Alkali Metal Vapor Research Articles

A Method of Laser Frequency Stabilization Based on the Effect of Linear Dichroism in Alkali Metal Vapors in a Modulated Transverse Magnetic Field

We present a method of laser frequency stabilization based on the linear dichroism signal in a transverse magnetic field. This method is similar to the DAVLL (Dichroic Atomic Vapor Laser Lock) method. It differs from DAVLL and from its existing modifications primarily by the fact that it uses signals of linearly polarized light caused by alignment, rather than circular refraction caused by orientation, and therefore allows us to obtain error signals at the magnetic field modulation frequency (or its second harmonic) by extremely simple means. The method allows the laser frequency to be stabilized in the vicinity of the low-frequency transition in the D1 line of Cs; it does not require strong magnetic fields or careful shielding of cells containing cesium atoms. Although the absorption line in a gas-filled cell is typically gigahertz wide, the achievable resolution, limited by the signal-to-noise ratio of photon shot noise, can reach units or tens of kilohertz in a one hertz bandwidth.

Alkali metal depletion in the deep Jovian atmosphere: The role of anions

The Juno Microwave Radiometer has allowed observation of Jupiter's atmosphere down to previously inaccessible depths, although the complexity of the atmospheric dynamics has complicated analysis. The longest-wavelength channel (600 MHz) is sensitive to pressure levels of hundreds of bars, and has observed opacity sources other than the known gaseous and cloud components, likely caused by thermally ionized free electrons from alkali metal vapor. We extend previous analysis of limb darkening at these wavelengths, using radiative transfer and thermal equilibrium modeling, by considering the effect of anions in the deep Jovian atmosphere, which act as a sink for free electrons and will thus decrease opacity for a given alkali metal abundance. We show that MWR observations are consistent with a sodium and potassium abundance on the order of 0.1× solar around the 1-kilobar level, higher than previously estimated but still substantially depleted compared to other heavy elements, a value that would be within the range of observed alkali metal abundances on giant exoplanets; alternatively, MWR observations may be consistent with 3× solar sodium abundance, but only if potassium is even more strongly depleted. Such depletion may be the result of either chemical processes yet deeper in the atmosphere, such as in the silicate clouds, or of a long-lived stable layer shallower than the alkali salt clouds.

Microfabricated vapor cells with chemical polishing and two-step low-temperature anodic bonding for single-beam magnetometer

With the miniaturization of quantum sensors, the fabrication of alkali metal vapor cells based on micro-electromechanical systems (MEMS) has become increasingly important. However, the fabrication of MEMS vapor cells remains a challenge. In this study, a microfabricated millimeter-level vapor cell was fabricated and tested. The silicon cavity was fabricated by dry etching and chemical polishing. After injection of alkali metal and buffer gas, a two-step low-temperature anodic bonding was employed. The first step was non-isothermal to keep the alkali metal in the low-temperature pole plate and the second step was isothermal to improve the bonding strength, enhancing the hermeticity. Then, the vapor cell was tested for sidewall roughness, bonding strength, and leakage rate. Subsequently, Rb D1 line absorption spectroscopy, stability, and the single-beam magnetometer signal were tested. The results show the fabricated vapor cell had high stability, providing a basis for the miniaturization of quantum devices.

Determining the absolute number density of a thermal vapor via photon correlations

We propose a technique to determine the absolute number density N of an alkali-metal vapor confined within a nanocell. This method is based on near-resonant driving of the vapor's constituent atoms and determining the mean interparticle distance (or equivalently the temperature) from the power spectrum associated with the offset photon intensity-intensity correlation function g(2)(τ)−1. In our investigation, we treat the atoms as an average of interacting and radiating dipole pairs randomly positioned within the nanocell. We observe that the power spectrum of the emitted light has a central dip for interparticle distances corresponding to ∼λ/5 and that this result is robust to variations in the driving Rabi frequency and the average detuning. With our proposed method, we can overcome the limitations in defining the absolute number density N, which is currently typically deduced from the temperature of heating elements applied externally to the cell. Published by the American Physical Society 2024

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.

The influence of multi-step modification method on the adsorption effect of alkali metal adsorbent in high temperature corrosive environment

In order to mitigate the serious high-temperature corrosion problem caused by gaseous alkali metal salts on the heating surface of boilers, the adsorption effect of kaolinite for alkali metal vapor stood out among many silicon-aluminum-based adsorbents. To improve the adsorption capacity of kaolinite for alkali metal vapor, a novel multi-step modification method was proposed in this reported work. The multi-step modification method consisted of the following four steps, which were pre-intercalation, replacement intercalation, multiple intercalation, and exfoliation, and it was found that the multi-step modification method resulted in the increase of the particle size, the expansion of the interlayer spacing, and the expansion of the porosity structure of kaolinite. The physicochemical analyses showed that the multi-step modification method made it easier to dehydrate kaolinite to produce metakaolinite. In addition, it was found that the multi-step modification method led to the generation of a new crystalline phase of kaolinite, which was formed by the dehydration and condensation of methanol and AlOH groups. It also led to the pre-removal of some of the hydroxyl groups in kaolinite. The insoluble sodium loading capacity of the multi-step modified kaolinite was enhanced by 64.47% and the insoluble potassium loading capacity was enhanced by 29.68% under the adsorption condition of 850 °C for 45 min. Therefore, the multi-step modification method enhanced the alkali metal vapor adsorption capacity of raw kaolinite under high-temperature environment, which laid a certain foundation for subsequent research in this field.

Corrosion, permeation and mass transfer mechanisms of alkali metals in corundum refractories

During the operation of the waste liquid incinerator, the alkali metal slag might adhere to the surface of the refractory material to form an inhomogeneous solid slag layer, causing the furnace lining to be simultaneously corroded by three phases of alkali metals: vapor, molten salt, and slag. This phenomenon intensifies the damage to the refractory material. Therefore, this paper investigates the mass transfer and permeation process, phase change process and corrosion rate of Na2CO3 inside corundum refractory materials in three phases: Na vapor, molten Na2CO3 and Na-slag. The results showed that alkali vapor penetrated the interior through the pores, and the NaAlO2 and β-Al2O3 generated by the reaction led to the volume expansion and microcracks. Na vapor continued to penetrate the interior along the cracks and eroded the sample, and the higher the temperature the greater Na vapor penetration. In vapor phase corrosion, the effect of corrosion time on the erosion resistance of corundum refractories is less than that of temperature, and the increase in corrosion time does not lead to the formation of additional new phases. In the molten salt corrosion experiments, it was found that the molten salt corrosion was accompanied by vapor phase corrosion at 1100 °C, and the amount of Na2CO3 has a greater effect on the corroded mass than the temperature. Comparing the corrosion of refractory materials by the three phases of Na2CO3, the molten salt corrosion rate was the highest, followed by the vapor phase corrosion, and finally the slag corrosion. It is concluded that the slag layer can effectively prevent the corrosion of the refractory material by alkali metal molten salts and vapors, thus prolonging the service life of refractories.

Real-time measurement of pump beam polarization through the transmitted light intensity in a polarized alkali metal vapor cell

A laser beam with left-/right-handed circular polarization is generally used to create the oriented atomic spins for precision measurements in a spin-exchange relaxation-free (SERF) co-magnetometer. The fluctuation of laser polarization interferes with the spin polarization of alkali metal atoms, leading to the system performance degradation. Here, we report a method for real-time polarization state measurement by using the transmitted light intensity of the pump beam passing through the vapor cell. Based on the principle of circular dichroism, the optical absorption model of polarized alkali metal atoms is established. The simulation results of the transmittance of the pump laser with different polarization states through the alkali metal vapor cell are given and verified by experiments. The experimental results show that the circularly polarized beam has the weakest absorption, while the linearly polarized laser beam is absorbed the strongest. The achieved measurement accuracy stands at an impressive 98.83 %. This work provides a simple and easy-to-use way to measure the polarization state of the laser beam used in the vapor cell devices, particularly the microfabricated prototypes with limited space.

Exploration of applying Cesium–Xenon system as a sensitive broadband frequency up-conversion photodetecting medium

Alkali metal vapor was applied as sensitive frequency-conversion photodetectors, however, their application was limited by their natural narrow (∼100 MHz) sensing frequency range. Here, Cs–Xe excimer is used to break this limitation. Demonstration of Cs–Xe frequency up-conversion photodetector is given. Using excimer transition, 550–552 nm light is converted to 541 nm fluorescence, this conversion wavelength band is much larger than a typical atom energy level transition. The frequency up-conversion background is mainly energy pooling fluorescence, which varies with Cs 6P3/2 state number density on roughly a cubic function. Atom collision generated Cs 12DJ, 9FJ levels are measured and discussed. By switching excimer transitions selected, e.g. using Cs nPJ (n = 7, 8, …) related upward excimer transitions, it is possible to detect light with other wavelength from infrared to terahertz band. The concept in this work may get application in photon detection with high sensitivity.

Optical control and coherent coupling of spin diffusive modes in thermal gases

Quantum science and technology devices exploiting collective spins in thermal gases are extremely appealing due to their simplicity and robustness. This comes at the cost of dealing with the random thermal motion of the atoms which is usually an uncontrolled source of decoherence and noise. There are however conditions, for example, when diffusing in a buffer gas, where thermal atoms can occupy a discrete set of stable spatial modes. Diffusive modes can be extended or localized, have different magnetic properties depending on boundary conditions, and can react differently to external perturbations. Here, we selectively excite, manipulate, and interrogate the longest-lived of these modes by using laser light. In particular, we identify the conditions for the generation of modes that are exceptionally resilient to detrimental effects such as light induced frequency shifts and power-broadening, which are often the dominant sources of systematic errors in atomic magnetometers and comagnetometers. Moreover, we show that the presence of spatial inhomogeneities in the pump introduces a coupling that leads to a coherent exchange of excitation between the two longest-lived modes. Our results demonstrate that systematic engineering of the multi-mode nature of diffusive gases has great potential for improving the performance of quantum sensors based on alkali-metal thermal vapors, and opens new perspectives for quantum information applications. Published by the American Physical Society 2024

Anomalous noise spectra in a spin-exchange-relaxation-free alkali-metal vapor

Anomalous noise spectra in a spin-exchange-relaxation-free alkali-metal vapor

Towards low-temperature anodic bonding for RbN3-filled MEMS atomic vapor cells and defect inspection by OCT

From the different methodologies to fill microfabricated alkali-metal vapor cells, the rubidium azide (RbN3) decomposition by UV radiation is a cost-effective solution to produce rubidium (Rb) and nitrogen (N2). The typical fabrication of the vapor cells is based on silicon and glass bonding, in which the substrates are heated to temperatures between 300 and 450 ºC along with a high electrostatic field (400 – 1000 V) to establish the solid-state connection. However, the RbN3 compound has been reported to undergo thermal decomposition within the temperature range of 355–395 ºC. In this work, a systematic variation of the bonding temperature (from 200 to 300 ºC) in silicon cavity-based vapor cells is presented to prevent the RbN3 decomposition during the cell fabrication. Considering that the anodic bonding process can be well represented by a simplified equivalent electrical circuit model, we report a maximum bond strength of 6.05 MPa and a lowest time constant τ of 180.03 s for 300 ºC with a simple electrode configuration. A total transferred charge of ≥0.250 mC.mm−2 for temperatures above 225 ºC are indicative of a good quality bond. Interestingly, distinct differences in the failure mode of bonding are observed, in which undamaged bonded interfaces are only observed for temperatures of 275 ºC and above. As a result, a 3 mm diameter vapor cell was successfully fabricated using anodic bonding at 275 ºC. Moreover, and for the first time, optical coherence tomography (OCT) was implemented as an effective and novel technique to investigate the glass-silicon-glass bonding in a MEMS vapor cell, providing a cross-sectional image of the device, in a non-destructive and contactless manner, to ensure the production of reliable and defect-free devices.

Optimized optical tomography of quantum states of a room-temperature alkali-metal vapor

Optimized optical tomography of quantum states of a room-temperature alkali-metal vapor

Development of a negative helium ion source with non-metallic charge exchange

Negative helium ion (He−) beams are required for tandem accelerators used at research centers and implanter facilities. The common production method of such beams involves charge exchange between a positive helium ion (He+) source or beam, with a low pressure alkali metal vapour. However, the efficiency of this reaction is poor (only a small percentage) and the alkali metal vapour increases the potential flammability of the interior surfaces of the reaction chamber, as well as contributing to unwanted electrostatic discharge and target contamination downstream. In this paper we describe initial results from experiments using a focused He+ ion microscope (HIM) to measure charge uptake efficiencies after transmission through carbon membranes. The He+ beam (15 keV to 30 keV, 50 fA to 10 pA) collides with a non-metallic foil, and transmitted particles (He+, He0, and He−) are separated electrostatically into discrete beam spots and detected using a radiation camera (AdvaCam MiniPIX). A He− percentage uptake of 0.05 was observed using a 20 nm exfoliated graphite membrane consistent with previous reports. Future work will investigate the efficiency of charge exchange through other suitable membrane materials.

Investigation of Ash Deposition Characteristics in Full-Scale Circulating Fluidized Bed Boiler Burning Zhundong Coal

ABSTRACT Due to the high content of alkali and alkaline earth metals (AAEMs) in Zhundong coal, serious ash-related problems such as slagging and fouling occurred in the process of utilization, which has attracted wide attention. In this work, the ash deposition characteristics in a new type circulating fluidized bed boiler burning Zhundong coal was studied. The effect of steam cooled flue on alleviating slagging problems was evaluated, and the influence of flue gas temperature and wall temperature on ash deposition characteristics were studied. The results showed that the mass of ash deposition and the concentration of submicron particles decrease gradually along the direction of flue gas flow in the steam cooled flue. The deposited ash changed from tightly bound to wall to loosely deposited, which indicated that the arrangement of the steam cooled flue greatly alleviated the slagging problems. Compared with flue gas temperature, the wall temperature had a greater impact on ash deposition characteristics. Mineral elements such as Na, S, Fe and Mg were more likely to enrich on the high temperature wall, while the Ca showed the opposite trend. On the low temperature wall, Na existed in the form of Na2SO4 and Na2Si2O5, while on the high temperature wall, the direct condensation of alkali metal vapor was prevented and the thermophoresis disappeared, so Na mainly existed in the form of Na2Si2O5.

Temperature Sensing via Electromagnetically Induced Transparency Vapor

The behavior of multilevel atomic ensembles (e.g., alkali-metal atoms) can be influenced significantly by the intensity of a driving field (or controlling/coupling field). The phase coherence between two transition pathways driven by a probe light and a driving field can lead to the effect known as electromagnetically induced transparency (EIT). In EIT, the probe light can pass through a three-level alkali-metal atomic vapor without absorption or reflection when two coherent resonances (transition pathways driven by the driving and probe fields) are present and the linewidths of the transparency windows are sufficiently narrow. The optical characteristics of atomic systems can also be affected by the Doppler broadening of the absorption profile in a spectroscope. Our analysis indicates that both broadenings (related to the transitions excited by the driving and probe fields) can be expanded, leading to an increase in the transmittance and reflectance broadenings when a coupling field with adaptive strength is applied; the corresponding temperature would, thus, be implemented and readable. We show that the most suitable preparation for temperature sensing via an EIT vapor is to provide 80 times the spontaneous decay rate (SDR) of the excited atomic levels. This configuration results in reflectance and transmittance values that range between zero and one and cover a temperature range of 0 K to 600 K. As an example, we demonstrate the integration of specialized coating technologies with EIT ensembles for temperature sensing in the range of dozens of kelvins at and above room temperature. A key advantage of this temperature-sensing system is its ability to use adaptive resonant visible light as the probe field. This novel approach may find applications in providing unprecedented levels of precision and control in temperature sensing for coating processes and in the design of other photonic or optical devices. It can also be used to determine the temperature-dependent behavior of the specific heat of alkali-metal solids and gases (including the latent heats of vaporization or sublimation of alkali-metal solids) through the reflection and transmission spectra of the vaporized EIT atomic vapors.

Optical pumping of rubidium isotopes by Cr<sup>3+</sup>:BeAl<sub>2</sub>O<sub>4</sub> laser radiation

We consider the use of a Cr3+:BeAl2O4 laser in free-running operating as a source of emission for optical pumping rubidium alkali metal vapors. The use of dispersive elements in the composition of the laser cavity makes it possible to smoothly tune lasing wavelength and to realize generation at wavelengths corresponding to the D1 and D2 lines of the 85Rb and 87Rb isotopes. Optical pumping of rubidium isotopes by laser emission with wavelengths of 795 and 780 nm, respectively, is experimentally implemented, and their fluorescence is demonstrated. The question of using a wavelength-tunable laser in the method of spin-exchange optical pumping of noble gases is discussed.

Plasma-activated high-strength non-isothermal anodic bonding for efficient fabrication of the micro atomic vapor cells

Microfabricated alkali metal vapor cells are the core components for miniaturizing quantum devices, such as atomic clocks, atomic gyroscopes, and atomic magnetometers. Alkali metals are prone to thermal migration at the bonding interface during the sealing of the vapor cell, which leads to bonding failure. In this study, a non-isothermal anodic bonding process based on plasma activation was developed to achieve high-strength bonding at low temperatures using the temperature gradient to keep the alkali metal away from the bonding interface. This procedure eliminates interference from the alkali metal and significantly increases the success rate of anodic bonding of vapor cells. Simulation results for the temperature field and the residual stresses during anodic bonding demonstrate that the procedure is theoretically feasible. The effect of plasma treatment on the contact angle and roughness of the bonding surface revealed a mechanism for improving the bond strength. In addition, the construction of an integrated processing platform made it possible to perform non-isothermal anode bonding, injection of alkali metal, filling of inert gases, and recovery and recycling of noble gases. The high strength and good hermeticity of the bonding interface were demonstrated by the bond strength, cross-sectional high-resolution transmission electron microscopy observations, and leakage rate tests. Finally, the absorption spectrum and free-induction decay signal of the fabricated vapor cell were measured.

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.

Plasma-activated silicon–glass high-strength multistep bonding for low-temperature vacuum packaging

Si-glass bonding is one of the most crucial vacuum packaging methods in semiconductor devices; however, high temperatures (>300 °C) are required to achieve a high bonding strength, which limits its applications. Current low-temperature Si-glass bonding requires a humid environment and is not compatible with vacuum environments. In this study, a low-temperature vacuum multistep bonding method based on plasma activation was proposed. Traditional plasma-activated low-temperature bonding relies on water-molecule bridging in the atmosphere to lower the bonding temperature. In contrast, the proposed multistep bonding process uses plasma-activated vacuum anodic bonding for initial bonding, and then utilizes direct bonding to balance the bonding surface energy and sufficiently eliminate the bonding surface micro gap, thus achieving a high bonding strength of 18 MPa at 150 °C under vacuum. The effects of plasma activation on the hydrophilicity, surface morphology, and chemical conditions of the wafer surfaces were investigated. The morphologies of the bonding interfaces under different bonding conditions were characterized, and cross-sectional elemental scanning was used to analyze the microscopic factors. Based on the aforementioned investigations and the gap closure theory, the mechanism of multistep bonding was analyzed. Finally, the proposed bonding method was validated by fabricating micro alkali metal vapor cells. The results demonstrate that the proposed method meets the requirements for vacuum device packing at low temperatures.