Active and Time-modulated Flat Optics

In this work, we will review our recent progress in development of active optical metasurfaces in the quasi-static and time-modulated operation regimes. We will first present methodologies to achieve widely tunable phase responses with high amplitude efficiencies by exploiting high-Q resonant modes supported by all-dielectric metasurfaces. We will then discuss unique opportunities for light manipulation with extended degree of freedom enabled by active time-modulated metasurfaces. In particular, we will introduce a tunable geometric phase shift in time-modulated metasurfaces which allows for dispersionless dynamic wavefront engineering of higher-order sidebands. It will be emphasized that this feature can be used to overcome several limitation of quasi-static metasurfaces and yields several distinctive capabilities for time-modulated platforms including wideband all-angle beam scanning and topological space-time photonic transitions. Various implementations of time-modulated metasurfaces at optical frequencies will be presented and their applicability as optical phased arrays with flexible pattern synthesis will be highlighted. Moreover, design approaches for achieving higher efficiency in sideband generation and broadband performance will be proposed.