Photonic nodal lines with quadrupole Berry curvature distribution

TL;DR

This paper introduces a new type of photonic straight nodal lines protected by roto-PT symmetry, exhibiting unique topological features and enabling diffraction-less surface wave propagation for advanced photonic devices.

Contribution

It proposes and demonstrates a novel photonic nodal line protected by roto-PT symmetry with quadrupole Berry curvature, expanding topological photonics understanding.

Findings

Nodal lines are protected by roto-PT symmetry in a D2d meta-crystal.

Surface states evolve with frequency, enabling diffraction-less propagation.

Surface wave contours become straight at a critical frequency.

Abstract

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.