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arXiv:2201.06201

Focusing of an Atomic Beam

Doak, R. Bruce

Published Jan 16, 2022• 0 citations
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Abstract

A bent-crystal mirror has been used to focus an atomic helium beam. The mirror is made from gold deposited onto a mica substrate to form a thin film (~5000 Å thick) of large single crystal domains (domain size ~4000 Å ). The mica sheet is then bent in situ to form a cylindrical mirror of variable radius of curvature. Measurement of scattered beam intensity and angular distribution as a function of curvature demonstrate excellent focusing to within the mosaic spread of the surface. The reflectivity of the room temperature mirror is about ten percent. Potential uses are discussed, ranging from immediate applications in intensity and resolution enhancement of helium scattering experiments, to such long term possibilities as hyper-cooling of the beam or fabrication of a scanning atom microscope.

DOI
10.48550/arxiv.2201.06201

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A bent-crystal mirror has been used to focus an atomic helium beam. The mirror is made from gold deposited onto a mica substrate to form a thin film (~5000 Å thick) of large single crystal domains (domain size ~4000 Å ). The mica sheet is then bent in situ to form a cylindrical mirror of variable radius of curvature. Measurement of scattered beam intensity and angular distribution as a function of curvature demonstrate excellent focusing to within the mosaic spread of the surface. The reflectivity of the room temperature mirror is about ten percent. Potential uses are discussed, ranging from immediate applications in intensity and resolution enhancement of helium scattering experiments, to such long term possibilities as hyper-cooling of the beam or fabrication of a scanning atom microscope.
A bent-crystal mirror has been used to focus an atomic-helium beam. The mirror is made from gold deposited onto a mica substrate to form a thin epitaxial film (5000 Å thick) of large single-crystal domains (domain size ~ 4000 Å). The mica sheet is then bent in situ to form a cylindrical mirror of variable radius of curvature. Measurement of scattered beam intensity and angular distribution as a function of curvature demonstrates focusing to within the mosaic spread of the surface. The reflectivity of the room temperature mirror is about 10%. Potential uses are discussed, ranging from immediate applications in intensity and resolution enhancement of helium-scattering experiments to such long-term possibilities as hypercooling of the beam or fabrication of a scanning atom microscope.
A large-aperture liquid-He coated spherical mirror and a room-temperature bent-gold crystal have been used to focus atom beams of H and He, respectively. They are described elsewhere in this symposium. Many of the new optical elements for atoms rely on the mechanical forces of light; mirrors are one example. When atoms approach the surface of glass that has intense blue-detuned light inside, they recoil from the evanescent wave that extends several wavelengths past the surface and into the vacuum. The atoms are never close enough to the surface to be affected by its atomic attraction or its small-scale irregularities. When a highly collimated beam of sodium atoms is directed at a standing wave of light near its resonance frequency, the angular scattering pattern of the atoms exhibits the characteristics of diffraction from a short sinusoidal grating. Diffraction orders up to ~20 have been observed at high light intensities. In contrast to phase gratings made with light, material absorption gratings are amplitude gratings.
Manipulation of atomic and molecular beams is essential to atom optics applications including atom lasers, atom lithography, atom interferometry and neutral atom microscopy. The manipulation of charge-neutral beams of limited polarizability, spin or excitation states remains problematic, but may be overcome by the development of novel diffractive or reflective optical elements. In this paper, we present the first experimental demonstration of atom focusing using an ellipsoidal mirror. The ellipsoidal mirror enables stigmatic off-axis focusing for the first time and we demonstrate focusing of a beam of neutral, ground-state helium atoms down to an approximately circular spot, (26.8±0.5) μm × (31.4±0.8) μm in size. The spot area is two orders of magnitude smaller than previous reflective focusing of atomic beams and is a critical milestone towards the construction of a high-intensity scanning helium microscope.
Neutral Helium atoms have significant potential for use as a probe in a matter-wave microscope. Their low energy scattering interactions with surfaces are well understood within the field of Helium Atom Scattering, where they have been shown to be sensitive to the presence of adsorbates, surface phonons and metalisation of surfaces. A focused probe of neutral Helium atoms could exploit this sensitivity and provide spatially resolved information for surfaces when scanned in two dimensions, thereby realising a Scanning Helium Microscope (SHeM). The ultimate resolution of a SHeM is set by the size of the Helium focus, and is ultimately limited by the sub-angstrom de Broglie wavelength of thermal Helium atoms. This thesis describes progress towards the demonstration of a SHeM using optical potential lenses to focus ground state neutral Helium atoms. The AC Stark effect is exploited to create an optical potential lens to focus a supersonic Helium beam. This is the first experimental demonstration of neutral Helium focusing using far off resonant fields. In addition, a high efficiency detection scheme for neutral Helium atoms (50%) using multiphoton ionisation is developed for use in SHeM. This represents a significant improvement on the very poor efficiency of electron bombardment detection schemes commonly used in Helium experiments. This detection scheme offers potentially sub-micron spatial resolution, and nanosecond temporal resolution.
We use a concave spherical mirror to focus at 18-mm-diam beam of H atoms down to 0.5 mm. The mirror consists of a fused-quartz substrate polished to optical precision and coated with a liquid${\mathrm{\ensuremath{-}}}^{4}$He film to obtain high reflectivity. The temperature dependence of the focused beam intensity enables us to study the influence of the dynamic surface roughness on the reflection of the H atoms. Both zero-point fluctuations and thermal excitations turn out to be of importance. A monolayer of 3He^{3}\mathrm{He}3He does not significantly affect the results.
Focusing optics for neutral molecules finds application in shaping and steering molecular beams. Here we present an electrostatic elliptical mirror for polar molecules consisting of an array of microstructured gold electrodes deposited on a glass substrate. Alternating positive and negative voltages applied to the electrodes create a repulsive potential for molecules in low-field-seeking states. The equipotential lines are parallel to the substrate surface, which is bent in an elliptical shape. The mirror is characterized by focusing a beam of metastable CO molecules and the results are compared to the outcome of trajectory simulations.