Bergin M, Ward D J, Ellis J, Jardine A P
Cavendish Laboratory, JJ Thomson Avenue, Cambridge, UK.
Cavendish Laboratory, JJ Thomson Avenue, Cambridge, UK.
Ultramicroscopy. 2019 Dec;207:112833. doi: 10.1016/j.ultramic.2019.112833. Epub 2019 Aug 29.
We describe a method for obtaining the optimal design of a normal incidence Scanning Helium Microscope (SHeM). Scanning helium microscopy is a recently developed technique that uses low energy neutral helium atoms as a probe to image the surface of a sample without causing damage. After estimating the variation of source brightness with nozzle size and pressure, we perform a constrained optimisation to determine the optimal geometry of the instrument (i.e. the geometry that maximises intensity) for a given target resolution. For an instrument using a pinhole to form the helium microprobe, the source and atom optics are separable and Lagrange multipliers are used to obtain an analytic expression for the optimal parameters. For an instrument using a zone plate as the focal element, the whole optical system must be considered and a numerical approach has been applied. Unlike previous numerical methods for optimisation, our approach provides insight into the effect and significance of each instrumental parameter, enabling an intuitive understanding of effect of the SHeM geometry. We show that for an instrument with a working distance of 1 mm, a zone plate with a minimum feature size of 25 nm becomes the advantageous focussing element if the desired beam standard deviation is below about 300 nm.
我们描述了一种用于获得垂直入射扫描氦显微镜（SHeM）最佳设计的方法。扫描氦显微镜是一种最近开发的技术，它使用低能中性氦原子作为探针来对样品表面进行成像而不会造成损伤。在估计了源亮度随喷嘴尺寸和压力的变化后，我们进行了约束优化，以确定给定目标分辨率下仪器的最佳几何结构（即强度最大化的几何结构）。对于使用针孔形成氦微探针的仪器，源光学和原子光学是可分离的，并且使用拉格朗日乘数来获得最佳参数的解析表达式。对于使用波带片作为聚焦元件的仪器，必须考虑整个光学系统并应用数值方法。与以前的数值优化方法不同，我们的方法能够深入了解每个仪器参数的作用和重要性，从而直观地理解SHeM几何结构的影响。我们表明，对于工作距离为1毫米的仪器，如果所需的光束标准偏差低于约300纳米，最小特征尺寸为25纳米的波带片将成为有利的聚焦元件。
