Focusing of a neutral helium beam below one micron

Abstract

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 µm. The main result of this paper is the focusing of a helium atom beam to a diameter below 1 µm. 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 µm is an important step towards nanometre resolution neutral helium microscopy.

Zero-order filter for diffractive focusing of de Broglie matter waves

“…The larger the source (skimmer) and the smaller the speed ratio, the larger this effect will be. It has previously been shown that the analytical model provides good agreement for small skimmers in the micrometer range (see, for example, [9]). The MCSTAS simulations predict only a very small change in focus size with speed ratio and fit the data very well except for the footprint to the right, which is slightly raised.…”

“…A simplified model for the final focused spot size is given by approximating the source image and w with Gaussian functions. The skimmer diameter d sk is taken to be the FWHM of the object function [9]. We thus obtain for the theoretical final spot size d focus ,…”

“…The first helium focusing experiments of neutral helium were carried out by Carnal et al [6] using a metastable beam and later by Doak et al [7] using a ground-state helium beam. The first neutral helium microscopy images were obtained using a diffractive focusing element: A Fresnel-Soret zone plate (zone plate with a square-wave amplitude transmission function) was used to focus a helium beam down to a few micrometers [8] and later even below 1 μm [9]. An alternative pinhole helium microscopy setup was subsequently introduced [10][11][12][13].…”

Communication: Neutral atom imaging using a pulsed electromagnetic lens

“…Fresnel zone plates have larger apertures but still block large portions of the incident beam. [4][5][6] Zone plate focusing has recently achieved resolution on the order of 1 µm. Standing light waves, which interact with atoms via the AC Stark effect, are amenable to a variety of shaping and tuning techniques.…”

Nanostructuring of free-standing, dielectric membranes using electron-beam lithography

“…The smallest free-standing features made so far for such applications are around 50 nm, 7 and even less would be desirable. 12 This calls for alternative approaches, such as UV or deep UV lithography, electron-beam (e-beam) lithography, nanoimprint lithography, or scanning probe lithography.…”