  We numerically and experimentally demonstrate efficient light couplers
between topological slow light waveguides in valley photonic crystals (VPhCs)
and wire waveguides. By numerical simulations, we obtained a high coupling
efficiency of -0.84 dB/coupler on average in the slow light regime of a group
index ng = 10 - 30. Experimentally, we fabricated the couplers in a Si slab and
measured the transmitted power of the devices. We realized a high coupling
efficiency of approximately -1.2 dB/coupler in the slow light region of ng = 10
- 30, which is close to the result from the numerical simulations. These
demonstrations will lay the groundwork for low-loss photonic integrated
circuits using topological slow light waveguides.
