Focusing of a neutral helium beam with a photon-sieve structure

Abstract

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 p m in diameter and has 31 991 holes. The diameter of the holes varies from 1840 to 150 nm. Using abeam 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 pm . The focus size is limited by the intrinsic velocity spread of the helium beam.

A Green's function approach to modeling molecular diffraction in the limit of ultra‐thin gratings

“…Babinet's theorem predicts also quantum fringes in diffraction at opaque obstacles, and Poisson's spot was observed with atoms [10] and diatomic molecules [11] -with an application also in Fresnel zone plates [12]. In all these experiments attractive interactions between the grating and the matter-wave play an important role.…”

Optimal Design of Grid-Based Binary Holograms for Matter-Wave Lithography

“…For a real appliation a lens may need to be introduced to bring the pattern closer and to increase resolution. Several experiments have been carried out focussing molecular beams using zoneplates [10,[20][21][22][23] or electromagnetic [24] lenses and this principle could be applied here.…”

Multiple scattering in scanning helium microscopy

“…Contrast in scanning helium microscopy has similarities with the origins of contrast in scanning electron microscopy, both of which involve rastering a focused or collimated beam across the sample and the collection of a fraction of the backscattered signal. In the case of helium atoms, a narrow spot can be generated via simple pinhole collimation, as used in the current work; 3 via diffractive focusing with a Fresnel zone plate or similar; [6][7][8] or through the use of atom mirrors. [9][10][11][12] Since the local surface position and orientation affect the resulting distribution of scattered particles, topographic contrast is evident in both cases.…”