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Physical Review Letters

Quantum reflection: Focusing of hydrogen atoms with a concave mirror

J. J. Berkhout, O.J. Luiten, Irwan D. Setija, Tom W. Hijmans, T. Mizusaki, Jook T. M. Walraven

Published Oct 16, 1989• 141 citations
Publisher

Abstract

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 $^{3}\mathrm{He}$ does not significantly affect the results.

DOI
10.1103/physrevlett.63.1689

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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\ensuremath−4{\mathrm{\ensuremath{-}}}^{4}\ensuremath−4He 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.
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 studied the low-energy limit of atomsurface scattering and the phenomenon of quantum reflection by using hydrogen atoms and the surface of liquid helium. Measuring the specular reflectivity of a liquid-helium-covered hemispheric mirror for a beam of hydrogen atoms we obtained information on elastic and inelastic scattering phenomena. At 100 mK, reflectivities larger than 90% were achieved; this can be used to demonstrate atomic beam focusing. Approximately three-fourths of the atoms escaping from a small orifice in a cell were reflected back into the cell. The dominant inelastic processes at work here are surface absorption and direct inelastic scattering over small angles under excitation of surface ripplons (quantized capillary waves). Helium-covered surfaces maybe useful for atomic-beam manipulation of atomic hydrogen.
We formed the first ``atomic-optics'' beam of electron-spin-polarized hydrogen atoms using a quasiparabolic polished copper mirror coated with a hydrogen-atom-reflecting film of superfluid 4He^{4}\mathrm{He}4He. The mirror was located in the gradient of an 8-T solenoidal magnetic field and mounted on an ultracold cell at 350 mK. After the focusing by the mirror surface, the beam was again focused with a sextupole magnet. The mirror, which was especially designed for operation in the magnetic field gradient of our solenoid, increased the focused beam intensity by a factor of about 7.5.
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.
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 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.