Alkali Vapor Cells Research Articles

Eddy current imaging with an atomic radio-frequency magnetometer

We use a radio-frequency 85Rb alkali-vapor cell magnetometer based on a paraffin-coated cell with long spin-coherence time and a small, low-inductance driving coil to create highly resolved conductivity maps of different objects. We resolve sub-mm features in conductive objects, we characterize the frequency response of our technique, and by operating at frequencies up to 250 kHz we are able to discriminate between differently conductive materials based on the induced response. The method is suited to cover a wide range of driving frequencies and can potentially be used for detecting non-metallic objects with low DC conductivity.

Raman and nuclear magnetic resonance investigation of alkali metal vapor interaction with alkene-based anti-relaxation coating.

The use of anti-relaxation coatings in alkali vapor cells yields substantial performance improvements compared to a bare glass surface by reducing the probability of spin relaxation in wall collisions by several orders of magnitude. Some of the most effective anti-relaxation coating materials are alpha-olefins, which (as in the case of more traditional paraffin coatings) must undergo a curing period after cell manufacturing in order to achieve the desired behavior. Until now, however, it has been unclear what physicochemical processes occur during cell curing, and how they may affect relevant cell properties. We present the results of nondestructive Raman-spectroscopy and magnetic-resonance investigations of the influence of alkali metal vapor (Cs or K) on an alpha-olefin, 1-nonadecene coating the inner surface of a glass cell. It was found that during the curing process, the alkali metal catalyzes migration of the carbon-carbon double bond, yielding a mixture of cis- and trans-2-nonadecene.

Widefield microwave imaging in alkali vapor cells with sub-100 μm resolution

We report on widefield microwave vector field imaging with sub- resolution using a microfabricated alkali vapor cell. The setup can additionally image dc magnetic fields, and can be configured to image microwave electric fields. Our camera-based widefield imaging system records 2D images with a 6 × 6 mm2 field of view at a rate of 10 Hz. It provides up to spatial resolution, and allows imaging of fields as close as above structures, through the use of thin external cell walls. This is crucial in allowing us to take practical advantage of the high spatial resolution, as feature sizes in near-fields are on the order of the distance from their source, and represent an order of magnitude improvement in surface-feature resolution compared to previous vapor cell experiments. We present microwave and dc magnetic field images above a selection of devices, demonstrating a microwave sensitivity of per voxel, at present limited by the speed of our camera system. Since we image 120 × 120 voxels in parallel, a single scanned sensor would require a sensitivity of at least to produce images with the same sensitivity. Our technique could prove transformative in the design, characterization, and debugging of microwave devices, as there are currently no satisfactory established microwave imaging techniques. Moreover, it could find applications in medical imaging.

Laser light routing in an elongated micromachined vapor cell with diffraction gratings for atomic clock applications.

This paper reports on an original architecture of microfabricated alkali vapor cell designed for miniature atomic clocks. The cell combines diffraction gratings with anisotropically etched single-crystalline silicon sidewalls to route a normally-incident beam in a cavity oriented along the substrate plane. Gratings have been specifically designed to diffract circularly polarized light in the first order, the latter having an angle of diffraction matching the (111) sidewalls orientation. Then, the length of the cavity where light interacts with alkali atoms can be extended. We demonstrate that a longer cell allows to reduce the beam diameter, while preserving the clock performances. As the cavity depth and the beam diameter are reduced, collimation can be performed in a tighter space. This solution relaxes the constraints on the device packaging and is suitable for wafer-level assembly. Several cells have been fabricated and characterized in a clock setup using coherent population trapping spectroscopy. The measured signals exhibit null power linewidths down to 2.23 kHz and high transmission contrasts up to 17%. A high contrast-to-linewidth ratio is found at a linewidth of 4.17 kHz and a contrast of 5.2% in a 7-mm-long cell despite a beam diameter reduced to 600 μm.

Measuring excited state lifetime of Rb atoms with pump-probe technique

A technique for measuring the excited state lifetime τex of optical transitions in alkali atoms is presented. It is a form of pump-probe technique based on time-resolved optical transmission through the atomic vapor cell. This technique can serve as an alternative to the traditionally used time-resolved photofluorescence methods when measuring alkali vapor cells with heavily quenched fluorescence, where the τex is expected to be on the order of a few nanoseconds, and the highly sensitive fluorescence detectors with sub-nanosecond temporal resolution that are required may not be available. We use this technique to measure the τex of Rb atoms in vapor cells with different buffer gas pressures.

Aging studies on micro-fabricated alkali buffer-gas cells for miniature atomic clocks

We report an aging study on micro-fabricated alkali vapor cells using neon as a buffer gas. An experimental atomic clock setup is used to measure the cell's intrinsic frequency, by recording the clock frequency shift at different light intensities and extrapolating to zero intensity. We find a drift of the cell's intrinsic frequency of (−5.2 ± 0.6) × 10−11/day and quantify deterministic variations in sources of clock frequency shifts due to the major physical effects to identify the most probable cause of the drift. The measured drift is one order of magnitude stronger than the total frequency variations expected from clock parameter variations and corresponds to a slow reduction of buffer gas pressure inside the cell, which is compatible with the hypothesis of loss of Ne gas from the cell due to its permeation through the cell windows. A negative drift on the intrinsic cell frequency is reproducible for another cell of the same type. Based on the Ne permeation model and the measured cell frequency drift, we determine the permeation constant of Ne through borosilicate glass as (5.7 ± 0.7) × 10−22 m2 s−1 Pa−1 at 81 °C. We propose this method based on frequency metrology in an alkali vapor cell atomic clock setup based on coherent population trapping for measuring permeation constants of inert gases.

Algorithm for evaluation of temperature distribution of a vapor cell in a diode-pumped alkali laser system (part II).

With high efficiency and small thermally-induced effects in the near-infrared wavelength region, a diode-pumped alkali laser (DPAL) is regarded as combining the major advantages of solid-state lasers and gas-state lasers and obviating their main disadvantages at the same time. Studying the temperature distribution in the cross-section of an alkali-vapor cell is critical to realize high-powered DPAL systems for both static and flowing states. In this report, a theoretical algorithm has been built to investigate the features of a flowing-gas DPAL system by uniting procedures in kinetics, heat transfer, and fluid dynamic together. The thermal features and output characteristics have been simultaneously obtained for different gas velocities. The results have demonstrated the great potential of DPALs in the extremely high-powered laser operation.

Microfabricated alkali vapor cell with anti-relaxation wall coating

We present a microfabricated alkali vapor cell equipped with an anti-relaxation wall coating. The anti-relaxation coating used is octadecyltrichlorosilane and the cell was sealed by thin-film indium-bonding at a low temperature of 140 °C. The cell body is made of silicon and Pyrex and features a double-chamber design. Depolarizing properties due to liquid Rb droplets are avoided by confining the Rb droplets to one chamber only. Optical and microwave spectroscopy performed on this wall-coated cell are used to evaluate the cell's relaxation properties and a potential gas contamination. Double-resonance signals obtained from the cell show an intrinsic linewidth that is significantly lower than the linewidth that would be expected in case the cell had no wall coating but only contained a buffer-gas contamination on the level measured by optical spectroscopy. Combined with further experimental evidence this proves the presence of a working anti-relaxation wall coating in the cell. Such cells are of interest for applications in miniature atomic clocks, magnetometers, and other quantum sensors.

基于受抑全反射的碱金属气室镀膜厚度测量

基于受抑全反射的碱金属气室镀膜厚度测量

Investigation of thermal features of two types of alkali-vapor cells pumped by a laser diode

We develop a new algorithm to evaluate the thermal features of a rubidium-vapor cell and a cesium-vapor cell pumped by the laser diode. The theoretical model is based on the principles of both heat transfer and laser kinetics. The obtained population density distribution and the radial temperature distribution are analyzed for both types of cells. It is thought that the theoretical results are logically reasonable and the mathematical precision is satisfactory in designing a real diode-pumped alkali laser (DPAL). The methodology is valuable in the construction of a high-powered DPAL in the future.

Lifetime improvement of micro-fabricated alkali vapor cells by atomic layer deposited wall coatings

Alumina (Al2O3), silicon dioxide (SiO2) and titanium dioxide (TiO2) films were examined for their suitability as a protection coating for micro-fabricated cesium vapor cells. The layers were deposited by atomic layer deposition (ALD). This technique enables the deposition of pinhole-free passivation layers with a low roughness, a high conformality and a high homogeneity, even at three-dimensional surfaces. It is shown that Al2O3 and SiO2 films, in contrast to TiO2 films, do not hamper anodic bonding up to thicknesses of 20nm. The resistance enhancement of the vapor cells by means of the implementation of the ALD films is demonstrated by absorption measurements of the cesium D1 line. SiO2 coatings showed no effects on the resistance of the vapor cells against the reducing behavior of cesium. In contrast, Al2O3 coatings improved the resistance of the vapor cells by a factor of ~100.

Low-temperature indium-bonded alkali vapor cell for chip-scale atomic clocks

A low-temperature sealing technique for micro-fabricated alkali vapor cells for chip-scale atomic clock applications is developed and evaluated. A thin-film indium bonding technique was used for sealing the cells at temperatures of ≤140 °C. These sealing temperatures are much lower than those reported for other approaches, and make the technique highly interesting for future micro-fabricated cells, using anti-relaxation wall coatings. Optical and microwave spectroscopy performed on first indium-bonded cells without wall coatings are used to evaluate the cleanliness of the process as well as a potential leak rate of the cells. Both measurements confirm a stable pressure inside the cell and therefore an excellent hermeticity of the indium bonding. The double-resonance measurements performed over several months show an upper limit for the leak rate of 1.5 × 10−13 mbar·l/s. This is in agreement with additional leak-rate measurements using a membrane deflection method on indium-bonded test structures.

A remotely interrogated all-optical 87Rb magnetometer

Atomic magnetometry was performed at Earth’s magnetic field over a free-space distance of ten meters. Two laser beams aimed at a distant alkali-vapor cell excited and detected the 87Rb magnetic resonance, allowing the magnetic field within the cell to be interrogated remotely. Operated as a driven oscillator, the magnetometer measured the geomagnetic field with ≲3.5 pT precision in a ∼2 s data acquisition; this precision was likely limited by ambient field fluctuations. The sensor was also operated in self-oscillating mode with a 5.3 pT/Hz noise floor. Further optimization will yield a high-bandwidth, fully remote magnetometer with sub-pT sensitivity.

Extraction of a strongly focusing He+ beam from three-stage concave electrodes for alpha particle measurement system in ITER

A strongly focusing He(+) ion beam source equipped with concave multi-aperture electrodes was developed for production of He(-) through a charge exchange cell. The beam was extracted at a voltage less than 20 kV from 301 apertures distributed in an area of 100 mm φ, and focused at 750 mm distance. The beam current and the beam size of 2 A and 20 mm in diameter, respectively, were achieved with an arc power less than 10 kW. The optimum perveance was obtained at 0.02 A∕kV(1.5) at the beam energy less than 20 keV which is suitable for the conversion to He(-) in an alkali vapor cell.

チップスケール原子磁気センサのためのガラスフリットリフローによる犠牲マイクロ流路気密封止技術

This paper reports on a novel and simple packaging technique to enhance the atmosphere controllability of the microfabricated alkali vapor cell used for CSAMs (Chip Scale Atomic Magnetometers), in which sacrificial microchannels at the bonded interface are utilized as gas feedthrough for evacuation and filling, subsequently sealed by glass-frit reflow. By applying the proposed method, a 10 mm3 potassium vapor cell with 0.1 MPa helium buffer gas was fabricated. Sealing of the microchannels by glass-frit reflow were successfully demonstrated adopting the optimized parameters as 460–480 °C at bonding pressure of more than 250 kPa. The leak rate of less than 3.1×10−14 Pa·m3/s, which is required sealing quality for CSAM, was verified through a high resolution helium leak test.

High-temperature alkali vapor cells with antirelaxation surface coatings

Antirelaxation surface coatings allow long spin relaxation times in alkali-metal cells without buffer gas, enabling faster diffusion of the alkali atoms throughout the cell and giving larger signals due to narrower optical linewidths. Effective coatings were previously unavailable for operation at temperatures above 80 °C. We demonstrate that octadecyltrichlorosilane (OTS) can allow potassium or rubidium atoms to experience hundreds of collisions with the cell surface before depolarizing, and that an OTS coating remains effective up to about 170 °C for both potassium and rubidium. We consider the experimental concerns of operating without buffer gas and with minimal quenching gas at high vapor density, studying the stricter need for effective quenching of excited atoms and deriving the optical rotation signal shape for atoms with resolved hyperfine structure in the spin-temperature regime. As an example of a high-temperature application of antirelaxation coated alkali vapor cells, we operate a spin-exchange relaxation-free atomic magnetometer with sensitivity of 6 fT/Hz and magnetic linewidth as narrow as 2 Hz.

Slow light in narrow paraffin-coated vapor cells

Alkali vapor cells with antirelaxation coated walls can have long atomic coherence times. However, using such coated cells in the hyperfine configuration for electromagnetically induced transparency (EIT) requires longitudinal atomic motion to be confined to less than the hyperfine wavelength. We employed a narrow (1 mm) coated cell geometry to study hyperfine EIT and slow and stored light in warm R87b vapor, with results comparable to those in buffer gas cells and showing the promise of such cells for several applications.

Optical characterization and manipulation of alkali metal nanoparticles in porous silica

Rubidium and cesium metal nanoparticles were grown in nanoporous silica samples placed in alkali vapor cells. Their size and shape were investigated by measuring the sample optical transmittance. Spectral changes due to photodesorption processes activated by weak light were also analyzed. Alkali atoms photoejected from the silica walls diffuse through and out of the nanopores, modifying both the nanoparticle distribution in the silica matrix and the atomic vapor pressure in the cell volume. The number of rubidium and cesium atoms burst out of the samples was measured as a function of photon energy and fluence. The optical absorption measurements together with the analysis of the photodesorption yield give a complete picture of the processes triggered by light inside the nanopores. We show that atomic photodesorption, upon proper choice of light frequency and intensity, induces either growth or evaporation of nanosized alkali metal clusters. Cluster size and shape are determined by the host-guest interaction.

Method for characterizing self-assembled monolayers as antirelaxation wall coatings for alkali vapor cells

We describe a method for characterizing self-assembled monolayers (SAMs) in terms of their performance as antirelaxation wall coatings for alkali atom vapor cells. A combination of initial surface analysis and subsequent laser spectroscopy is used to provide insight into the quality of the coating, as well as its performance under the exposure to alkalis as it occurs, for example, when used in applications such as atomic magnetometers or clocks. Fused silica plates coated with octadecyltrichlorosilane SAMs were used to make cubic R87b gas cells. The surface was characterized by x-ray diffraction, contact angle measurements, and atomic force microscopy. Measurements of hyperfine resonance linewidths and frequency shifts show that the rubidium vapor atoms collide up to 40 times with the walls of the cells before their coherence relaxes and their adsorption energy is around 0.065 eV. Chemical analysis of the cell indicates some weak reactions between the coating and the rubidium atoms.

New approach of fabrication and dispensing of micromachined cesium vapor cell

We describe the fabrication of wafer-scale alkali vapor cells based on silicon micromachining and anodic bonding. The principle of the proposed micromachined alkali cell is based on an extremely compact sealed vacuum cavity of a few cubic millimeters containing caesium vapors, illuminated by a high-frequency modulated laser beam. The alkali cells are formed by sealing an etched silicon wafer between two glass wafers. The technique of cell filling involves the use of an alkali dispenser. The activation of cesium vapors is made by local heating of the dispenser below temperature range causing degradations of cesium vapor purity. Thus, the procedure avoids negative effects of cesium chemistry on the quality of cell surfaces and sealing procedure. To demonstrate the clock operation, cesium absorption as well as coherent population trapping resonance was measured in the cells.