A metalens-based focus variation instrument exploiting focal plane scanning via illumination wavelength

Here we present a focus variation microscope without moving parts, utilizing the chromatic aberration characteristic of the single-surface metalens. By varying the illumination wavelength filtered through an acousto-optic tuneable filter, scanning of the focal plane can be realised. Imaging is achieved using basic hyperbolic metalens composed of pillars etched from GaN on an Al_2 O_3 substrate. Varying the illumination wavelength from 650 nm to 670 nm shifts the focal plane by 75μm, allowing for capturing the required image stack. Depth information can be extracted by a focus detection algorithm, and the surface topography can be reconstructed. The compact design of this device allows for its use in spaces where traditional instruments cannot fit. We will demonstrate the results from our initial device, including the successful measurement of a stepped artefact, and discuss improvements, such as designing complex multi-element chromatic metalens with enhancing off-axis imaging.