Arbitrary Control over Reflectance with Freeform Phase-Change Metasurfaces

Active optical control has long been a pursuit in optical engineering, where external stimuli offer tunability of the properties of optical components post-fabrication. While well-established reconfigurable optical components, such as deformable mirrors and spatial light modulators, have been widely used in various fields, they usually suffer from a large pixel pitch and an increased system size, leading to a small field of view and large driving voltages. Active metasurfaces, which integrate active materials with metasurfaces, could give rise to active optical components of the next generation, featuring compactness, high spatial resolution, and low power consumption. Over the past decade, despite the integration of various active materials in metasurfaces, common issues such as small tuning range and functionality degradation have limited the dynamic optical performance of active metasurfaces due to the vast design complexity caused by the presence and interplay of different optical states. As a result, arbitrary control over the reflectance of the phase-change metasurface has yet to be achieved.<br/><br/>We introduce a freeform phase-change metasurface platform by integrating freeform Si nanostructures onto a vanadium dioxide (VO2) layer to realize arbitrary reflectance control at both the semiconducting and metallic states of VO2. The remarkably increased tuning capability and elimination of crosstalk are achieved by embracing the unprecedented design space and resonant diversity enabled by the freeform nanostructures, which are inverse-designed by a generic global optimization approach consisting of a generative neural network and the adjoint method. This inverse design method also considers performance robustness, ensuring desirable dynamic functionalities with feasible nanostructures and moderate computational cost. The ability of arbitrary reflectance control of the inverse-designed metasurface is then experimentally demonstrated with two exemplified dynamic functionalities: switchable independent grayscale nanoprinting and reconfigurable varifocal focusing. Our work provides a generic avenue for active metasurfaces to fully unleash the dynamic benefits of the active material through leveraging diverse resonances offered by inverse-designed freeform nanostructures, facilitating intriguing active optical applications in optical display, imaging, and light detection and ranging.