Three‐cavity photonic molecules have been realized in several platforms, but their structures and configurations are mostly fixed with limited tunability in the coupling strengths and individual resonant frequencies. Here, we demonstrate an experimental realization of a coupled triple‐cavity photonic molecule (TCPM) composed of three independently‐selectable ultrahigh quality microtoroids. By precisely tuning the inter‐cavity coupling strengths (via distances) and the resonant frequencies (via fine temperature control), as well as coupling the tapered optical fiber onto either the side or middle cavity, evolutions of the TCPM's supermodes are fully mapped and analyzed. Interesting phenomena, such as dark state and double anti‐crossing, emerge in this TCPM system. When the quality factors of the cavities are properly chosen, transition from double electromagnetically‐induced transparency to double electromagnetically‐induced absorption phenomena happens. Such fully‐controlled TCPM system sets a stage for future investigations of new physical effects including cavity‐QED with multiple coupled cavities and topologically protected photonic states.

Realization of chip-scale nonreciprocal optics such as isolators and circulators is highly demanding for all-optical signal routing and protection with standard photonics foundry process. Owing to the significant challenge for incorporating magneto-optical materials on chip, the exploration of magnetic-free alternatives has become exceedingly imperative in integrated photonics. Here, a chip-based, tunable all-optical isolator at the telecommunication band is demonstrated, which is based upon bulk stimulated Brillouin scattering (SBS) in a high-Q silica microtoroid resonator. This device exhibits remarkable characteristics over most state-of-the-art implements, including high isolation ratio, no insertion loss, and large working power range. Thanks to the guided acoustic wave and accompanying momentum-conservation condition, bulk SBS also assist in realizing the nonreciprocal parity-time symmetry in two directly coupled microresonators. The breach of time-reversal symmetry further makes the design a versatile arena for developing many formidable ultra-compact devices such as unidirectional single-mode Brillouin lasers and supersensitive photonic sensors.

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