Nonlocal Metasurface Lens for Long-Wavelength Infrared Radiation.

Dielectric metasurfaces are structured thin films with a thickness smaller than the operating wavelength aiming at replacing and enhancing conventional bulk optical components. At visible and near-infrared frequencies, titania or silicon is routinely used as substrates to realize these ultrathin devices by structuring local resonances across an aperture. Unfortunately, directly scaling the same design and material approaches to long-wave infrared frequencies is unpractical, due to the resulting thickness and the presence of phonon absorption lines. Nonlocal metasurfaces based on extended resonances with a local geometric phase provide a compelling design platform that can address these challenges, they enable ultrathin metasurfaces and offer multi-functionalities, polarization- and frequency-selectivity, and can be implemented in several low-loss material platforms. Here, nonlocal metalenses are demonstrated based on germanium thin films on a zinc-selenide substrate, operating at ≈10.3µm within a deeply subwavelength device thickness of 1.45µm (14% the free-space wavelength). A novel meta-unit geometry is showcased based on a square lattice with highly isotropic dispersion features, supporting a resonant geometric phase that is highly stable in frequency, simplifying the rational design of complex metasurface operations. The introduced platform promises multi-functional, low-profile meta-optics with enhanced meta-unit designs, compatible with the challenging thermal spectral region for imaging and sensing applications.