Varifocal Metalens Using Tunable and Ultralow-loss Dielectrics.

The field of flat optics that uses nanostructured, so-called metasurfaces, has seen remarkable progress over the last decade. Chalcogenide phase-change materials (PCMs) offer a promising platform for realizing reconfigurable metasurfaces, as their optical properties can be reversibly tuned. Yet, demonstrations of phase-change metalenses to date have employed material compositions such as Ge2 Sb2 Te5 , which show high absorption in the visible to near-IR wavelengths particularly in their crystalline state, limiting the applicability. Here, by using a low-loss PCM Sb2 Se3 , for the first time, active polarization-insensitive phase-change metalenses at near-IR wavelengths with comparable efficiencies in both material states are shown. An active metalens with a tunable focusing intensity of 95% and a focusing efficiency of 23% is demonstrated. A varifocal metalens is then demonstrated with a tunable focal length from 41 to 123 µm with comparable focusing efficiency (5.7% and 3%). The ultralow-loss nature of the material introduces exciting new possibilities for optical communications, multi-depth imaging, beam steering, optical routing, and holography.

Electric-Driven Polarization Meta-Optics for Tunable Edge-Enhanced Images.

In this study, we demonstrate an electrically driven, polarization-controlled metadevice to achieve tunable edge-enhanced images. The metadevice was elaborately designed by integrating single-layer metalens with a liquid-crystal plate to control the incident polarization. By modulating electric-driven voltages applied on the liquid-crystal plate, the metalens can provide two polarization-dependent phase profiles (hyperbolic phase and focusing spiral phase). Therefore, the metalens can perform two-dimensional focusing and spatial differential operation on an incident optical field, allowing dynamic switching between the bright-field imaging and the edge-enhanced imaging. Capitalizing on the compactness and dynamic tuning of the proposed metadevice, our scheme carves a promising path to image processing and biomedical imaging technology.

Ultrathin van der Waals Metalenses.

Ultrathin and flat optical lenses are essential for modern optical imaging, spectroscopy, and energy harvesting. Dielectric metasurfaces comprising nanoscale quasi-periodic resonator arrays are promising for such applications, as they can tailor the phase, amplitude, and polarization of light at subwavelength resolution, enabling multifunctional optical elements. To achieve 2π phase coverage, however, most dielectric metalenses need a thickness comparable to the wavelength, requiring the fabrication of high-aspect-ratio scattering elements. We report ultrathin dielectric metalenses made of van der Waals (vdW) materials, leveraging their high refractive indices and the incomplete phase design approach to achieve device thicknesses down to ∼λ/10, operating at infrared and visible wavelengths. These materials have generated strong interest in recent years due to their advantageous optoelectronic properties. Using vdW metalenses, we demonstrate near-diffraction-limited focusing and imaging and exploit their layered nature to transfer the fabricated metalenses onto flexible substrates to show strain-induced tunable focusing. Our work enables further downscaling of optical elements and opportunities for the integration of metasurface optics in ultraminiature optoelectronic systems.