Symmetric Time Symmetric And Tunable Optical Isolation Based On Active - Passive Optical Microcavity Coupling System

Compound-photonic structures with gain and loss provide a powerful platform for testing various theoretical proposals on non-Hermitian parity-time (PT) symmetric quantum mechanics, and initiate new possibilities for shaping optical beams and pulses beyond conservative structures. Such structures can be designed as optical analogues of complex PT-symmetric potentials with real spectra; however, the beam dynamics can exhibit unique features distinct from conservative systems due to nontrivial wave interference and phase-transition effects.Here, we experimentally realize PT-symmetric optics on-chip at the 1550 nm wavelength in two directly-coupled high-Q silica-microtoroid resonators with balanced effective-gain and loss.We explained the nonlinearity observed in a single active microtoroid by introducing the concept of gain saturation. With the system of coupled active-passive microtoroids, we further implement a series of experiments on achieving switchable optical isolation with nonreciprocal isolation ratio from-8 dB to +8 dB, by breaking time-reversal symmetry with gain-saturated nonlinearity in a large tunable parameter space. Using coupled-mode theory, our theoretical simulations can well match the experimental data. Of importance, our scheme represents a significant advance in synthesizing novel microscale photonic structures for potential applications in optical isolators, on-chip light control and optical communications.