Observation of Bloch Flatbands and Localized States in Moiré Bilayer Grating

Here, we explore the formation of Bloch flatbands in photonic moiré bilayer grating. The stacking-induced interlayer coupling contrast serves as a crucial condition for flatband formation. The number of Bloch flatbands is related to the number of antinodes present within one unitcell of monolayer grating. By applying the band-unfolding approach to moiré supercell states, we identify that the associated localized states in moiré bilayer grating arise from the "beating" mode between Bloch standing-waves associated with two similar periods. Experimentally, we established a resonance-enhanced photon scattering setup to directly measure Bloch flatbands and observe the corresponding localized states in the moiré photonic system. Furthermore, we present the experimental demonstration of third-harmonic generation enhancement facilitated by moiré-induced flatbands. Our work provides insights into moiré-induced modulation of photonic band structures, opening up possibilities for novel optical phenomena and device applications in photonic crystals.