Coherent control of (non-)Hermitian mode coupling: tunable chirality and exceptional point dynamics in photonic microresonators.

This work introduces a novel on-chip integrated photonic device, the Dynamically Reconfigurable Unified Microresonator (DRUM), enabling full and dynamic control of Hermitian and non-Hermitian modal coupling between counter-propagating modes in a microresonator. The DRUM consists of a microresonator coupled to two tunable side waveguides, each incorporating a Mach-Zehnder Interferometer and a phase shifter, allowing for independent manipulation of the amplitude and phase of the coupling coefficients. This unique architecture facilitates a continuous and arbitrary transition between diabolic points (DPs) and exceptional points (EPs). We experimentally demonstrate the versatility of the DRUM through several key functionalities: dynamic tuning of the resonance spectral lineshape, coherent suppression of backscattering to achieve an ideal DP, and operation in both Hermitian and non-Hermitian states, enabling continuous chirality tuning and dynamic steering between two EPs. The device achieves a chirality of ±1 at the EPs, indicating strong directionality in light propagation. The experimental results, supported by a theoretical model based on Temporal Coupled Mode Theory, pave the way for reconfigurable photonic devices that exploit (non-)Hermitian dynamics for advanced functionalities, with potential applications ranging from high-sensitivity sensors to neuromorphic computing. The DRUM overcomes the limitations of previous implementations by offering unprecedented control over the coupling between counter-propagating modes within a single integrated device.