Observation of an unpaired photonic Dirac point

TL;DR

This paper reports the first experimental observation of an unpaired photonic Dirac point in a gyromagnetic 2D photonic crystal, enabled by breaking time-reversal symmetry and tuning a topological transition, with implications for non-reciprocal photonic devices.

Contribution

It demonstrates the realization of an unpaired photonic Dirac point in a planar photonic crystal with broken time-reversal symmetry, a significant advancement over previous paired Dirac points.

Findings

Observation of an unpaired Dirac point in a gyromagnetic photonic crystal

Evidence from transmission and field-mapping experiments

Demonstration of strongly non-reciprocal reflection

Abstract

At photonic Dirac points, electromagnetic waves are governed by the same equations as two-component massless relativistic fermions. However, photonic Dirac points are known to occur in pairs in "photonic graphene" and other similar photonic crystals, which necessitates special precautions to excite only states near one of the Dirac points. Systems hosting unpaired photonic Dirac points are significantly harder to realize, as they require broken time-reversal symmetry. Here, we report on the first observation of an unpaired Dirac point in a planar two-dimensional photonic crystal. The structure incorporates gyromagnetic materials, which break time-reversal symmetry; the unpaired Dirac point occurs when a parity-breaking parameter is fine-tuned to a topological transition between a photonic Chern insulator and a conventional photonic insulator phase. Evidence for the unpaired Dirac point is provided by transmission and field-mapping experiments, including a demonstration of strongly non-reciprocal reflection. This photonic crystal is suitable for investigating the unique features of two-dimensional Dirac states, such as one-way Klein tunneling.