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Weyl fermions are hypothetical two-component massless relativistic particlesin three-dimensional (3D) space, proposed by Hermann Weyl in 1929. Theirband-crossing points, called 'Weyl points,' carry a topological charge and aretherefore highly robust. There has been much excitement over recentobservations of Weyl points in microwave photonic crystals and the semimetalTaAs. Here, we report on the first experimental observation of Weyl points oflight at optical frequencies. These are also the first observations of'type-II' Weyl points for photons, which have strictly positive group velocityalong one spatial direction. We use a 3D structure consisting of laser-writtenwaveguides, and show the presence of type-II Weyl points by (1) observingconical diffraction along one axis when the frequency is tuned to the Weylpoint; and (2) observing the associated Fermi arc surface states. Therealization of Weyl points at optical frequencies allow these novelelectromagnetic modes to be further explored in the context of linear,nonlinear, and quantum optics.

Experimental observation of optical Weyl points