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.

High-Q guided resonance modes in two-dimensional photonic crystals, enable high field intensity in small volumes that can be exploited to realize high performance sensors. We show through simulations and experiments how the Q-factor of guided resonance modes varies with the size of the photonic crystal, and that this variation is due to loss caused by scattering of in-plane propagating modes at the lattice boundary and coupling of incident light to fully guided modes that exist in the homogeneous slab outside the lattice boundary. A photonic crystal with reflecting boundaries, realized by Bragg mirrors with a band gap for in-plane propagating modes, has been designed to suppress these edge effects. The new design represents a way around the fundamental limitation on Q-factors for guided resonances in finite photonic crystals. Results are presented for both simulated and fabricated structures.

No 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.

Supersonic beams have numerous applications in research fields ranging from spectroscopy with nanodroplets\ud to surface science and matter-wave microscopy. Thus, measurement and prediction of their properties is of\ud considerable interest. In this paper we present measurements of the virtual-source size and its brightness, as well as the terminal speed and terminal speed ratio of a supersonic deuterium (D2) beam. The speed distribution\ud data were measured with time-of-flight experiments and Fresnel zone-plate imaging was used to measure virtual\ud source size. The point-spread function of the zone plate was simulated based on the measured wavelength\ud distribution and used to extract the width of the virtual source and its brightness from the focus measurement.\ud The experiments were carried out with a 10-μm-diameter nozzle and a source temperature of T0 = 310 K in the\ud pressure range p0 = 3–171 bars and for T0 = 106 K in the pressure range p0 = 3–131 bars.We found that using\ud deuterium as opposed to helium results in a virtual source that is about a factor 2 brighter under similar stagnation\ud conditions. A comparison between the measured data and the predictions from a theoretical model based on\ud the Boltzmann equation, which explicitly include the coupling between translational and rotational degrees of\ud freedom as well as the real-gas properties of D2, resulted in good correspondence for the two different interaction\ud potentials we tried. A careful comparison with the experimental results shows that the potential by Buck et al.\ud [J. Chem. Phys. 78, 4439 (1983)] is moderately better than the Lennard-Jones potential at describing the expansion\ud dynamics