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arXiv:0806.4847 · Physical Review A

Quantum reflection of helium atom beams from a microstructured grating

Bum Suk Zhao, Stephan A. Schulz, Samuel A. Meek, Gerard Meijer, Wieland Schöllkopf

Published Jul 29, 2008• 48 citations
arXivHTMLPDF

Abstract

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 $20\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}$. 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.

DOI
10.1103/physreva.78.010902

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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\ensuremath-\ensuremathμm10\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}10\ensuremath-\ensuremathμm-wide Cr strips patterned on a quartz substrate and has a periodicity of 20\ensuremathμm20\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}20\ensuremathμm. 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.
The quantum reflection measured previously by Zhao et al. ( Phys. Rev. A 2008 , 78 , 010902(R) ) for the scattering of He atoms off of a microstructured grating is described and analyzed theoretically. Using the close-coupling formalism with a complex absorbing potential and describing the long-range interaction in terms of the Casimir-van der Waals potential, we find probabilities and diffraction patterns that are in fairly good agreement with the experimental results. The central outcomes of this study are two-fold. First is the theoretical confirmation that, indeed, the phenomenon of quantum reflection may be detected not only through the elastic peak but also in terms of a quantum reflected diffraction pattern. Second, we demonstrate that the phenomenon of quantum reflection is the result of a coherent process where all of the potential regions are involved on an equal footing. It is a nonlocal property and cannot be related only to the long-range badlands region of the potential of interaction.
We report on the non-destructive scattering and diffraction of He, He2 and He3 from a plane ruled reflection grating. At grazing incidence the normal component of the particle’s wave-vector is sufficiently small to allow for quantum reflection at the attractive Casimir–van der Waals particle–surface interaction potential. Quantum reflection occurs tens of nanometres in front of the surface, before the dimers and trimers reach the region where the surface-induced forces would inevitably cause the breakup of the fragile bonds. The reflected particles are identified via their mass-dependent diffraction angles and by mass spectrometry. The intensity distributions of the observed diffraction patterns are discussed in terms of the grating’s blaze angle.
We report coherent reflection of thermal He atom beams from various microscopically rough surfaces at grazing incidence. For a sufficiently small normal component k(z) of the incident wave vector of the atom the reflection probability is found to be a function of k(z) only. This behavior is explained by quantum reflection at the attractive branch of the Casimir-van der Waals interaction potential. For larger values of k(z) the overall reflection probability decreases rapidly and is found to also depend on the parallel component k(x) of the wave vector. The material specific k(x) dependence for this classic reflection at the repulsive branch of the potential is discussed in terms of an averaging out of the surface roughness under grazing incidence conditions.
Thermal-energy atom scattering at a surface under grazing incidence conditions is an innovative method for investigating dispersive atom-surface interactions with potential application in quantum sensing interferometry. The complete establishment of this technique requires a detailed peak analysis, which is yet to be achieved. We examine peak-width fluctuations in atomic and molecular beams diffracted by a grating under grazing incidence conditions. Careful measurements and analyses of the diffraction patterns of He atoms and D2{\mathrm{D}}_{2}D2​ molecules from three square-wave gratings with different periods and radii of curvature enable the identification of factors influencing the peak-width variations as a function of incidence angle. The effects of macroscopic surface curvature, grating magnification, and beam emergence are substantial under these conditions but negligible for incidence angles close to the normal. Our results shed light on the phenomena occurring in grazing incidence thermal-energy atom scattering.
Abstract Molecular beams of He and D2 are scattered from a ruled diffraction grating in conical‐mount geometry under grazing‐incidence conditions. Fully resolved diffraction patterns as a function of detection angle are recorded for different grating azimuth angles and for two different kinetic energies of the particle beams. Variations in diffraction peak widths are traced back to different velocity spreads of He and D2 determined by time‐of‐flight measurements. A comprehensive analysis of diffraction intensities confirms universal diffraction, that is, for identical de Broglie wavelengths, the relative diffraction intensities for He and D2 are the same. Universal diffraction results from peculiarities of quantum reflection of the atoms and molecules from the diffraction grating. In quantum reflection particles scatter many nanometers in front of the surface from the long‐range attractive branch of the particle–surface interaction potential without probing the potential well and the short‐range repulsive branch of the potential.
The coherent reflection of ultra-cold neon and helium atoms from silicon surfaces and surface structures is measured and analyzed. When the silicon surface is flat, the reflection is explained by the quantum reflection due to van der Waals attractive potential near the silicon surface. When the silicon surface is modified to form a grating structure, the reflectivity increases dramatically. For certain ranges of surface structure parameters, the reflection may be interpreted as a coherent summation of Fresnel diffraction of the atomic wave at the ridges of the grating structure.