  Topological photonics sheds light on some of the surprising phenomena seen in
condensed matter physics that arise with the appearance of topological
invariants. Optical waveguides provide a well-controlled platform to
investigate effects that relate to different topological phases of matter,
providing insight into phenomena such as topological insulators and
superconductors by direct simulation of the states that are protected by the
topology of the system. Here, we observe a mode associated with a topological
defect in the bulk of a 2D photonic material by introducing a vortex distortion
to an hexagonal lattice and analogous to graphene. These observations are made
possible by advances in our experimental methods. We were able to manufacture
uniform large two-dimensional photonic crystal structures, containing thousands
of identical waveguides arranged in two dimensions, and we developed a new
method to excite multiples of these waveguides with a well-defined light field.
This allows us to probe the detailed spatial features of topological defect
modes for the first time. The observed modes lie mid-gap at zero energy and are
closely related to Majorana bound states in superconducting vortices. This is
the first experimental demonstration of a mode that is a solution to the Dirac
equation in the presence of a vortex, as proposed by Jackiw and Rossi.
