Thermally reconfigurable dielectric metalens

Metasurfaces rely on a lattice of nano-structures patterned on a two-dimensional layer to control the light properties in an unique manner. Indeed, they can combine reconfigurable capabilities and nanoscale control of the light properties exploiting sub-wavelength thick layers. Engineering their constituent nano-building-blocks (meta-atoms) is thus fundamental to advance their functionality and to extend their applicability range in nano-photonic and nanoscale optic. In optical metasurface/metalens design, the identification of specific geometrical parameters, meta-atoms arrangement and materials can enable tuneable wavefront control of the light, beam steering capabilities or polarization-based switching mechanisms. In this work, we report the design of an ultrathin (300 nm thick) and thermo-optically reconfigurable silicon metalens operating in the visible regime (632 nm). Importantly, in our design, we rely on the thermo-optical effects to demonstrate that it is possible to achieve continuous variation of the focal-length at a fixed wavelength overcoming the need for a spatially-varying modulation input and potentially enabling an all-optical photo-thermal modulation. Operating under right-circularly polarized light, our metalens exhibits a linear focal shift from 165 μm at 20°C to 135 μm at 260°C. The average conversion efficiency of the lens is 26%, close to mechanically modulated devices, while its Strehl ratio is 0.99, confirming a diffraction-limited performance.