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New Journal of Physics

Focusing a helium atom beam using a quantum-reflection mirror

H. Christian Schewe, Bum Suk Zhao, Gerard Meijer, Wieland Schöllkopf

Published Nov 17, 2009• 16 citations
Publisher

Abstract

We demonstrate one-dimensional (1D) focusing of a thermal helium atom beam by quantum reflection from a cylindrical concave quartz mirror at near-grazing incidence. The smallest width of the focus achieved is 1.8 μm, essentially limited by spherical aberration. The various effects that contribute to the finite focal width have been investigated. We propose to apply near-grazing reflection from two concave elliptical mirrors in a Kirkpatrick–Baez arrangement for two-dimensional (2D) focusing of a helium atom beam, paving the way for a helium atom microprobe.

DOI
10.1088/1367-2630/11/11/113030

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We demonstrate one-dimensional (1D) focusing of a thermal helium atom beam by quantum reflection from a cylindrical concave quartz mirror at near-grazing incidence. The smallest width of the focus achieved is 1.8 μm, essentially limited by spherical aberration. The various effects that contribute to the finite focal width have been investigated. We propose to apply near-grazing reflection from two concave elliptical mirrors in a Kirkpatrick–Baez arrangement for two-dimensional (2D) focusing of a helium atom beam, paving the way for a helium atom microprobe.
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.
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.
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.
We observe high-resolution diffraction patterns of a thermal-energy helium atom beam reflected from a microstructured surface grating at grazing incidence. The grating consists of $10\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-wide Cr strips patterned on a quartz substrate and has a periodicity of $20\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Fully resolved diffraction peaks up to the seventh order are observed at grazing angles up to 20mrad20\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}20mrad. With changes in de Broglie wavelength or grazing angle the relative diffraction intensities show significant variations which shed light on the nature of the atom-surface interaction potential. The observations are explained in terms of quantum reflection at the long-range attractive Casimir--van der Waals potential.
Reflective atom optics has been experimentally realized with a continuous flux of cold atoms by using a magnetized videotape. We have studied the behaviour of a slow atomic beam with a velocity of 20 m s(-1) incident at different angles on the surface of a curved magnetic mirror. Focusing of the beam has been demonstrated and this induces an amplification up to a factor 3 of the fluorescence emitted by the atoms around the focal point. We deduced the shape of our reflector from the focal length measurements and show that the result is completely consistent with those obtained from the reflection of a red diode laser beam on the curved mirror.
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.