensation of the background magnetic field and the acquisition of three-axis magnetic field information can be achieved using only a pair of pump–probe beams. By optimizing the bias magnetic field Bx, Bz and optical parameters, we achieved bandwidths of 85 Hz, 42 Hz, and 48 Hz and magnetometer sensitivities of 18 fT/Hz1/2, 12 fT/Hz1/2, and 26 fT/Hz1/2 in the x-, y-, and z-axis, respectively. The proposed miniaturized configuration will be beneficial to developing magnetocardiography and magnetoencephalography instrumentation based on atomic magnetometers.

Design of a Fiber Alkali Vapor Cell for Atomic Magnetometer for Magnetoencephalography Applications

Spin exchange relaxation free (SERF) atomic magnetometer (AM), based on the Larmor precession of alkali atoms, is considered a promising candidate for magnetoencephalography (MEG) systems with the advantages of high sensitivity and no need for cryogenic devices. The footprint of the sensor header contains alkali vapor cell and bulk optical elements determining the spatial resolution of the MEG system. Optical fiber could separate the vapor cell far from other parts of the sensor header to improve the spatial resolution. However, coupling between glass cell and fibers limits the coupling loss of the light. Here, we describe the design of a fiber-based alkali vapor cell that could alleviate these issues. A pair of fiber cables combining a polarization maintaining fiber (PMF) and hollow-core photonic crystal fibers (HC-PCFs) are enclosed in a vacuum-sealed T-shape glass tube filled with alkali atoms. The fiber cell ensures a flexible integration with most fiber systems. The fiber structure, with an air gap between HC-PCFs, provides a large interaction volume between light and atoms. The vapor of the alkali atoms diffuses into the air core of the HC-PCF from the glass tube by heating. The alkali atoms still contained in SERF regime are within the wall relaxation rates of 12,764 s−1 in the coating fiber cell. The insertion loss due to fiber coupling is analyzed. The coupling efficiency could be 91%, with the fiber structure consisting of a 40 μm diameter HC-PCF and a 1 mm air gap. The limit sensitivity under this condition is simulated at 14.7 fT/Hz1/2. The fabrication technique and the light insertion loss are discussed. The fiber alkali vapor cell is of compact size and has flexible integration with the fiber atomic spin precession detection system.

Cross-Axis Dynamic Field Compensation of Optically Pumped Magnetometer Arrays for MEG

We present dynamic field compensation (DFC), whereby three-axis field measurements from reference magnetometers are used to dynamically maintain null at the alkali vapor cells of an array of primary sensors that are proximal to a subject's scalp. Precision measurement of the magnetoencephalogram (MEG) by zero-field optically pumped magnetometer (OPM) sensors requires that sensor response is linear and sensor gain is constant over time. OPMs can be operated in open-loop mode, where the measured field is proportional to the output at the demodulated photodiode output, or in closed-loop, where on-board coils are dynamically driven to maintain the internal cell at zero field in the measurement direction. While OPMs can be operated in closed-loop mode along all three axes, this can increase sensor noise and poses engineering challenges. Uncompensated fluctuations in the ambient field along any statically nulled axes perturb the measured field by tipping the measurement axis and altering effective sensor gain – a phenomenon recently referred to as cross-axis projection error (CAPE). These errors are particularly problematic when OPMs are allowed to move in the remnant background field. Sensor gain-errors, if not mitigated, preclude precision measurements with OPMs operating in the presence of ambient field fluctuations within a typical MEG laboratory. In this manuscript, we present the cross-axis dynamic field compensation (DFC) method for maintaining zero field dynamically on all three axes of each sensor in an array of OPMs. Together, DFC and closed-loop operation strongly attenuate errors introduced by CAPE. This method was implemented by using three orthogonal reference sensors together with OPM electronics that permit driving each sensor's transverse field coils dynamically to maintain null field across its OPM measurement cell. These reference sensors can also be used for synthesizing 1st-gradient response to further reduce the effects of fluctuating ambient fields on measured brain activity and compensate for movement within a uniform field. We demonstrate that, using the DFC method, magnetic field measurement errors of less than 0.7% are easily achieved for an array of OPM sensors in the presence of ambient field perturbations of several nT.

Simple fabrication of miniaturized glass cells for atomic vapor spectroscopy

Recent technological applications like atomic clocks, magnetometers or gyroscopes require the miniaturization of alkali vapor cells. We propose a procedure to manufacture miniaturized glass cells by cutting them from a larger one, previously filled with the required vapor. The proof of concept was demonstrated by fabricating millimeter-size cells with pure Rb vapor. We perform measurements of the mean lifetime of the $$^{85}$$ Rb(5D $$_{5/2}$$ ) excited state to sensitively probe the presence of impurities inside the miniaturized cells. We find that the proposed cutting procedure seals the cells without compromising the Rb vapor purity. This demonstrates that the proposed method can be successfully used for vapor cell miniaturization.

Collimated versatile atomic beam source with alkali dispensers

Alkali metal dispensers have become an indispensable tool in the production of atomic vapors for magnetometry, alkali vapor cell clocks, and laser cooling experiments. A primary advantage of these dispensers is that they contain alkali metal in an inert form that can be exposed to air without hazard. However, their high temperature of operation (>600 °C) is undesirable for many applications, as it shifts the atomic speed distribution to higher values and presents a radiative heat source that can raise the temperature of its surroundings. For this reason, dispensers are typically not used in line-of-sight applications, such as atomic beam generation. In this work, we present an integrated rubidium dispenser collimating device with a thickness of only 2 mm that produces a beam of atoms traveling primarily in the forward direction. We find that the collimator plate serves to both shield the dispenser's radiation and moderate the velocity of the atomic beam so that the measured longitudinal speed distribution is comparable to that of an ordinary alkali oven at only a slightly elevated temperature of 200 °C. To confirm our theory, we also constructed another compact apparatus consisting of a dispenser and a silicon collimator and the measurements support our conclusion. Our integrated dispenser collimator will particularly be useful in integrated photonics and cavity QED on-chip, where a localized, directed source of Rb vapor in small quantities is needed.

Recent Progress on Micro-Fabricated Alkali Metal Vapor Cells.

Alkali vapor cells are the core components of atomic sensing instruments such as atomic gyroscopes, atomic magnetometers, atomic clocks, etc. Emerging integrated atomic sensing devices require high-performance miniaturized alkali vapor cells, especially micro-fabricated vapor cells. In this review, bonding methods for vapor cells of this kind are summarized in detail, including anodic bonding, sacrificial micro-channel bonding, and metal thermocompression bonding. Compared with traditional through-lighting schemes, researchers have developed novel methods for micro-fabricated vapor cells under both single- and double-beam schemes. In addition, emerging packaging methods for alkali metals in micro-fabricated vapor cells can be categorized as physical or chemical approaches. Physical methods include liquid transfer and wax pack filling. Chemical methods include the reaction of barium azide with rubidium chloride, ultraviolet light decomposition (of rubidium azide), and the high-temperature electrolysis of rubidium-rich glass. Finally, the application trend of micro-fabricated alkali vapor cells in the field of micro-scale gyroscopes, micro-scale atomic clocks, and especially micro-scale biomagnetometers is reviewed. Currently, the sensing industry has become a major driving force for the miniaturization of atomic sensing devices, and in the near future, the micro-fabricated alkali vapor cell technology of atomic sensing devices may experience extensive developments.

Coherent optical processes on Cs D2 line magnetically induced transitions

The increased spectral resolution allowed by the use of extremely thin vapor cells has led to the observation of interesting behavior of alkali transitions when placed in a magnetic field. Particularly, the probabilities of transitions obeying an apparent Fe−Fg≡ΔF=±2 selection rule, referred to as magnetically-induced (MI) transitions, largely increase in the intermediate interaction regime while being null at zero and higher magnetic fields. With an 800 nm-thick Cs vapor cell placed in a field up to 1.5 kG, we show here that the generation of electromagnetically induced transparency (EIT), realized in Λ-systems involving ΔF=−2 MI transitions, is only possible when both the coupling and probe beams are σ−-circular polarized, demonstrating that EIT is affected by magnetic circular dichroism. A similar rule of thumb can be extrapolated for ΔF=+2 MI transitions and σ+ polarization. Because of the high frequency shift slope (typ. 4 MHz/G), the generation of EIT resonances involving MI transitions is interesting, especially in the context of growing attention towards micro-machined alkali vapor cell sensors.

The Microfabricated Alkali Vapor Cell with High Hermeticity for Chip-Scale Atomic Clock

Herein, a microfabricated millimeter-level vapor alkali cell with a high hermeticity is fabricated through a wet etching and single-chip anodic bonding process. The vapor cell, containing Rb and N2, was investigated in a coherent population trapping (CPT) setup for the application of a chip-scale atomic clock (CSAC). The contrast of CPT resonance is up to 1.1% within the only 1 mm length of light interacting with atom. The effects of some critical external parameters on the CPT resonance, such as laser intensity, cell temperature, and buffer gas pressure, are thoroughly studied and optimized. The improved microfabricated vapor cell also exhibited great potential for other chip-scale atomic devices.

Single-step alkaline etching of deep silicon cavities for chip-scale atomic clock technology

This paper presents the results of experiments on the development of the technology of MEMS alkali vapor cells for a miniature quantum frequency standard. The classical design of a two-chamber silicon cell containing an optical chamber, shallow filtration channels and a technical container for a solid-state alkali source was implemented in a single-step process of wet anisotropic silicon etching. To prevent the destruction of the filtration channels during etching of the through silicon cavities, the shapes of the compensating structures at the convex corners of the silicon nitride mask were calculated and the composition of the silicon etchant was experimentally found. The experiments results were used in the manufacture of chip-scale atomic clock cells containing vapors of 87Rb or 133Cs isotopes in the neon atmosphere.

Nonlinear enhanced-absorption resonances in compact alkali-vapor cells for applications in quantum metrology

We review three laser spectroscopy schemes developed recently for observation of high-contrast enhanced-absorption nonlinear resonances in small alkali-vapor cells. In our experiments, optical transitions within the cesium D1 line are involved and a probe beam transmission is analyzed. The first spectroscopy method is based on the configuration with two-frequency counter propagating beams, which are linearly polarized in orthogonal directions. This configuration provides observation of high-contrast natural-linewidth resonances superimposed on broad Doppler profiles when the laser frequency is scanned. These resonances have good prospects for developing a miniature optical frequency reference. The second scheme involves two-frequency counter propagating beams with equal circular polarizations and provides observation of subnatural-linewidth resonances when the Raman frequency detuning is scanned. We use these resonances for stabilizing the microwave frequency of a local oscillator (≈ 4.6 GHz). Frequency stability of around 6 × l0-12 is achieved at 1-s averaging using a 5-mm length cell. This result makes the technique attractive for developing a miniature frequency standard in thr microwave range. The third configuration exploits single-frequency counter-propagating beams with linear orthogonal polarizations. The ultrahigh-contrast subnatural-linewidth resonances can be observed when the longitudinal magnetic field is scanned around zero. The possible application is discussed of these resonances in vector atomic magnetometry.

Magnetocardiography Measurements by Microfabricated Atomic Magnetometer With a 3-D Spherical Alkali Vapor Cell

In the field of biomagnetic applications, including magnetocardiography (MCG), portable magnetic measurement is promising. This article demonstrates simulated MCG measurements using a microfabricated spin-exchange relaxation-free (SERF) atomic magnetometer with a 3-D chip-scale spherical rubidium vapor cell. The vapor cell temperature is optimized from 100 °C to 160 °C, and the magnetometer response is recorded under different magnetic flux densities in the range of 10–122 pT. Then simulated MCG measurements are implemented in a magnetic shield. The original MCG signals are denoised by a two-step processing to obtain the featured waveforms of the MCG signals. The experimental result shows that the microfabricated magnetometer in the magnetic shield exhibits a sensitivity of 125 fT/Hz1/2 at 15 Hz, and the signal-to-noise ratio is increased to 48.3 after denoising. The magnetometer enabled by a 3-D chip-scale spherical rubidium vapor cell has the ability to obtain morphologically clear cardiomagnetic signals with distinct P-, QRS-, and T-waves. With further optimizations, the microfabricated atomic magnetometers based on chip-scale 3-D alkali vapor cells have the potential to enable magnetoencephalography (MEG) measurements and nuclear magnetic resonance imaging (NMRI).

Improvement of spin polarization spatial uniformity in optically pumped atomic magnetometers based on counter-propagating pump beams and atomic diffusion

In an optically pumped alkali vapor cell with a high density of atoms, the attenuation of the pump light generates a spatially non-uniform distribution of the electronic spin polarization of alkali atoms, which is detrimental to biomagnetism applications of magnetometers as well as the hyperpolarization of noble gas atoms. Therefore, in this study, we propose a new scheme to generate a nearly uniform, unsaturated spin polarization region based on counter-propagating pump beams and atomic diffusion. A finite element method-based simulation is used to demonstrate the three-dimensional distribution of the spin polarization in a spherical cell. The effects of cell temperature and pump light power on the homogeneity of the spin polarization are studied. The distribution of spin polarization near the center of the cell is experimentally measured and a 1 cm uniform spin polarization region is achieved in the center of the cell. The uniformity of spin polarization in the center region of the cell increased by 50% compared with single beam pumping. The advantage of our proposed scheme is that it can generate an unsaturated uniform region of spin polarization in the center of a cell using a single species of alkali atoms.

Laser cooling in a chip-scale platform

Chip-scale atomic devices built around micro-fabricated alkali vapor cells are at the forefront of compact metrology and atomic sensors. We demonstrate a micro-fabricated vapor cell that is actively pumped to ultra-high-vacuum (UHV) to achieve laser cooling. A grating magneto-optical trap (GMOT) is incorporated with a 4 mm-thick Si/glass vacuum cell to demonstrate the feasibility of a fully miniaturized laser cooling platform. A two-step optical excitation process in rubidium is used to overcome surface-scatter limitations to the GMOT imaging. The unambiguous miniaturization and form-customizability made available with micro-fabricated UHV cells provide a promising platform for future compact cold-atom sensors.