Topological photonic devices with dynamically tunable functions
are highly on demand in practice, but the majority of previously proposed
photonic systems have been limited to fixed performances, once fabricated.
Although several approaches have been proposed for obtaining the tunability
in topological photonic systems, they are limited to first-order topological
states and require rather complicated structures. Herein, second-order
topological properties of rhombic photonic crystals (PCs) are revealed, for
the first time, enabling to realize tunable photonic devices. For this
purpose, the conventional square lattice PCs composed of four rigid
dielectric rods are reshaped to rhomboid ones with preserved inversion
symmetry, which exhibit well-quantized bulk polarizations. Since the
eigenfrequencies of topological edge and corner states depend on the angle
between the neighboring sides of unit cells, the second-order topological
systems exhibit dynamic tunability, being useful for diverse applications
such as optical switching and flexible beam control. Unlike the previous
results for reconfigurable routing limited to special angles, this
lattice-reshaping mechanism has the ability to realize dynamically tunable
routing, extending the realm of applications of topological photonics. For
its simplicity and feasibility, this mechanical lattice-reshaping approach
paves the way toward higher-order topological photonic devices with
dynamically controlled functions.