Exciting topological edge states by using plane waves in valley-Hall photonic crystal slabs

Abstract

Topological edge states are commonly excited by dipole sources or waveguide couplers. In practice, however, such near-field excitations are unfavorable and the simplest way is to use far-field sources. In this report, we theoretically show that the topological edge modes inside the light cone can be directly excited by plane waves via momentum matching, taking valley-Hall topological photonic crystal slabs as examples. Due to the field distributions and mode symmetries, the edge states show different excitation behaviors. One of the edge modes behaves as symmetry-protected bound states in the continuum (BIC) and is excited only by oblique incidence, the Q-factor of which diverges to infinity for normal incidence and drastically decreases for the increase of incident angle. The other one, however, can be excited by far-field for arbitrary incident angle, which does not show the influence on the magnitude of Q-factor. By changing the incident angle, the resonant wavelengths of both modes are tunable and they are determined by the edge bandstructure. Due to the simple excitation scheme and high Q-factors of excited edge states, this proposal will find practical applications, including efficient frequency conversion, sensing, slow light, and optical communications.