High resolution large working distance scanning helium microscopy.

Scanning helium microscopy (SHeM) is attractive for imaging delicate and insulating surfaces because it combines a non-destructive neutral-atom probe with strong surface sensitivity. Recent pinhole microscopy experiments highlight that practical constraints relating to the equipment dimensions often limit resolution. Notably, both the intensity and the effective spot size of a divergent He beam are improved by a short working distance; but short working distances are restricted by the physical size of the vacuum system required to support, and detect a helium beam. Here, we report sub-micron resolution whilst maintaining a large-working-distance, with an intrinsic beamwidth of 340nm achieved at working distances of 770µm to 850µm. This sixfold improvement over our previous configuration is enabled by a new sample chamber with a movable source, which allows optimisation of the atom optics, together with a more compact pinhole mounted directly beside a larger detector aperture. Geometric contributions to the measured beamwidth from the pinhole are equal to those from the demagnified source size, and slightly larger than diffraction contributions, placing the instrument in a near-optimised regime. The resulting combination of sub-micron beam size, useful depth of field and practical sample access is demonstrated on bacterial specimens and eroded diamond. The work establishes large-working-distance pinhole SHeM as a viable sub-micron imaging platform and extends its usefulness for topographic imaging and micro-diffraction applications.