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

The manipulation of neutral atoms and molecules via their de Broglie wave properties, also referred to as de Broglie matter wave optics, is relevant for several fields ranging from fundamental quantum mechanics tests and quantum metrology to measurements of interaction potentials and new imaging techniques. However, there are several challenges. For example, for diffractive focusing elements, the zero-order beam provides a challenge because it decreases the signal contrast. Here we present the experimental realization of a zero-order filter, also referred to as an order-sorting aperture for de Broglie matter wave diffractive focusing elements. The zero-order filter makes it possible to measure even at low beam intensities. We present measurements of zero-order filtered, focused, neutral helium beams generated at source stagnation pressures between 11 and 81 bars. We show that for certain conditions the atom focusing at lower source stagnation pressures (broader velocity distributions) is better than what has previously been predicted. We present simulations with the software ray-tracing simulation package mcstas using a realistic helium source configuration, which gives very good agreement with our measurements.

Figure 1

Drawing of NEMI used to perform the measurements presented here. The focused helium atom spot profile is observed by moving a 10-$\mu \mathrm{m}$ slit aperture, mounted on a translation stage, across the focused spot and measuring the pressure increase in the Pitot tube detector. To the right the detector setup used for reflection imaging can be seen. It is not used for the experiments presented here.

Figure 2

Diagram of the zero-order filter setup. After the free-jet expansion through the nozzle, the central part of the helium beam is selected by the microskimmer. The beam hits the Fresnel zone plate. The central part of the zone plate has been blocked by a circular middle stop, which filters part of the zero-order beam. The rest of the zero-order beam (and most of the higher orders) is blocked by the order-sorting aperture with a diameter slightly smaller than the middle stop.

Figure 3

Experimental focusing results showing two focused beams with and without zero-order filtering together with mcstas simulations. The two intensity curves have been normalized relative to the maximum intensity measured with zero-order filtering and the background has been subtracted for both curves. As can be seen, the zero-order background is completely removed by our zero-order filter. These experiments were carried out at a pressure of 76 bars, using a 10-$\mu \mathrm{m}$ diameter skimmer.

Figure 4

Experimental focusing results for a 50-$\mu \mathrm{m}$ skimmer with pressures ranging from 11 to 81 bars, plotted together with simulations (sim.) and the analytical model (mod.); here $S$ denotes the speed ratio.