摘要: In periodic systems, nodal lines are loops in the three-dimensional momentum
space where two bands are degenerate with each other. Nodal lines exhibit rich
topological features as they can take various configurations such as rings,
links, chains and knots. These line nodes are usually protected by mirror or PT
symmetry. Here we propose and demonstrate a novel type of photonic straight
nodal lines in a D2d meta-crystal which are protected by roto-inversion time
(roto-PT) symmetry. The nodal lines are located at the central axis and hinges
of the Brillouin zone. They appear as quadrupole sources of Berry curvature
flux and allow for the precise control of the quadrupole strength.
Interestingly, there exist topological surface states at all three cutting
surfaces, as guaranteed by the pi-quantized Zak phases along all three
directions. As frequency changes, the surface state equi-frequency contours
evolve from closed to open contours, and become straight lines at a critical
transition frequency, at which diffraction-less surface wave propagation are
demonstrated, paving way towards development of super-imaging photonic devices.