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Quantum Electronics and Laser Science Conference

Light force manipulation of an atomic gallium beam

Steven J. Rehse, Karen M. Bockel, S. A. Lee

Published Nov 13, 2002• 0 citations
Publisher

Abstract

Summary form only given. Laser focusing of neutral atoms is a unique approach for fabricating periodic small structures. In a standing wave formed by two counter-propagating laser beams, the radiation force acts as a sequence of cylindrical lenses that focus the atoms into a periodic array of parallel lines during deposition onto a substrate. In this way, a pattern is written onto the substrate without the use of any masks. To minimize the feature width on the substrate, the atoms entering the focusing laser must be highly collimated transversely by optical cooling.

DOI
10.1109/qels.2001.962104

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Atom lithography is a technique to structure layers of atoms during deposition, using interactions of near-resonant light fields with neutral atoms. The basic scheme uses a standing wave light field, aligned just above a substrate, while a beam of atoms impinges perpendicularly on the standing wave light field. The interaction of the light field and the atoms focuses the atoms towards the crests or troughs of the standing wave intensity, dependent on the tuning of the light field. The focused atoms are subsequently deposited on the substrate, resulting in an array of lines, whose periodicity is determined by the wavelength of the light field. The standing wave light field can therefore be seen as an array of lenses, focusing the atom beam into an array of lines. The first experiments on atom lithography were performed in the 1990s with sodium and chromium. 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We show that light force lenses can be used to focus a thermal beam of neutral atoms with extremely high resolution (\ensuremath{\sim}20 nm) and high contrast (\ensuremath{\sim}10:1), finally making such atom-optical lenses viable for proposed applications. Our (cylindrical) lenses for sodium atoms are created by the dipole force within each period of a standing wave. We determine the spatial distribution of the focused atoms (with nanometer resolution) by allowing the atoms to deposit on a substrate and scanning with a UHV scanning tunneling microscope. We find that the requirements for optimum focusing are very large, positive detuning; short focal lengths; and a well-collimated atomic beam. These dependences can be thoroughly understood from a simple theoretical model. \textcopyright{}1996 The American Physical Society.

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Summary form only given. Recently it has been demonstrated that neutral atomic beams can be focused by use of a laser as an atom-optical lens. Standing wave atom-optical (SW) lenses have been of particular interest because of the potential for both high-resolution and high-throughput lithography. In this paper, using sub-Doppler optical cooling schemes in conjunction with mechanical collimation, and a SW with /spl sigma/=56 /spl mu/m, we have been able to achieve <20 nm resolution with contrast of at least 10:1, focusing a thermal atomic sodium beam using 589-nm light corresponding to the D/sub 2/ transition.

Quantum Electronics and Laser Science Conference•Jun 7, 1996•0 citations
Summary form only given. Laser focusing of neutral atoms is a unique approach for fabricating periodic small structures. In a standing wave formed by two counter-propagating laser beams, the radiation force acts as a sequence of cylindrical lenses that focus the atoms into a periodic array of parallel lines during deposition onto a substrate. In this way, a pattern is written onto the substrate without the use of any masks. To minimize the feature width on the substrate, the atoms entering the focusing laser must be highly collimated transversely by optical cooling.
We show that light can be used as a lens to focus a collimated neutral atomic beam to submicron dimensions during deposition onto a substrate. We have used an optical standing wave at 589 nm as an array of cylindrical lenses to focus a perpendicular sodium beam into a grating on a substrate, with a periodicity of 294.3\ifmmode\pm\else\textpm\fi{}0.3 nm. This result is the first direct evidence of submicron focusing of atoms by light, and represents a fundamentally new scheme for submicron lithography.
Slow neutral beams of metal atoms can be manipulated using the intensity gradient of near-resonant light-fields enabling the deposition of atoms onto a substrate in a processes often referred to an atom lithography. A suitably shaped light-field gradient is used to control the path of metal atoms using the dipole force created by the interaction between atoms and the strong, near-resonant optical intensity gradient. Relatively simple patterns such as lines and dots have been created using optical standing waves while more complex light-fields might be created using computer generated optical elements to manipulate the laser beam.
Fundamental interaction processes between atoms and photons are exploited to control external degrees of freedom of the atoms. Laser light, when properly tuned near an atomic resonance, exerts such forces that the atoms are repelled from or attracted to the regions with low light intensities. We use these forces to manipulate atom beams on nanometer-scale. In the simplest approach to nanofabrication via atom optics, a laser standing wave serves as a 'mask' through which the atoms are deposited onto a substrate. In this thesis we first present the modeling results on the focusing properties of a standing wave laser field and on the surface diffusion and growth phenomena in laser-focused atomic deposition. The development of our experimental facility for laser-manipulated deposition with chromium atoms (based on the obtained modeling results) is reported on. Thereafter, we demonstrate laser manipulation of chromium atoms and fabrication of nanostructures with these atoms. The results obtained in Nijmegen and also those obtained in collaboration with J.J. McClelland's group from the National Institute of Standards and Technology (Gaithersburg, Maryland, USA) are analyzed with the aim of understanding the role of atom-optical and surface growth phenomena in the fabrication process of laser-focused nanostructures
Atom lithography is a technique to structure layers of atoms during deposition, using interactions of near-resonant light fields with neutral atoms. The basic scheme uses a standing wave light field, aligned just above a substrate, while a beam of atoms impinges perpendicularly on the standing wave light field. The interaction of the light field and the atoms focuses the atoms towards the crests or troughs of the standing wave intensity, dependent on the tuning of the light field. The focused atoms are subsequently deposited on the substrate, resulting in an array of lines, whose periodicity is determined by the wavelength of the light field. The standing wave light field can therefore be seen as an array of lenses, focusing the atom beam into an array of lines. The first experiments on atom lithography were performed in the 1990s with sodium and chromium. In this thesis experiments with iron are described, which is a magnetic element and thus allows for the creation of magnetic nanostructures. To focus atom beams into structures using atom lithography, well-colimated atom beams are required. The most common technique to create collimated atom beams is transverse laser cooling, but iron has no closed optical transition suitable for laser cooling. Fortunately Smeets and Te Sligte found that even then atom lithography of iron is possible. If the standing wave light field is considered to be an array of lenses, the focusing of the atom beam on a single lens is determined by the local collimation of the atom beam at that position. This local collimation is determined by the geometry of the beam source and the distance of the beam source to the standing wave lens. Using a small beam source (e.g. 1 mm diameter) positioned far from the standing wave (e.g. 1 m), the geometrically collimated atom beam is suitable for atom lithography. Chapter 3 shows what the implications of geometric collimation are on the structure formation in atom lithography. The geometric effects that locally collimate the atom beam on the sample also introduce local offset angles of the atom beam with respect to the focusing light field. Structure formation has been observed with lines of up to 7 nm high on a background layer of 15 nm average height, over areas of up to 400 µm × 6 mm. This indicates that the offset angles can vary over an 8 mrad range, which is more than an order of magnitude larger than the local angle of collimation. The offset angle influence the local geometry of the deposited structures: an increasing offset angle decreases the local line height, increases the width and creates a skewness in the local line shape. The local geometry of the lines has been modelled with a Monte Carlo particle tracking model and the results were compared to experiments. It was found that the highest and narrowest lines suffer from significant broadening due to surface diffusion in experiments, limiting the achievable structure width of iron on SiOx samples to about 80 nm. We have also found that in atom lithography without laser cooling, each standing wave lens is effectively imaging the source geometry onto the substrate. We therefore propose using structured atom beam sources to image more complex patterns on sub-wavelength scales in a parallel way. The reason to use iron in atom lithography, is the option to create magnetic nanostructures. However, in previous experiments no clear magnetic signature of the atom lithographic structuring of iron was found. Therefore in Chapter 4 firstly the magnetic properties of unstructured layers of iron (Fe), nickel (Ni) and FexNi1-x are investigated. These layers are all ferromagnetic, but with a reduced magnetic moment compared to bulk values due to contaminations and crystal growth effects. As a reference for periodic atom lithography structures, periodic line structures are created with interference lithography in a polymer layer and onto these structures thin layers of FexNi1-x are shadow deposited, thereby creating a layer periodically modulated in both height and elemental composition. This periodic modulation in composition has been observed with SEM-EDX on sub-micron scales. The structures showed a clear magnetic anisotropy with an easy axis along the direction of the lines, in accordance with expectations. In Chapter 5, the magnetic properties of atom lithographic iron line structures are presented. Using MOKE microscopy, direct comparison of structured and unstructured parts of a sample is possible, which allows for the observation of clear magnetic signatures due to the atom lithographic structuring. In layers of an average thickness of about 15 nm, no anisotropy is induced by the line structures, while a magnetic easy axis along the direction of the lines is expected. The line structures introduce an isotropic increase in the coercivity of the layers, indicating that the line structures can be considered as isotropic corrugations instead of directional line structures. For layers of 30 nm average thickness and thus higher line structures, a clear magnetic anisotropy is observed for the highest line structures. This anisotropy can be seen as a magnetic easy axis along the direction of the lines, as expected for line structures. However, we also observe a sharp increase in coercivity for applied fields perpendicular to the lines. This phenomenon is intriguing and may be explained by a pinning of head-to-head domain walls along the direction of the lines. In Chapter 5, we also report on the first results of co-deposition of FexNi1-x, where the Fe is structured into lines using atom lithography, while the Ni is deposited uniformly, thereby creating an alloy of modulating composition. Magnetic analysis of these structures indicates that these structures are anisotropic, both the coercivity and the shape of domain walls is dependent on the angle of the applied field relative to the line structures. Finally in Chapter 6, we investigate the possibilities to focus a thermal beam of atoms into a single 100 nm spot using light fields, thereby creating a nanopencil suitable for deposition of nanostructures. A Monte Carlo particle tracking program was developed to model the all optical focusing or funneling of 10 µm sized atom beams to a 100 nm spot. This model included effects of initial beam divergence, magnetic substructure, laser cooling, and spontaneous as well as stimulated diffusion and it was applied to a number of promising light field configurations. The best results obtained for experimentally realistic settings are a beam focused to a full width at half maximum of FWHM = 0.55 µm in a blue detuned hollow beam where atoms are focused towards the dark center, or a FWHM = 0.31 µm in a red detuned axicon light field, where atoms are focused towards the maximum intensity at the heart line. However, in the latter case the focused beam has a flux only 25 times larger than the background flux, which is unpractical for applications. The limiting factors of all schemes are heating due to stimulated diffusion and the limited interaction time available for a thermal beam of atoms. A nanopencil could be possible if it were based on the focussing of a slow and / or monochromatic atom beam, but this would seriously complicate the practical application of such a device.
We show that light force lenses can be used to focus a thermal beam of neutral atoms with extremely high resolution (\ensuremath{\sim}20 nm) and high contrast (\ensuremath{\sim}10:1), finally making such atom-optical lenses viable for proposed applications. Our (cylindrical) lenses for sodium atoms are created by the dipole force within each period of a standing wave. We determine the spatial distribution of the focused atoms (with nanometer resolution) by allowing the atoms to deposit on a substrate and scanning with a UHV scanning tunneling microscope. We find that the requirements for optimum focusing are very large, positive detuning; short focal lengths; and a well-collimated atomic beam. These dependences can be thoroughly understood from a simple theoretical model. \textcopyright{}1996 The American Physical Society.
Summary form only given. Recently it has been demonstrated that neutral atomic beams can be focused by use of a laser as an atom-optical lens. Standing wave atom-optical (SW) lenses have been of particular interest because of the potential for both high-resolution and high-throughput lithography. In this paper, using sub-Doppler optical cooling schemes in conjunction with mechanical collimation, and a SW with /spl sigma/=56 /spl mu/m, we have been able to achieve <20 nm resolution with contrast of at least 10:1, focusing a thermal atomic sodium beam using 589-nm light corresponding to the D/sub 2/ transition.