Dielectric metasurfaces have become efficient tools for creating ultrathin optical components with various functionalities for imaging, holography, quantum optics, and topological photonics. While static all-dielectric resonant metaphotonics is reaching maturity, challenges remain in the design and fabrication of efficient reconfigurable and tunable metasurface structures. A promising pathway towards tunable metasurfaces is by incorporating phase-transition materials into the photonic structure design. Here we demonstrate Mie-resonant silicon-based metasurfaces tunable via the insulator-to-metal transition of a thin VO2 layer with reversible properties at telecom wavelengths. We experimentally demonstrate two regimes of functional tunability driven by the VO2 transition: (i) two orders of magnitude modulation of the metasurface transmission, (ii) spectral tuning of near-perfect absorption. Both functionalities are accompanied by a hysteresislike behavior that can be exploited for versatile memory effects. Beyond this demonstration of multifunctional properties, this work provides a general framework to efficiently use the full complex refractive index tuning of VO2, both for its refractive index modulation and optical absorption tuning. Tunable dielectric metasurfaces may find their applications in various photonics technologies including optical communications, information storage, imaging, detectors, and sensors.

The study of topological phases of light underpins a promising paradigm for engineering disorder-immune compact photonic devices with unusual properties. Combined with an optical gain, topological photonic structures provide a novel platform for micro-and nanoscale lasers, which could benefit from nontrivial band topology and spatially localized gap states. Here, we propose and demonstrate experimentally active nanophotonic topological cavities incorporating III-V semiconductor quantum wells as a gain medium in the structure. We observe room-temperature lasing with a narrow spectrum, high coherence, and threshold behaviour. The emitted beam hosts a singularity encoded by a triade cavity mode that resides in the bandgap of two interfaced valley-Hall periodic photonic lattices with opposite parity breaking. Our findings make a step towards topologically controlled ultrasmall light sources with nontrivial radiation characteristics.

We study active dielectric metasurfaces composed of two-dimensional arrays of split-nanodisk resonators fabricated in InGaAsP membranes with embedded quantum wells. Depending on the geometric parameters, such split-nanodisk resonators can operate in the optical anapole regime originating from an overlap of the electric dipole and toroidal dipole Mie-resonant optical modes, thus supporting strongly localized fields and high-Q resonances. We demonstrate room-temperature lasing from the anapole lattices of engineered active metasurfaces with low threshold and high coherence.

Nonlinear structured illumination microscopy (nSIM) is an effective approach for super-resolution wide-field fluorescence microscopy with a theoretically unlimited resolution. In nSIM, carefully designed, highly-contrasted illumination patterns are combined with the saturation of an optical transition to enable sub-diffraction imaging. While the technique proved useful for two-dimensional imaging, extending it to three-dimensions is challenging due to the fading of organic fluorophores under intense cycling conditions. Here, we present a compressed sensing approach that allows 3D sub-diffraction nSIM of cultured cells by saturating fluorescence excitation. Exploiting the natural orthogonality of speckles at different axial planes, 3D probing of the sample is achieved by a single two-dimensional scan. Fluorescence contrast under saturated excitation is ensured by the inherent high density of intensity minima associated with optical vortices in polarized speckle patterns. Compressed speckle microscopy is thus a simple approach that enables 3D super-resolved nSIM imaging with potentially considerably reduced acquisition time and photobleaching.