Fabrication and optimization of dielectric metalenses

An optical metasurface is composed of building blocks – nanostructures – which allow for control of the transmitted light with high spatial resolution and replication of the functionalities held by traditional optical components simultaneous to significant miniaturization of their absolute thickness. Plasmonic optical metasurfaces composed of metallic building blocks have limited possibilities in steering the phase of the light, which is transmitted through them and also, they suffer from losses caused by light absorption. Dielectric optical metasurfaces have no such limitations and generally show higher effectivity than their plasmonic alternatives – however, the fabrication of the dielectric building blocks is often challenging. To fabricate the dielectric optical metasurface with the functionality of a focusing lens, we first deposited the 550 nm thick layer of titanium dioxide on the transparent fused silica substrate using magnetron sputtering. This dielectric layer was subsequently transformed into the metasurface building blocks using the selective reactive ion etching through a chromium mask created with electron beam lithography. The complete fabrication process is shown in Fig.1. The shapes and sizes of the final metasurface building blocks were checked with scanning electron microscopy (see Fig. 2).

The metasurface was designed for light with wavelength of 650 nm. The building blocks from titanium dioxide – cylinders with the height of 550 nm and diameters of 100 nm and 175 nm – are located on the metasurface in a square array (period 400 nm) and they are alternated in concentric-circle-shaped zones, which supports the functionality of the metasurface – focusing lens. The area of the created focusing lens is 100 μm x 100 μm and its focal length was experimentally found as (63±5) μm, as shown in Fig. 3.

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Fig. 1: Fabrication of the metalens. (a) Description of the used layers and materials. (b) After sputtering of the titanium dioxide layer on the substrate, the chromium mask is created with electron beam lithography on top of it, which allows for the selective reactive ion etching (RIE) of the dielectric layer. The redundant chroumium is removed via wet etching.

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Fig. 2: Scanning electron microscopy micrographs of the dielectric metalens and its building blocks, tilt 40°. (a) Square-shaped metalens with the size of 100 μm x 100 μm. (b) Close-up to the placement of metasurface building blocks. The zones composed of concentric circles are filled with building blocks arranged in a square array with 400 nm period. The building blocks have the height of 550 nm and diameters of 100 nm and 175 nm.

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Fig. 3: Imaging of a square-shaped aperture with the created focusing metalens – metasurface. (a) The dependence of the intensity of light transmitted through the focusing metalens while aperture imaging on the distance z from the metalens in the image half-space of the metalens. (b)-(f) Deformations of the aperture image in planes parallel to the metalens – in (b) the metalens can be seen and in (d) we show the focused image of the aperture.