  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.
