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New Journal of Physics

An ellipsoidal mirror for focusing neutral atomic and molecular beams

Katrin Fladischer, Herbert Reingruber, Thomas J.G. Reisinger, V Mayrhofer, Wolfgang Ernst, A. Ross, Donald A. MacLaren, William Allison, Dariusz Litwin, Jacek Galas, Stefan Sitarek, Pablo Nieto, Daniel Barredo, Daniel Farı́as, Rodolfo Miranda, B. Surma, A Miros, B. Piątkowski, Elin Søndergard, Bodil Holst

Published Mar 11, 2010• 31 citations
Publisher

Abstract

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.

DOI
10.1088/1367-2630/12/3/033018

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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.
The manipulation of low-energy beams of neutral atoms and molecules via their de Broglie wavelength is a branch of atom optics often referred to as de Broglie matter wave optics. The application areas include fundamental quantum mechanics, atom interferometry, and the development of new microscopy instrumentation. The focusing of de Broglie matter waves with a Fresnel zone plate was used to demonstrate the first neutral helium microscopy imaging. The ultimate resolution of such a microscope is limited by the width of the outermost zone. Because a Fresnel zone plate for atoms cannot be fabricated on a substrate (the low-energy atom beams would not be able to penetrate the substrate material), this gives a fabrication determined limit for the first-order focus of around 30--50 nm. Therefore, it is important to search for alternative optical elements that enable higher resolution. Photon sieves consist of a large number of pinholes, arranged suitably relative to the Fresnel zones. The great advantages are that the width of the pinholes can be larger than the respective Fresnel zones and a free-standing pinhole is much easier to fabricate than a free-standing zone. Thus, with a photon-sieve structure applied for de Broglie matter wave manipulation, the fabrication limit for focusing is reduced to potentially around 3--5 nm. Here we present a realization of such an ``atom sieve,'' which we fabricated out of a silicon nitride (SiN) membrane, using electron-beam lithography and reactive ion etching. Our atom sieve is 178 $\ensuremath{\mu}\mathrm{m}$ in diameter and has 31 991 holes. The diameter of the holes varies from 1840 to 150 nm. Using a beam of neutral, ground-state helium atoms with an average wavelength of 0.055 nm, we demonstrate helium atom focusing down to a spot size of less than 4 $\ensuremath{\mu}\mathrm{m}$. The focus size is limited by the intrinsic velocity spread of the helium beam.
In 2008 we presented the first images obtained with a new type of matter wave microscope: NEutral Helium Atom MIcroscopy (NEMI). The main features in NEMI are the low energy of the atoms (<0.1 eV) and the fact that they are neutral. This means that fragile and/or insulating samples can be imaged without surface damage and charging effects. The ultimate resolution limit is given by the de Broglie wavelength (about 0.06 nm for a room-temperature beam), but reaching a small focus spot is still a major challenge. The best result previously was about 2 μ m. The main result of this paper is the focusing of a helium atom beam to a diameter below 1 μ m. A particular challenge for neutral helium microscopy is the optical element for focusing. The most promising option is to manipulate neutral helium via its de Broglie wavelength, which requires optical elements structured to nanometre precision. Here we present an investigation of the helium focusing properties of nanostructured Fresnel zone-plates. Experiments were performed by varying the illuminated area and measuring the corresponding focused spot sizes and focused beam intensities. The results were fitted to a theoretical model. There is a deviation in the efficiency of the larger zone plate, which indicates a distortion in the zone-plate pattern, but nevertheless there is good agreement between model and experiments for the focus size. This together with the demonstration of focusing to below 1 μ m is an important step towards nanometre resolution neutral helium microscopy.
Slow atoms in Rydberg states can exhibit specular reflection from a cylindrical surface upon which an azimuthally periodic potential is imposed. We have constructed a concave mirror of this type, in the shape of a truncated oblate ellipsoid of revolution, which has a focal length of (1.50±0.01) m measured optically. When placed near the center of a long vacuum pipe, this structure brings a beam of n=32 positronium (Ps) atoms to a focus on a position sensitive detector at a distance of (6.03±0.03) m from the Ps source. The intensity at the focus implies an overall reflection efficiency of ∼30%. The focal spot diameter (32±1) mm full width at half maximum is independent of the atoms' flight times from 20 to 60 μs, thus indicating that the mirror is achromatic to a good approximation. Mirrors based on this principle would be of use in a variety of experiments, allowing for improved collection efficiency and tailored transport or imaging of beams of slow Rydberg atoms and molecules.
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.
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.
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.