We present a theoretical study on photonic topological crystals whose symmetry is governed by quantum valley-Hall topological insulators and whose propagating edge modes are strongly confined due to bowtie geometries. Dielectric bowtie structures exploit the field discontinuities at the boundaries between materials with different refractive indices, and here bowties emerge at the topological interface due to the close proximity of two triangular features in the underlying crystal. The topological bowtie mode features a unit-cell mode volume down to $8\times {10}^{-4}$ cubic wavelengths at the center of the bowtie bridge of width $10\mathrm{nm}$. Valley-Hall photonic topological insulators have often been assumed to be immune to backscattering, but we show that perturbed versions of the topological unit cells can form highly reflective mirrors. This enables us, in turn, to design topological heterostructure bowtie cavities with quality factors exceeding ${10}^7$ and a Purcell factor of $3\times {10}^6$, implying that unprecedented regimes of light-matter interaction may be in reach with topological bowtie modes.

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• Cavity quantum electrodynamics
• Quantum description of light-matter interaction
• Topological effects in photonic systems