Buffer Gas Pressure Research Articles

On the reduction of gas permeation through the glass windows of micromachined vapor cells using Al2O3 coatings

Stability and precision of atomic devices are closely tied to the quality and stability of the internal atmosphere of the atomic vapor cells on which they rely. Such an atmosphere can be stabilized by building the cell with low permeation materials such as sapphire or aluminosilicate glass in microfabricated devices. Recently, we have shown that permeation barriers made of Al2O3 thin-film coatings deposited on standard borosilicate glass could be an alternative for buffer gas pressure stabilization. In this study, we, hence, investigate how helium permeation is influenced by the thickness, ranging from 5 to 40 nm, of such Al2O3 thin films coated by atomic layer deposition. Permeation rates are derived from long-term measurements of the pressure-shifted transition frequency of a coherent population trapping (CPT) atomic clock. From thicknesses of 20 nm onward, a significant enhancement of the cell hermeticity is experienced, corresponding to two orders of magnitude lower helium permeation rate. In addition, we test cesium vapor cells filled with neon as a buffer gas and whose windows are coated with 20 nm of Al2O3. As for helium, the permeation rate of neon is significantly reduced, thanks to alumina coatings, leading to a fractional frequency stability of 4×10−12 at 1 day when the cell is used in a CPT clock. These features outperform the typical performances of uncoated Cs–Ne borosilicate cells and highlight the significance of Al2O3 coatings for buffer gas pressure stabilization.

The use of He buffer gas for moderating the plume kinetic energy during Nd:YAG-PLD growth of EuxY2−xO3 phosphor films

We have investigated the He buffer gas process of moderating the kinetic energy of the pulsed laser deposition (PLD) plume during EuxY2−xO3 phosphor film growth. When using a neodymium yttrium aluminum garnet laser for PLD thin film growth, the kinetic energy of the ablation plumes can be high enough to cause the formation of point defects in the film. The buffer gas pressure is an important process parameter in PLD film growth. We find that the presence of the He buffer gas reduces the kinetic energy of the laser deposition plume through many low-angle collisions in the gas phase by a factor of 7 without reducing the deposition rate. This is because He is much lighter than any of the elements in the plume and it does not affect the composition of the oxide films. Consequently, the resputtering of the Y2O3 film surface by the plume was significantly suppressed in the presence of the He gas moderator, leading to a decrease of the defect density in the Y2O3 films. The improvement of the film quality was verified by a systematic analysis of time-resolved photoluminescence (PL) data for EuxY2−xO3 composition–gradient films. The PL lifetime and intensity of Eu0.2Y1.8O3, which shows the highest PL intensity, increased by 13.3% and 36.4%, respectively, when the He gas moderation process was used. The He buffer gas process is applicable to the PLD growth of the other oxide materials as well, where the reduction of the kinetic energy of the plume would bring the PLD process closer to the molecular beam epitaxy growth condition.

Dynamic laser ablation loading of a linear Paul trap

We present a detailed method for accumulating Ca+ ions controllably in a linear Paul trap. The ions are generated by pulsed laser ablation and dynamically loaded into the ion trap by switching the trapping potential on and off. The loaded ions are precooled by buffer gas and then laser-cooled to form Coulomb crystals for verifying quantity. The number of ions is controlled by manipulating the trapping potential of the ion trap, partial pressure of buffer gas and turn-on time of the entrance end cap voltage. With single-pulse laser ablation, the number of trapped ions ranges from tens to ten thousand. The kinetic energy of loaded ions can be selected via the optimal turn-on time of the entrance end cap. Using multiple-pulse laser ablation, the number is further increased and reaches about 4×104 . The dynamic loading method has wide application for accumulating low-yielding ions via laser ablation in the ion trap.

A Miniature Orthogonal Injection Ion Funnel (MO-IF) Providing Enhanced Performance for the Miniature Mass Spectrometer.

The sensitivity of the miniature mass spectrometer (mini-MS) is largely restricted by the ion transmission in rough vacuum region. Even though various "in-line" ion transfer devices have improved mini-MS sensitivity, the severe dynamic gas is still weakening the efficiency of ion transmission in this region. Inspired by the "off-axis" ion funnel design in the lab-scale mass spectrometers, a miniature orthogonal injection ion funnel (MO-IF) was developed in this study for the mini-MS with a continuous atmospheric pressure interface. Capable of directing injected ions by 90° and then transport them forward to the downstream skimmer, the MO-IF enabled the separation of ions from the dynamic gas flow jetted out of the inlet capillary. The key factors were optimized for the MO-IF, including the effects of RF amplitude, DC electric fields, and the position of the repeller. Under optimized conditions, the MO-IF minimized the negative effects of dynamic gas and improved the ion transmission efficiency by ∼2-fold in comparison with the in-line injection ion funnel. As a result, a lower limit of detection of 0.5 ng/mL were obtained with good linearity for hypaconitine. Additionally, the MO-IF further decreased the buffer gas pressure in the second vacuum chamber and improved the mass resolution by 1.1-1.5 times at different scan rates.

Double-Cycle Alternating-Flow Diode Pumped Potassium Vapor Laser

A novel double-cycle alternating-flow diode-pumped potassium vapor laser is proposed, theoretically modeled and simulated. The results show that the optical-to-optical efficiency of the laser increases with increasing gas flow rates, although at high flow rates the rate of increase in efficiency decreases. The optical-to-optical efficiency reaches 74.8% at a pump power density of 30 kW/cm2 and a gas flow rate of 50 m/s. The optical-to-optical efficiency of the laser is greater with a narrow linewidth pump and high buffer gas pressure. The optical-to-optical efficiency of a flow gas cell is higher than that of a static gas cell. There is an optimal gas cell length that provides the highest optical-to-optical efficiency. At higher pump power densities, higher flow rates are required to obtain higher optical-to-optical efficiencies.

Contrast of Ramsey-CPT Fringes in Quenching and Depolarizing Gases

Molecular nitrogen is often used as a buffer gas in cells with alkali metals due to its known ability to quench resonant fluorescence. It is widely believed that the suppression of spontaneous emission decreases the width of coherent population trapping resonance. However, our recent results have not confirmed this positive action of molecular nitrogen within the typical range of 87Rb concentrations and buffer gas pressures. On the opposite, we have observed the negative influence of quenching, the decrease in contrast of the coherent population trapping resonance in $${{\sigma }^{ + }}$$–$${{\sigma }^{ + }}$$ configuration. In this work, we further confirm these results implementing the Ramsey spectroscopy, and compare the characteristics of the central fringe in nitrogen and neon, and show that the latter provides a significantly better contrast-to-width ratio.

Empirical Calibration of a Cylindrical Ion Trap for Mass-Selected Gas-Phase Fluorescence Spectroscopy.

The ion motion in a quadrupole ion trap of hyperbolic geometry is well described by the Mathieu equations. A simpler cylindrical ion trap has also gained significance and has been used by us for fluorescence-spectroscopy experiments. This design allows for the easy replacement of the end-cap with a mesh, enhancing the photon collection. It is crucial to obtain a firm understanding of the ion motion in cylindrical ion traps and their capability as mass spectrometers. We present here an empirical method of calibrating a cylindrical ion trap based on fluorescence detection. This can be done nearly background-free in a pulsed experiment. The ions are located at the center of the trap, where the field is primarily quadrupolar, and here an effective Mathieu description is found through an effective geometry parameter. In spectroscopy experiments, high buffer-gas pressures are needed to efficiently cool the ions, which complicates the ions' motion and hence their stability. Still, simulations show that the stability diagram closely aligns with the Mathieu diagram, albeit shifted due to collisions. We map the stability diagram for six molecular ions by fluorescence collection from four cations and two anions spanning m/z from 212 to 647. The stability diagram is parametrized through the Mathieu functions with an m/z-dependent effective geometry parameter and a q-dependent shrinkage of the diagram. Based on the calibration, we estimate the mass resolution to be +7/-3 Da for ions with masses in the hundreds of Da.

Dependence of Rydberg-atom-based sensor performance on different Rydberg atom populations in one atomic-vapor cell

The atomic-vapor cell is a vital component for Rydberg atomic microwave sensors, and impacts on overall capability of Rydberg sensors. However, the conventional analysis approach on effect of vapor-cell length contains two implicit assumptions, that is, the same atomic population density and buffer gas pressure, which make it unable to accurately capture actual response about effect of Rydberg-atom-based sensor performance on different Rydberg atom populations. Here, utilizing a stepped cesium atomic-vapor cell with five different dimensions at the same atomic population density and buffer gas pressure, the height and full width at half maximum of electromagnetically induced transparency (EIT) signal, and the sensitivity of the atomic superheterodyne sensor are comprehensively investigated under conditions of the same Rabi frequencies (saturated laser power). It is identified that EIT signal height is proportional to the cell length, full width at half maximum and sensitivity grow with the increment of cell length to a certain extent. Employing the coherent integration signal theory and atomic linear expansion coefficient method, theoretical analysis of the EIT height and sensitivity are further investigated. The results could shed new light on understanding and design of ultrahigh-sensitivity Rydberg atomic microwave sensors and find promising applications in quantum measurement, communication, and imaging.

A Conjoined Rectilinear Collision Cell and Pulsed Extraction Ion Trap with Auxiliary DC Electrodes.

Ion traps are routinely directly coupled to mass analyzers, where they serve to suitably cool and shape an ion population prior to pulsed extraction into the analyzer proper. Such devices benefit from high duty cycle and transmission but suffer slow ion processing times caused by a compromise in the buffer gas pressure range that suitably dampens the ion kinetic energy without causing excessive scatter during extraction or within the analyzer. A rectilinear RF quadrupole ion trap has been characterized, conjoining a pressurized collision region with a pumped extraction region, and an unbroken RF interface for seamless ion transfer between them. Auxiliary electrodes mounted between the RF electrodes provide DC voltage gradients that serve to both guide ions through the device and position them at the extraction slot. The influence of the auxiliary DC upon the trapping RF field was measured, and suitable parameters were defined. A mode of operation was developed that allowed parallel processing of ions in both regions, enabling a repetition rate of 200 Hz when the device was coupled to a high-resolution accurate-mass analyzer.

Electric arc synthesis of Al2O3 nanoparticles of various sizes

This article discusses the synthesis of hollow Al2O3 nanoparticles via electric arc spraying and subsequent material calcination in air. The effect of buffer gas pressure on the size and thickness of the synthesised hollow nanoparticle enclosure is investigated. Based on experimental data and numerical modelling data for the respective processes, a mechanism for the formation of various-sized nanoparticles based on changes in the residence time of carbon is proposed.

Nitrogen buffer gas pressure tuning in a micro-machined vapor cell

We demonstrate a controllable depletion of the nitrogen buffer gas pressure in a micro-machined cesium (Cs) vapor cell from the dynamic heating of an alkali dispenser pill. When the alkali source is laser activated, the gettering compounds within the alkali pill dispenser reduce the nitrogen (N2) content from the vapor for fine-tuning of the alkali to buffer gas pressure ratio, with a demonstrated pressure step size as low as 1 Torr. Additionally, we decrease the buffer gas pressure below 100 mTorr to evaluate the presence of other potential broadening mechanisms. Real-time control of the gas pressure ratio in the vapor cell will have notable benefits for refining atomic sensor performance and provide a routine to achieve various target pressures across a wafer bonded with a uniform back-filled buffer gas pressure.

An improved source of spin-polarized electrons based on spin exchange in optically pumped rubidium vapor

We have improved a polarized electron source in which unpolarized electrons undergo collisions with a mixture of buffer gas molecules and optically spin-polarized Rb atoms. With a nitrogen buffer gas, the source reliably provides spin polarization between 15% and 25% with beam currents >4 μA. Vacuum pump upgrades mitigate problems caused by denatured diffusion pump oil, leading to longer run times. A new differential pumping scheme allows the use of higher buffer gas pressures up to 800 mTorr. With a new optics layout, the Rb polarization is continuously monitored by a probe laser and improved pump laser power provides more constant high polarization. We have implemented an einzel lens to better control the energy of the electrons delivered to the target chamber and to preferentially select electron populations of higher polarization. The source is designed for studies of biologically relevant chiral molecule samples, which can poison photoemission-based GaAs polarized electron sources at very low partial pressures. It operates adjacent to a target chamber that rises to pressures as high as 10−4 Torr and has been implemented in a first experiment with chiral cysteine targets.

Dynamics of a laser plume plasma in an atmosphere of the He, Xe, Ar, and N2 buffer gases during ablation of a SiMn target

Investigations of the dependence of the ferromagnetism of MnxSi1-x (x ∼ 0.5) films on the energy spectrum of target plume particles are topical for spintronics. The time-of-flight (TOF) method using a Langmuir probe has been used to study the dynamics of a laser plume during ablation of a SiMn target in vacuum and in an atmosphere of He, Xe, Ar, N2 buffer gases in a wide pressure range from 10−6 to 4∙10−1 mbar. The TOF curves of the probe signal have been obtained by ablation of the SiMn target by the second harmonic of the Q-switched Nd:YAG laser. A nonmonotonic dependence of the probe signal amplitude on the buffer gas pressure was established. A feature of the delay of the TOF curve signal from the moment of target ablation as a function of the N2 pressure in the chamber was revealed, which was explained by the resonant scattering of the Si + ion and the N2 nitrogen molecule. The most suitable gases for control of the ion velocity were selected based on the dynamics of the laser plume. The dependence of the electrical characteristics of MnxSi1-x (x ∼ 0.5) films on the pressure of helium and argon has been established.

Analytical model of a diode-pumped cesium laser for investigation of upper-state mixing and quenching reactions.

A one-dimensional analytical model of a diode-pumped cesium vapor laser was constructed, and equations were derived to explain the dependence of the laser power on the hydrocarbon gas partial pressure. By varying the hydrocarbon gas partial pressure over a wide range and measuring the laser power, the corresponding mixing and quenching rate constants were validated. A gas-flow Cs diode-pumped alkali laser (DPAL) was operated with methane, ethane, and propane as buffer gases, with the partial pressures varied from 0 to 2 atmospheres. The experimental results were found to be in good agreement with the analytical solutions, confirming the validity of our proposed method. Separate 3-D numerical simulations were performed, and the output power effectively reproduced the experimental results over the entire buffer gas pressure range.

Optimizing 129Xe and 131Xe relaxation in an NMR gyroscope using buffer gas pressure and wall coating

The accuracy of inertial measurement performed by the nuclear magnetic resonance gyroscope (NMRG) with two isotopes depends on the duration of transverse relaxation. Extending the relaxation of the xenon isotopes at the same time plays a very important role in the accuracy of gyro. The relaxation time of 129Xe and 131Xe can be increased to about 15–20 s by optimizing the buffer gas pressure of N2 at about 0.57 amg and coating RbH, respectively. According to the results of theoretical analysis and experimentation, the gyro stability reaches 0.6°/h, and the active measurement volume is 3 × 3 × 3 ∼ mm3.

Temperature characteristics of Rb-N2 single-beam magnetometer with different buffer gas pressures

We demonstrate the temperature characteristics of Rb-N2 single-beam spin-exchange relaxation-free (SERF) magnetometer with different buffer gas pressures. An optimization model is proposed to reveal that the optimal condition is determined by cell temperature and gas pressure simultaneously, where the magnetometer response is maximized. Three Rb-N2 vapor cells under different gas pressures are constructed with interior volume of 3 × 3 × 3 mm3, and a single-beam zero-field modulation magnetometer is carried out. For each individual vapor cell, the optical power density and cell temperature are optimized to obtain the maximum response signal. The unique optimal condition including cell temperature and gas pressure is determined with 1.55 amg at 148 ℃, which verifies the proposed optimization model. This study is significative for optimizing the miniature vapor cell parameters and contributes to further improving the performance of SERF magnetometer.

Pulsed-CPT Cs-Ne microcell atomic clock with frequency stability below 2 × 10-12 at 105 s.

We report on the mid-term stability progress of a table-top coherent population trapping (CPT) microcell atomic clock, previously limited by light-shift effects and variations of the cell's inner atmosphere. The light-shift contribution is now mitigated through the use of a pulsed symmetric auto-balanced Ramsey (SABR) interrogation technique, combined with setup temperature, laser power, and microwave power stabilization. In addition, Ne buffer gas pressure variations in the cell are now greatly reduced through the use of a micro-fabricated cell built with low permeation alumino-silicate glass (ASG) windows. Combining these approaches, the clock Allan deviation is measured to be 1.4 × 10-12 at 105 s. This stability level at one day is competitive with the best current microwave microcell-based atomic clocks.

A high resolution Fourier transform ion trap enabled by image current splicing: a theoretical study.

The relatively high work pressure within an ion trap has limited the implementation of the Fourier transform technique for high resolution mass analysis. The main reason is that high buffer gas pressure will cause the rapid decay of ion oscillations. In this study, an image current splicing method based on the filter diagonalization method (FDM) and the Hilbert transform was developed to increase the resolving power of nondestructive mass analysis in a linear ion trap. First, multiple repeated experiments (or ion trajectory simulations) were performed to collect multiple sets of data. Using the FDM, the frequency component distribution was extracted from short image current transients collected from each experiment. The Hilbert transform was then applied to calculate and normalize the decay envelope of each transient. The relative abundance was calculated by counting the envelopes. Finally, image current transients collected from these multiple experiments were spliced and merged into a whole signal with much longer duration and continuous phase. This splicing method could effectively increase the duration of the image current, and thus improve the mass resolution of the ion trap mass analyzer. The mass resolution (m/Δm) was improved from 183.5 to 5.8 × 103, and the average relative difference was 2.8%. The proposed method resolved 3 adjacent peaks which originally could not be resolved from the raw signal by the fast Fourier transform (FFT). Besides simulated data, this method was also applied to the experimental data collected from a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The influence of electronic noise on the proposed method was also discussed in this study.

Optimal buffer gas pressure in dual-beam spin-exchange relaxation-free magnetometers

Optically pumped magnetometers (OPMs) operated in the spin-exchange relaxation-free (SERF) regime have emerged as prevailing biomagnetism measurement devices. The buffer gas in vapor cells can suppress wall collision relaxation but introduce unexpecting destruction relaxation, so as to be crucial to determine the magnetometer performance. In this study, we demonstrated the optimal buffer gas pressure of a dual-beam high sensitivity SERF magnetometer theoretically and experimentally. An optimization model was proposed revealing that the maximum response signal was dependent on the combined effects of the spin-relaxation rate and the pressure broadened linewidth of optical absorption. Six 3 mm × 3 mm × 3 mm Rb-N 2 vapor cells with different gas pressures were fabricated and constructed in a dual-beam SERF magnetometer to verify the optimal gas pressure model. The experimental results coincided well with the theoretical analysis, and a sensitivity of 4 fT/Hz 1/2 was achieved. • An optimal gas pressure exists to optimize the dual-beam SERF magnetometer performance. • The optimal pressures of the buffer and quench gases by establishing a calculation model for the Rb-N 2 magnetometer. • The comprehensive effects of nitrogen pressure on spin relaxation and pressure broadening are experimentally verified.

A Multi-Pass Optically Pumped Rubidium Atomic Magnetometer with Free Induction Decay.

A free-induction-decay (FID) type optically-pumped rubidium atomic magnetometer driven by a radio-frequency (RF) magnetic field is presented in this paper. Influences of parameters, such as the temperature of rubidium vapor cell, the power of pump beam, and the strength of RF magnetic field and static magnetic field on the amplitude and the full width at half maximum (FWHM) of the FID signal, have been investigated in the time domain and frequency domain. At the same time, the sensitivities of the magnetometer for the single-pass and the triple-pass probe beam cases have been compared by changing the optical path of the interaction between probe beam and atomic ensemble. Compared with the sensitivity of ∼21.2 pT/ in the case of the single-pass probe beam, the amplitude of FID signal in the case of the triple-pass probe beam has been significantly enhanced, and the sensitivity has been improved to ∼13.4 pT/. The research in this paper provids a reference for the subsequent study of influence of different buffer gas pressure on the FWHM and also a foundation for further improving the sensitivity of FID rubidium atomic magnetometer by employing a polarization-squeezed light as probe beam, to achieve a sensitivity beyond the photo-shot-noise level.