Line nodes, Dirac points and Lifshitz transition in 2D nonsymmorphic photonic crystals

TL;DR

This paper introduces simple 2D photonic crystal designs with symmetry-protected band degeneracies, enabling topological phase transitions, Dirac points, and Lifshitz transitions for potential applications in photonic devices.

Contribution

It presents experimentally feasible 2D photonic crystals with symmetry-protected degeneracies, demonstrating Lifshitz transitions and dynamic control of Dirac points via lattice symmetry switching.

Findings

Line degeneracies protected by glide reflection symmetry.

Presence of Dirac points protected by a Z2 topological number.

Lifshitz transition enabling anomalous refraction.

Abstract

Topological phase transitions, which have fascinated generations of physicists, are always demarcated by gap closures. In this work, we propose very simple 2D photonic crystal lattices with gap closure points, i.e. band degeneracies protected by nonsymmorphic symmetry. Our photonic structures are relatively easy to fabricate, consisting of two inequivalent dielectric cylinders per unit cell. Along high symmetry directions, they exhibit line degeneracies protected by glide reflection symmetry, which we explicitly demonstrate for $pg,pmg,pgg$ and $p4g$ nonsymmorphic groups. In the presence of time reversal symmetry, they also exhibit point degeneracies (Dirac points) protected by a $Z_2$ topological number associated with crystalline symmetry. Strikingly, the robust protection of $pg$-symmetry allows a Lifshitz transition to a type II Dirac cone across a wide range of experimentally accessible parameters, thus providing a convenient route for realizing anomalous refraction. Further potential applications include a stoplight device based on electrically induced strain that dynamically switches the lattice symmetry from $pgg$ to the higher $p4g$ symmetry. This controls the coalescence of Dirac points and hence the group velocity within the crystal.