Experimental observation of optical Weyl points

TL;DR

This paper reports the first experimental observation of optical Weyl points, including type-II Weyl points, in a 3D laser-written waveguide structure, confirmed by conical diffraction and Fermi arc surface states.

Contribution

It demonstrates the first optical frequency realization of Weyl points and their type-II variant using a 3D waveguide system, enabling further optical studies.

Findings

Observation of conical diffraction at Weyl points

Detection of Fermi arc surface states

Confirmation of type-II Weyl points in optics

Abstract

Weyl fermions are hypothetical two-component massless relativistic particles in three-dimensional (3D) space, proposed by Hermann Weyl in 1929. Their band-crossing points, called 'Weyl points,' carry a topological charge and are therefore highly robust. There has been much excitement over recent observations of Weyl points in microwave photonic crystals and the semimetal TaAs. Here, we report on the first experimental observation of Weyl points of light at optical frequencies. These are also the first observations of 'type-II' Weyl points for photons, which have strictly positive group velocity along one spatial direction. We use a 3D structure consisting of laser-written waveguides, and show the presence of type-II Weyl points by (1) observing conical diffraction along one axis when the frequency is tuned to the Weyl point; and (2) observing the associated Fermi arc surface states. The realization of Weyl points at optical frequencies allow these novel electromagnetic modes to be further explored in the context of linear, nonlinear, and quantum optics.