Resonant Polarization Response in Phononic Moiré Metamaterials

Extensive studies on twisted van der Waals materials have led to the emergence of twistronics as a new paradigm in materials design. However, electronic Moiré structures are constrained by various factors, including natural lattice dependencies, symmetry constraints, and mismatches in length scales. Recently, its optical analog, metasurface-based photonic Moiré structures with designed lattice misorientations have emerged as a promising platform for new optical phenomena [1, 2]. These structures can mirror the design space of electronic Moiré systems of two-dimensional (2D) materials, demonstrating photonic flat bands with low group velocity, optical chirality, and enhanced light-matter interactions. Further, integrating photonic moiré structures with 2D materials or nanostructured media could unlock new functionalities and applications. Our work aims to use grating-based photonic structures to achieve a generalized understanding of twist physics in photonic Moiré structures.<br/><br/>To do so, we probe twisted metamaterials via far-field polarized Fourier-transform infrared (FTIR) microscopy and finite element method (FEM) simulations. We utilize a silicon carbide (SiC) substrate with a photolithographically patterned single-layer metal grating formed by combining two gratings at different twist angles. This structure excites resonant surface phonon polariton modes with twisted properties. This structure is coated with dielectric ZnO coating, enhancing light-matter coupling with the surface phonon polariton and supporting guided mode resonances. Our FEM simulations indicate that this metasurface supports both surface phonon polaritons and guided-mode resonances, which are strongly dependent on twist angle and incoming polarization state. Optical measurements of the twisted photonic structures reveal a strong polarization-dependent response, uncovering a resonant cross-polarization effect highly dependent on lattice parameters and twist angle. This effect offers almost perfect polarization conversion from the metasurface, aligns with the Brillouin zone of the 2D twisted grating, and arises from guided wave coupling in the device. Both surface phonon polariton modes and guided wave resonances are consistent with the calculated grating vector parameters and dependent on the lattice period. Our investigation provides insights into twist photonic physics and opens pathways for developing novel devices with tunable and enhanced functionalities. In the long term, such metamaterials could be leveraged for the generation of chiral thermal light sources, chiral sensing, and energy harvesting.<br/><br/>References:<br/>1. Lou, B., et al., <i>Tunable guided resonance in twisted bilayer photonic crystal.</i> Science Advances, 2022. <b>8</b>(48): p. eadd4339.<br/>2. Zhu, H. and B.I. Yakobson, <i>Creating chirality in the nearly two dimensions.</i> Nature Materials, 2024. <b>23</b>(3): p. 316-322.