  Slow light in topological valley photonic crystal structures offers new
possibilities to enhance light-matter interaction. We report a topological
cavity based on slow light topological edge mode for broadband Purcell
enhancement. The topological edge modes with large group indices over 100 can
be realized with a bearded interface between two topologically distinct valley
photonic crystals, featuring the greatly enhanced Purcell factor because of the
increased local density of states. In the slow light regime, the topological
cavity supports much more cavity modes with higher quality factor than that in
the fast light regime, which is both demonstrated theoretically and
experimentally. We demonstrate the cavity enables the broadband Purcell
enhancement together with substantial Purcell factor, benefiting from dense
cavity modes with high quality factor in a wide spectral range. It has great
benefit to the realization of high-efficiency quantum-dot-based single-photon
sources and entangled-photon sources with less restriction on spectral match.
Such topological cavity could serve as a significant building block toward the
development of photonic integrated circuits with embedded quantum emitters.
