Metasurfaces offer a unique playground to tailor the electromagnetic field at subwavelength scale to control polarization, wavefront, and nonlinear processes. Tunability of the optical response of these structures is challenging due to the nanoscale size of their constitutive elements. A long-sought solution to achieve tunability at the nanoscale is all-optical modulation by exploiting the ultrafast nonlinear response of materials. However, the nonlinear response of materials is inherently very weak, and, therefore, requires optical excitations with large values of fluence. We show that by properly tuning the equilibrium optical response of a nonlocal metasurface, it is possible to achieve sizable variation of the photoinduced out-of-equilibrium optical response on the picosecond timescale employing fluences smaller than 250 μJ / cm², which is 1 order of magnitude lower than previous studies with comparable reflectivity variations in silicon platforms. Our results pave the way to fast devices with large modulation amplitude.

The interest in dynamic modulation of light by ultra-thin materials exhibiting insulator–metal phase transition, such as VO2, has rapidly grown due to the myriad industrial applications, including smart windows and optical limiters. However, for applications in the telecommunication spectral band, the light modulation through a thin VO2 film is low due to the presence of strong material loss. Here, we demonstrate tailored nanostructuring of VO2 to dramatically enhance its transmission modulation, reaching a value as high as 0.73, which is 2 times larger than the previous modulation achieved. The resulting designs, including free-topology optimization, demonstrate the fundamental limit in acquiring the desired optical performance, including achieving positive or negative transmission contrast. Our results on nanophotonic management of lossy nanostructured films open new opportunities for applications of VO2 metasurfaces.

Infrared imaging is a crucial technique in a multitude of applications, including night vision, autonomous vehicle navigation, optical tomography, and food quality control. Conventional infrared imaging technologies, however, require the use of materials such as narrow bandgap semiconductors, which are sensitive to thermal noise and often require cryogenic cooling. We demonstrate a compact all-optical alternative to perform infrared imaging in a metasurface composed of GaAs semiconductor nanoantennas, using a nonlinear wave-mixing process. We experimentally show the upconversion of short-wave infrared wavelengths via the coherent parametric process of sum-frequency generation. In this process, an infrared image of a target is mixed inside the metasurface with a strong pump beam, translating the image from the infrared to the visible in a nanoscale ultrathin imaging device. Our results open up new opportunities for the development of compact infrared imaging devices with applications in infrared vision and life sciences.

Dielectric nanocavities are emerging as a versatile and powerful tool for the linear and nonlinear manipulation of light at the nanoscale. In this work, we exploit the effective coupling of electric and toroidal modes in AlGaAs nanodimers to locally enhance both electric and magnetic fields while minimizing the optical scattering, thereby optimizing their second-harmonic generation efficiency with respect to the case of a single isolated nanodisk. We also demonstrate that proper near-field coupling can provide further degrees of freedom to control the polarization state and the radiation diagram of the second-harmonic field.