Atomic Beam Research Articles

Contrast inversion in neutral-atom microscopy using atomic cluster beams

This work explores the possibility of using atomic cluster beams as a probe for neutral-atom microscopy (NAM) measurements. Using a beam of Kr clusters with mean size $\ensuremath{\sim}{10}^{4}$ atoms/cluster we demonstrate that topographical contrast can be obtained, similar to that in the case of monoatomic beams. Further, using atomically thin films of ${\mathrm{MoS}}_{2}$ grown on ${\mathrm{SiO}}_{2}\text{/}\mathrm{Si}$ substrate we show that NAM imaging using Kr clusters is also possible in domains where topographical contrast is not expected. Surprisingly, these images show an inverted contrast pattern when compared to the case of monoatomic beams. We attempt to understand these observations on the basis of angular distributions resulting from cluster-surface scattering. Finally, we discuss the implications of these results toward achieving a high lateral resolution neutral-atom microscope using atomic cluster beams as probe, with an estimated ultimate achievable lateral resolution up to 20 nm.

A chip-scale atomic beam clock

Atomic beams are a longstanding technology for atom-based sensors and clocks with widespread use in commercial frequency standards. Here, we report the demonstration of a chip-scale microwave atomic beam clock using coherent population trapping (CPT) interrogation in a passively pumped atomic beam device. The beam device consists of a hermetically sealed vacuum cell fabricated from an anodically bonded stack of glass and Si wafers in which lithographically defined capillaries produce Rb atomic beams and passive pumps maintain the vacuum environment. A prototype chip-scale clock is realized using Ramsey CPT spectroscopy of the atomic beam over a 10 mm distance and demonstrates a fractional frequency stability of ≈1.2 × 10−9/sqrt{tau } for integration times, τ, from 1 s to 250 s, limited by detection noise. Optimized atomic beam clocks based on this approach may exceed the long-term stability of existing chip-scale clocks, and leading long-term systematics are predicted to limit the ultimate fractional frequency stability below 10−12.

Removal of Wear-Resistant Coatings from Cutting Tools by Fast Argon Atoms

Wear-resistant coatings improve the machining capability of cutting tools and extend their useful life. However, when a tool needs to be reused, it is mandatory to remove the existing coating to facilitate resharpening and recoating. The existing technique uses electrochemical stripping, which is hazardous to the environment. The environmentally friendly pulsed laser stripping causes the melting and mixing of tools and coating materials, which makes it difficult to separate and remove the coating. This paper presents the results of coating stripping via a beam of fast argon atoms. Due to the twentyfold compression of the beam, a 3 µm thick AlTiN coating was removed from a rotating solid carbide end mill within 25 min. A subsequent one-hour-long irradiation of the cleaned tool with the same beam led to a decrease in the radius of the tool’s cutting edges from 10.5 to 3.5 µm. This allowed us to redeposit a 3.5 µm thick AlTiN coating and obtain a coated end mill with a cutting-edge radius of 7 µm.

Measurements of velocity-selective resonances from adiabatic rapid passage

The authors measure the velocity dependence of applied optical forces on a metastable He atom beam. The optical force implemented using adiabatic rapid passage shows unexpected, equally spaced peaks in velocity space. Such resonances may result from coherence established by atomic motion in traveling-wave laser fields.

Ammonia production in a dual crossed atom beam experiment

Production of ammonia by surface reactions of H and N atoms on surfaces not wetted by partially ionized plasma may represent an important technological issue in fusion reactors where puffing nitrogen is employed to cool plasma in the divertor region. The H and N atoms are likely to interact on such surfaces forming NH3 molecules. The interaction efficiency was studied in a laboratory setup consisting of two separate sources of either N or H atoms. Both sources enabled experiments with atoms at room temperature in the range of H-atom density of the order of 1021 m−3 and N-atom density of the order of 1020 m−3. The production of ammonia was measured with a calibrated residual gas analyser. The production depended on the fluxes of both atoms onto the surface of selected materials. As a general rule, the higher H-atom flux at a constant N-atom flux caused an increase in ammonia production. The highest efficiency of up to 50% was found for nickel. It was up to 30% for tungsten, whereas for P92 alloy, it was up to about 20%. The accuracy of these results is within about ±20% of the measured values. Methods for suppressing ammonia formation in fusion reactors will have to be invented in order to enable appropriate long-term operation.

Calibrating beam fluxes of a low-energy neutral atom beam facility.

Scientific detection and imaging instruments for low-energetic neutral atoms (ENA) onboard spacecraft require thorough pre-flight laboratory calibration against a well-characterized neutral atom beam source. To achieve this requirement, a dedicated test facility is available at the University of Bern, which is equipped with a powerful plasma ion source and an ion beam neutralization stage. Using surface neutralization, low-energy neutral atom beams of any desired gas species can be produced in the energy range from 3keV down as low as 10eV. As the efficiency of the neutralization stage is species and energy dependent, the neutralizer itself needs to be calibrated against an independent reference. We report on the calibration and characterization of this neutral atom beam source using our recently developed Absolute Beam Monitor (ABM) as a primary calibration standard. The ABM measures the absolute ENA flux independent of neutral species in the energy range from 10eV to 3keV. We obtain calibration factors of a few 100cm-2s-1 pA-1, depending on species at beam energies above about 100eV, and a power-law decrease for energies below 100eV. Furthermore, the energy loss of neutralized ions in the surface neutralizer is estimated from time-of-flight measurements using the ABM. The relative energy loss increases with ENA energy from low levels near zero up to 20%-35% at 3keV, depending on atomic species. Having calibrated our neutral beam source allows for accurate calibration of ENA space instruments.

Wall stabilization of high-beta anisotropic plasmas in an axisymmetric mirror trap

The stabilization of the ‘rigid’ flute and ballooning modes both with and without the effect of additional MHD anchors in an axisymmetric mirror trap, with the help of a perfectly conducting lateral wall, is studied using a model pressure distribution of a plasma with a steep-angle neutral beam injection at the magnetic field minimum. The calculations were performed for an anisotropic plasma in a model that simulates the pressure distribution during the injection of beams of fast neutral atoms into the magnetic field minimum. It is assumed that the lateral wall is an axisymmetric shell surrounding the plasma that follows the shape of the magnetic field line and is placed at a certain distance to the plasma border. It has been found that for the effective stabilization of the modes by the lateral wall only the parameter beta (β, ratio of plasma pressure to the magnetic field pressure) must exceed some critical value β crit. When combined with the conducting end plates imitating the MHD end stabilizers, there are two critical beta values and two stability zones and that can merge, making the entire range of allowable beta values stable. The dependence of the critical betas on the degree of plasma anisotropy, the mirror ratio, and the width of the vacuum gap between the plasma and the lateral wall is studied. In contrast to the previous studies focusing on a plasma model with a sharp boundary, we calculated the stability zones for a number of diffuse radial pressure profiles and several axial magnetic field profiles.

Magnetron sputter deposition of Ta2O5-SiO2 quantized nanolaminates.

Quantized nanolaminates are a type of optical metamaterials, which were discovered only recently. Their feasibility was demonstrated by atomic layer deposition and ion beam sputtering so far. In this paper, we will report on the successful magnetron sputter deposition of quantized nanolaminates based on Ta2O5-SiO2. We will describe the deposition process, show results and material characterization of films deposited in a very wide parameter range. Furthermore, we will show how quantized nanolaminates deposited by magnetron sputtering were used in optical interference coatings such as antireflection and mirror coatings.

Effect of Ar fast atom beam irradiation on alpha-Al2O3 for surface activated room temperature bonding

This study focuses on the surface-activated bonding of sapphire (alpha-Al2O3) wafers at RT. In the surface activation process, Ar fast atom beam (FAB) irradiation is used as a physical sputtering method. The bond strength estimated by the crack opening method is approximately 1.63 J m−2. The binding state of the activated alpha-Al2O3 surface is determined using angle-resolved X-ray photoelectron spectroscopy. The results reveal the existence of two binding energies of Al2p (73.9 and 74.0 eV) on the surface of the FAB-irradiated wafer, indicating that the surface activation changes the binding state of the utmost alpha-Al2O3 surface. This implies that the contact of the changed Al2O3 surface contributes to the formation of a strong bond interface.

Doppler-free spectroscopy of an atomic beam probed in traveling-wave fields

The authors propose and demonstrate a method for precision spectroscopy of atoms in a beam using two sequential rather than simultaneous laser pulses, thereby overcoming systematic challenges associated with standing-wave beams that plague other methods. Their results show that this method can effectively suppress the first-order Doppler effect and reduce systematic uncertainty.

Relative In-flight Response of IBEX-Lo to Interstellar Neutral Helium Atoms

The IBEX-Lo instrument on the Interstellar Boundary Explorer (IBEX) mission measures interstellar neutral (ISN) helium atoms. The detection of helium atoms is made through negative hydrogen (H−) ions sputtered by helium atoms from the IBEX-Lo’s conversion surface. The energy spectrum of ions sputtered by ISN helium atoms is broad and overlaps the four lowest IBEX-Lo electrostatic analyzer (ESA) steps. Consequently, the energy response function for helium atoms does not correspond to the nominal energy step transmission. Moreover, laboratory calibration is incomplete because it is difficult to produce narrow-energy neutral atom beams that are expected for ISN helium atoms. Here, we analyze the ISN helium observations in ESA steps 1–4 to derive the relative in-flight response of IBEX-Lo to helium atoms. We compare the ratios of the observed count rates as a function of the mean ISN helium atom energy estimated using the Warsaw Test Particle Model (WTPM). The WTPM uses a global heliosphere model to calculate charge exchange gains and losses to estimate the secondary ISN helium population. We find that the modeled mean energies of ISN helium atoms, unlike their modeled fluxes, are not very sensitive to the very local interstellar medium parameters. The obtained relative responses supplement the laboratory calibration and enable more detailed quantitative studies of the ISN helium signal. A similar procedure that we applied to the IBEX-Lo observations may be used to complement laboratory calibration of the next-generation IMAP-Lo instrument on the Interstellar Mapping and Acceleration Probe (IMAP) mission.

A new insight into the mechanical properties of nanobiofibers and vibrational behavior of atomic force microscope beam considering them as the samples

Nowadays, the utilization and production of polymeric nanofibers are expanding rapidly due to their unique and exclusive properties. The current study examined silk fibroin nanofibers, poly-L lactic acid (PLLA), and polycaprolactone (PCL) as three widely-used biopolymers. In the first section, the mechanical properties of nanobiofibers, including elastic modulus as the first research variable, were investigated using atomic force microscopy (AFM)-based nanoindentation and tensile testing (through a universal testing machine). The results indicated that increasing the concentration of the biopolymer solution increased its elastic modulus. The total amount of elastic modulus for silk fibroin (SF), PCL, and PLLA nanofibers with random and aligned scaffold formats ranged from 1.207 MPa to 3.71 MPa. Based on the molecular weight and concentrations of the solutions, PLLA, PCL, and silk fibroin nanofibers yielded the highest elastic modulus, respectively. In addition, the elastic modulus was greater in an aligned scaffold as opposed to a random format and in retraction as opposed to an extended stroke. During the second section of the study, resonant frequencies as the second research variable and the amplitude of the FRF of the AFM beam's vertical movements using nanobiopolymers with various scaffolds as the samples were investigated theoretically (using the Finite Element Method [FEM]) and experimentally (using AFM). For the first to third modes, the resonant frequency of the AFM beam considering biopolymer nanofibers as samples varied from 12658.43 Hz to 241208.4 Hz. The results revealed that increasing the elastic modulus of the samples increases the resonant frequency. The mechanical and vibrational properties of nanofibers calculated by various methods were subsequently compared separately. Overall, the comparison indicated good agreement between the properties.

A compact and highly collimated atomic/molecular beam source

We describe the design, characterization, and application of a simple, highly collimated, and compact atomic/molecular beam source. This source is based on a segmented capillary design, constructed using a syringe needle. Angular width measurements and free molecular flow simulations show that the segmented structure effectively suppresses atoms traveling in off-axis directions, resulting in a narrow beam of Helium atoms having a width of 7 mrad (full width half maximum). We demonstrate an application of this source by using it for monitoring real-time changes in surface coverage on a clean Cu(110) surface exposed to oxygen by measuring the specular reflectivity of the Helium beam generated using this source.

A new magnetic state selection method in high-performance optically detected compact cesium beam clocks

We perform a new scheme of magnetic state selection in optically detected compact cesium beam clocks. Unlike the conventional method, we select atoms in the ground state |F = 4, mF ≠ −4⟩ by pointing the atomic collimator to the convex pole of the magnet realizing the two-wire magnetic field and detect atoms in |F = 3⟩ after interacting with the microwave field using a distributed feedback laser. The fluorescence background is greatly reduced as the inherent residual atoms |F = 4, mF = −4⟩ are avoided in this reversed scheme. The velocity distribution is narrowed, and the most probable velocity is decreased, since atomic trajectories are close to the weak-field region. We also investigate the relationship between the position of the atomic collimator and the distributions of the atomic beam, which is consistent with the Monte Carlo-based simulation model. By applying the reversed scheme and setting the deviated position of the collimator to 1.3 mm, the signal contrast is improved from 0.7 to 3, and the short-term frequency stability reaches 3.0 × 10−12 τ−1/2, nearly three times better than that of the high-performance version of Microsemi 5071A.

Continuous-variable polarization mode entanglement in a V-type micromaser

The micromaser is an archetype experimental setting where a beam of excited two-level atoms is injected into a high-finesse cavity. It has played a pivotal role as a testbed for predictions of quantum optics. We consider a generalized micromaser setting consisting of a high-quality cavity pumped by a beam of three-level atoms. The atoms are assumed to be prepared to carry quantum coherence between their excited state doublet. Our objective is to produce quantum entanglement between the right-handed circular (RHC) and left-handed circular (LHC) polarized photons in the cavity, exploiting the quantum coherence in the pump atoms. For that aim, we derive the generalized micromaser master equation for our system. We find that the dynamics of the micromaser field driven by the pump beam is equivalent to two non-interacting RHC and LHC photonic systems sharing a common non-equilibrium environment. The effect of the shared bath is to mediate an incoherent interaction between the otherwise non-interacting cavity photons, which emerges only if the atoms carry quantum coherence. We take into account cavity losses as a source of quantum decoherence and characterize the quantum entanglement between the LHC and RHC polarized photons in terms of logarithmic negativity, Hillery–Zubairy and spin squeezing criterion, calculated using the dynamical solution of the master equation. We show that, in the same parameter regime, one of the criteria shows entanglement while for the other never detects entanglement. Our results reveal that LHC and RHC polarized photons can be entangled in the transient regime according to the logarithmic negativity criterion.

Correlations and linewidth of the atomic beam continuous superradiant laser

We propose a minimalistic model to account for the main properties of a continuous superradiant laser, in which a beam of atoms crosses the mode of a high-finesse Fabry-Perot cavity, and collectively emits light into the cavity mode. We focus on the case of weak single atom - cavity cooperativity, and highlight the relevant regime where decoherence due to the finite transit time dominates over spontaneous emission. We propose an original approach where the dynamics of atoms entering and leaving the cavity is described by a Hamiltonian process. This allows deriving the main dynamical equations for the superradiant laser, without the need for a stochastic approach. We derive analytical conditions for a sustained emission and show that the ultimate linewidth is set by the fundamental quantum fluctuations of the collective atomic dipole. We calculate steady-state values of the two-body correlators and show that the continuous superradiant regime is tied to the growth of atom-atom correlations, although these correlations only have a small impact on the laser linewidth.

Polychromatic atom optics for atom interferometry

Coherent manipulation of atoms with atom-optic light pulses is central to atom interferometry. Achieving high pulse efficiency is essential for enhancing fringe contrast and sensitivity, in particular for large-momentum transfer interferometers which use an increased number of pulses. We perform an investigation of optimizing the frequency domain of pulses by using tailored polychromatic light fields, and demonstrate the possibility to deliver high-efficiency and resilient atom-optic pulses even in the situation of inhomogeneous atomic clouds and laser beams. We find that this approach is able to operate over long interrogation times despite spontaneous emission and to achieve experimentally relevant pulse efficiencies for clouds up to 100~mu text{K}. This overcomes some of the most stringent barriers for large-momentum transfer and has the potential to reduce the complexity of atom interferometers. We show that polychromatic light pulses could enhance single-photon-based large-momentum transfer atom interferometry—achieving 850,hbar k of momentum splitting with experimentally accessible parameters, which represents a significant improvement over the state-of-the art. The benefits of the method extend beyond atom interferometry and could enable groundbreaking advances in quantum state manipulation.

Characterization of a Continuous Beam Cold Atom Ramsey Interferometer

The use of atom interferometers in inertial systems holds the promise of improvement of several orders of magnitude in sensitivity over sensors using current technology such as micro-electro-mechanical (MEMS) devices or ring laser gyroscopes (RLGs). This paper describes the construction and characterization of an atomic interferometry system for eventual use in a dual-atom-beam accelerometer/gyroscope sensor. In contrast with current state-of-the-art atomic sensors which use pulsed cold atom sources and pulsed laser beams, the investigated apparatus relies purely on continuous atomic and laser beams. These differences can result in a sensor with reduced complexity, a smaller physical footprint, and reduced power consumption. However, these differences also introduce challenges resulting from laser and atomic beam divergences and from velocity averaging due to both longitudinal and transverse velocity spreads. In this work, we characterize our rubidium-based atom beam system and show that Ramsey-style interference can still be observed. The implications for future research are also outlined and discussed.

Fabrication of 53.2 nm pitch self-traceable gratings by laser-focused atomic deposition combined with extreme ultraviolet interference lithography

The period of the self-traceable grating has extremely high accuracy and minimal uncertainty due to its traceability to natural constants. It is an essential task to shorten the period of self-traceable grating to below 100 nm. In this paper, the self-traceable Cr grating with a period of 212.8 nm prepared by laser-focused atomic deposition (LFAD) is used as the mask, and the second-order frequency doubling (k=2) of the Cr grating is performed by extreme ultraviolet interference lithography (EUV IL). The Cr grating is designed for the best second-order diffraction efficiency. By optimizing the orthogonality between the laser standing wave and the atomic beam, a Cr grating with a peak-to-valley height of 60 nm is prepared. After exposure and development with PMMA 672.01 photoresist, the Si grating is finally obtained by pattern transfer. And its average pitch is evaluated as (53.2 ± 0.1) nm. The exposure results of different photoresists and the uniformity of grating patterns are discussed. The peak-to-valley height of the Si grating is about 50 nm, and the full width at half maximum is about 20 nm. This research provides the possibility for the wider application of self-traceable gratings in advanced nanofabrication and precision measurement.

Grazing incidence fast atom diffraction in high-pressure conditions

Grazing Incidence Fast Atom Diffraction (GIFAD) is a recent technique for characterizing surface structures and real-time monitoring of thin film growth. Up to now, GIFAD has only been used in Ultra-High-Vacuum conditions, typically in the range of 10−10 to 10−8 mbar, and has therefore only been considered for high vacuum deposition methods like Molecular Beam Epitaxy or very low-pressure Chemical Vapor Deposition (CVD). At pressures exceeding 10−6 mbar, gas phase collisions along the atom beam trajectory not only reduce the mean free path but also degrade the beam coherence length and thus potentially suppress the diffraction signal. In addition, pressures lower than 10−5 mbar are required to maintain a low noise level on the scattered particle detector. In a new configuration, we demonstrate that GIFAD can operate at pressure as high as 10−2 mbar of argon with well-contrasted diffraction patterns. This opens wide avenues for the study of surface reactivity, thin film growth in Magnetron Sputtering Deposition, where electron diffraction is inevitably perturbed by the electromagnetic fields. This High-Pressure version of GIFAD could also be extended to Reactive Pulsed Laser Deposition and many CVD variants.