Nonlinear coupling of linearly uncoupled resonators through a Mach-Zehnder interferometer

Optical nonlinear processes in linearly uncoupled resonators are being actively studied as a convenient way to engineer and control the generation of non-classical light. In these structures, one can take advantage of the independent combs of resonances of two linearly uncoupled ring resonators for field enhancement, with the phase-matching condition being significantly relaxed compared to a single resonator. However, previous implementations of this approach have shown a limited operational bandwidth along with a significant reduction of the generation efficiency. Here, we experimentally demonstrate that a Mach-Zehnder interferometer can be used to effectively linearly uncouple two resonators and, at the same time, allows for their efficient nonlinear coupling. We demonstrate that this structure can lead to an unprecedented control over the rings' interaction and can operate over more than 160 nm, covering the S-, C-, and L-Telecom bands. In addition, we show that the photon pair generation efficiency is increased by a factor of four with respect to previous implementations.