Valley-Hall photonic crystal waveguides under non-Hermitian active defect

Photonic transport facilitated by topological protection is a proposed advantage of photonic topological waveguides based on valley photonic crystals (VPCs). Although topological protection significantly suppresses backscattering in these waveguides, it is often desirable to achieve active control over the transmission characteristics. We utilize photoexcited carriers in silicon to implement an active defect—a local, actively tunable, dissipative non-Hermitian perturbation in the path of a terahertz VPC waveguide—and systematically characterize the transport characteristics. We study waveguides constructed from different VPC interfaces (zigzag and bearded) and show that the high group index VPC waveguide modes are more strongly modulated by the phototunable defect. In both the waveguides, the faster modes exhibit approximately linear variation in transmission loss with increase in defect through enhanced photocarrier generation. However, for slower modes, the transmission loss varies nonlinearly, indicative of enhanced interaction with the active defect. We are able to model this behavior in terms of a group delay dependent loss. Our study not only highlights the superior performance of low index VPC waveguide modes but also paves the way for the systematic development of on-chip modulators based on active defects.