Ultra-narrow unidirectional transmission filter assisted by topological interface state in one-dimensional photonic crystal heterostructure

Abstract

A novel optical device with dual functions of ultra-narrow wave filtering and unidirectional transmission is proposed based on a topological interface state in a one-dimensional (1D) photonic crystal (PC) heterostructure. The structure is a 1D PC heterostructure connected with a dielectric grating. When light waves are incident from the PC side, topological interface states can be excited at a very narrow wavelength span. Within this span, light can transmit through the PC heterostructure easily and then be diffracted to the other side of the grating. However, for the reversed incidence, light will hardly transmit through the structure because the excitation conditions of the topological interface states are affected by the diffraction of the grating. Theoretical calculations demonstrate that the transmission difference between opposing incidences can reach 90%. Meanwhile, the high quality factor of the topological interface state resonance enables the structure to be an excellent filter. For operating frequencies in the communication range, the bandwidth of the unidirectional transmission can be as narrow as 0.2 nm. This ultra-narrow unidirectional transmission resonant filter has great potential in future all-optical integrated circuits.