Adjustable topological corner states in terahertz valley photonic crystals

Higher-order topological insulators (HOTIs) have emerged as unique topological materials hosting topological corner or hinge states. This work investigates terahertz (THz) higher-order topological states in ${C}_{3}$ symmetric valley photonic crystals (VPCs) in theory and experiment. Based on numerical simulations of photonic band structure, phase profiles, and Berry curvature, we realize the valley topological phase transition by rotating the scatterers in unit cells. Meanwhile, the higher-order topology of the VPCs is characterized by nontrivial bulk polarization. The topological corner states are demonstrated by calculated eigenenergy spectra, field distributions, and local density of states of nested triangular structures. In addition, the associated fractional corner charge is also employed to confirm the existence of topological corner states directly. Two topological corner states are identified, one staying within band gap and the other embedding in bulk bands. Both originate from quantized bulk dipole moments and are flexibly switchable by rotating the scatterers of the system. Experimentally angle-resolved transmittance measurements demonstrate the presence of the photonic band structure and the bulk band gap of the THz photonic crystals. The measured time-domain spectra reveal the valley edge states between distinct THz VPCs. Further, we directly observe the topological corner states in the measured spatial mapping of the electric field intensity. These investigations testify to the possibility of simultaneously employing topological corner, edge, and bulk states to guide THz waves, which may have potential applications in THz functional devices.