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Progress towards helium beam microscopy: focusing using optical potentials

Naomi Schofield

Published Jan 1, 2017• 0 citations
Publisher

Abstract

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.

DOI
10.26180/4664527

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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.
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.
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.
This dissertation presents progress on an atomic beam brightening and focusing experiment that has been shown to create a beam with higher brightness and greater resolution than that of any previous atomic focusing experiment. Our method of beam brightening relies upon using a permanent magnetic hexapole as a lens in conjunction with transverse laser cooling to focus and collimate a supersonic beam of metastable neon atoms. We then use an adjustable electromagnetic hexapole as a lens to focus the brightened atomic beam. Neutral atom beams have long been used as a tool to spectroscopically probe two dimensional surfaces, however the lack of a means to create bright, high-resolution neutral atom beams has limited their utility as a nanoscale resolution probing mechanism. While optical and electron microscopy have been the workhorses of microscopy for decades, we believe that our scheme for beam brightening and focusing will unleash the promise of neutral atom microscopy. Our scheme of beam brightening and focusing can be repeated in series and reveals a pathway to true high density nanoscale neutral atom focusing. Simulations suggest that when combined with aberration correction, our scheme will be able to produce an atomic beam with sufficient brightness and resolution to be useful as a surface sensitive nanoscale probe.
Matter-wave microscopy can be dated back to 1932 when Max Knoll and Ernst Ruska published the first image obtained with a beam of focussed electrons. In this paper a new step in the development of matter-wave microscopy is presented. We have created an instrument where a focussed beam of neutral, ground-state atoms (helium) is used to image a sample. We present the first 2D images obtained using this new technique. The imaged sample is a free-standing hexagonal copper grating (with a period of about 36 microm and rod thickness of about 8 microm). The images were obtained in transmission mode by scanning the focussed beam, which had a minimum spot size of about 2.0 microm in diameter (full width at half maximum) across the sample. The smallest focus achieved was 1.9 +/- 0.1 microm. The resolution for this experiment was limited by the speed ratio of the atomic beam through the chromatic aberrations of the zone plate that was used to focus. Ultimately the theoretical resolution limit is set by the wavelength of the probing particle. In praxis, the resolution is limited by the source and the focussing optics.
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.
A neutral, ground-state, 4He{}^{4}\mathrm{He}4He atom beam has been focused using Fresnel zone plates. At only moderate demagnification $(0.40\ifmmode\times\else\texttimes\fi{})$, a focused spot diameter of $\ensuremath{\le}2.0\ensuremath{\mu}\mathrm{m}$ is achieved at a signal intensity of 500 counts/s. This is an improvement over previous work by a factor of 10 in resolution, 103{10}^{3}103 in signal intensity, and 108{10}^{8}108 in focused beam density, and allows the full intrinsic intensity profile of a focused atomic Fresnel spot to be measured for the first time. Raster scans of 15 and $25\ensuremath{\mu}\mathrm{m}$ wide slits across the focused spot demonstrate transmission scanning helium de Broglie microscopy of micron resolution.