Chalcophosphate metasurfaces with multipolar resonances and electro-optic tuning

We computationally analyze the electro-optic response of metasurfaces consisting of interconnected nanoantennas with multipolar resonances using a chalcophosphate, Sn₂P₂S₆, as the active material. Sn₂P₂S₆ has large electro-optic coefficients and relatively low Curie temperature (<70 °C), allowing for strong changes in the refractive index of the material under moderate electric fields if the temperature can be finely controlled in proximity of the Curie point. Through numerical simulations, we show that metasurfaces designed with this nanostructured material demonstrate a significant shift of multipolar resonances upon biasing, despite moderate refractive-index values of the chalcophosphate and reduced mode localization due to this. The magnetic octupolar resonance of a dense array provides the strongest shift of spectral features upon changes in the refractive index, and we attribute it to the high mode localization of this higher-order multipole. We numerically demonstrate that narrow lattice resonances of collective nature do not provide an advantage in shifting the spectral features because of the nonlocal and delocalized nature of the modes, which are spread in the nanoantenna surrounding rather than confined inside nanoantennas. Both in-plane and out-of-plane biasing of the chalcophosphate crystals are similarly efficient with suitable electrode design, choice of electrode material, and crystal orientation within the nanoantennas. These designs exhibit optical properties similar to those of metasurfaces with isolated nanoantennas in the dense array.