  The introduction of topological concepts to the design of photonic crystal
cavities holds great promise for applications in integrated photonics due to
the prospect of topological protection. This study examines the signatures of
topological light confinement in the leakage radiation of two-dimensional
topological photonic crystal cavities. The cavities are implemented in an
all-dielectric platform that features the photonic quantum spin Hall effect at
telecom wavelengths and supports helical edge states that are weakly coupled to
the radiation continuum. The modes of resonators scaling down to single point
defects in the surrounding bulk lattice are characterized via spectral position
and multipolar nature of the eigenstates. The mode profiles in real and
momentum space are mapped using far field imaging and Fourier
spectropolarimetry, revealing how certain properties of the cavity modes
reflect on their origin in the topological bandstructure. This includes
band-inversion-induced confinement and inverted scaling of mode spectra for
trivial and topological defect cavities. Furthermore, hallmarks of topological
protection in the loss rates are demonstrated, which are largely unaffected by
cavity shape and size. The results constitute an important step toward the use
of radiative topological cavities for on-chip confinement of light, control of
emitted wave fronts, and enhancing light-matter interactions.
