Probing non-Hermitian zero modes in a nanophotonic trimer: From nonuniform to twisted coupling

Topological lasers based on optical cavity arrays, such as the Su-Schrieffer-Heeger array, in active media were recently realized on various platforms. In all these studies the coupling coefficient between any adjacent sites of the lattice was positive. Consequently, the lasing mode featured an out-of-phase oscillation which is typically impractical for any application because it weakens the far-field intensity. This problem can be mitigated by altering the coupling signs along the array. However, implementing negative evanescent coupling in microrings, microdisks and micropillars is not possible using standard designs and fabrication processes. In this work, we design and experimentally implement a photonic crystal cavity array that consists of three sites where the sign of one coupling parameter is inverted, which enables the observation of an in-phase symmetry-protected zero mode. The photonic crystal array presented here utilizes the recently proposed ``image-barrier'' technique, in which the photonic barriers are copied at opposite sides of the array, which mitigates chain-termination effects and at the same time enables sign flip from one barrier to the adjacent one, resulting in ``twisted coupling.'' Consequently, the overall symmetry of the zero mode is inverted and becomes even, which is experimentally demonstrated. Our work opens the door for implementing a new generation of phase-locked topological laser arrays that oscillate in the same phase with enhanced far-field optical intensities.