Large Photothermal Modulation of Dielectric Huygens' Metasurface Absorber

Mie-type resonators have been found to exhibit large nonlinearity of spectral responses under photoexcitation, which is applicable to all-optical modulation. The typical modulators, based on ring resonators are of size around 10-100 micrometers. To reduce the size of an optical modulator, one potential approach is to utilize Mie resonators. The state-of-the-art modulation depth of Mie resonators is 40-50%, which is still lower than the state-of-the-art all-optical modulator based on a silicon ring resonator. In this work, we demonstrate large modulation depth in Huygens‘ metasurface absorber. When multiple nanostructured resonators (units) are arranged periodically to form a metasurface, coupled electromagnetic interaction creates “lattice resonance” in addition to the resonances due to an individual unit itself. When the electric dipole lattice resonance (EDLR) matches with the magnetic dipole (MD) resonance and the materials possess moderate loss, each meta-atom (unit) could absorb most of the incoming electromagnetic wave due to the matched hybrid resonant modes, which is called Huygens‘ metasurface absorber. We used amorphous silicon-based Huygens’ metasurface absorber and ultrafast excitation to demonstrate two orders of magnitude enhancement of modulation depth, both theoretically and experimentally. We compared two sample structures with the EDLR matching and mismatching the MD of the single SiNB, respectively. At matched resonant condition, we found that the modulation depth was theoretically enhanced by two orders of magnitudes. Enhancement by an order of magnitudes and over 100% modulation depth were experimentally achieved at the matched resonant frequency. Since the optical spot covered tens of SiNBs in the array, our work manifested the feasibility to achieve a large modulation depth of a Mie-type all-optical modulator with size on the order of 1 micrometer. This work opens the avenue toward all-optical information processing via optimized modulation by careful spatiotemporal and spectral control over a metasurface.