Resonantly Enhanced Third Harmonic Up-conversion of 2.4 micron Excitation using Amorphous Germanium Zero Contrast Gratings

Non-linear optics in resonant metasurfaces opens a new paradigm for realizing active photonic functionalities using surface relief structures. One such compelling photonics functionality is the use of silicon or germanium sub-wavelength grating structures as frequency up-conversion surfaces in front of conventional visible/ near-IR detectors to detect longer wavelengths in the infrared range (> 2 μm). Third harmonic generation (THG) can be utilized for the up-conversion of photons to the visible/ near-IR wavelength range. Furthermore, THG on resonant metasurfaces can aid in achieving multi-fold enhancement in the nonlinear wavelength conversion process due to resonant field enhancement in these structures. The resonant field enhancement mechanism can be broadly classified as guided mode resonances (GMR) in periodic gratings [1] and Mie resonances in isolated structures [2] . Enhanced THG from single Germanium nanodisk has been reported for 1.65 μm by exciting anapolar mode [3] . In the present work, zero contrast gratings (ZCGs) [4] , consisting of partially etched GMR structures are utilized to achieve resonances in the 2.4-2.5 μm wavelength range. The resonant enhancement of THG up-conversion to the near-IR wavelength range at ~800 nm is experimentally demonstrated.