# Strong Nonlinear Coupling due to Induced Photon Interaction on a Si$_{3}$N$_{4}$ Chip

## TL;DR

This paper demonstrates a significant nonlinear optical interaction on a Si3N4 chip, achieving effective second-order processes through third-order nonlinearities, enabling potential single-photon level applications in integrated photonics.

## Contribution

It introduces a method to induce large effective second-order nonlinearity in Si3N4 resonators, enabling strong coupling and Rabi-like splitting at the chip scale.

## Key findings

- Achieved 74,000%/W efficiency in effective second-order upconversion.
- Observed a nonlinear coupling rate exceeding the energy dissipation rate.
- Provided a theoretical model explaining the ultrastrong nonlinear interactions.

## Abstract

Second-order optical processes lead to a host of applications in classical and quantum optics. With the enhancement of parametric interactions that arise due to light confinement, on-chip implementations promise very-large-scale photonic integration. But as yet there is no route to a device that acts at the single photon level. Here we exploit the $\chi^{(3)}$ nonlinear response of a Si$_{3}$N$_{4}$ microring resonator to induce a large effective $\chi^{(2)}$. Effective second-order upconversion (ESUP) of a seed to an idler can be achieved with 74,000 %/W efficiency, indicating that single photon nonlinearity is within reach of current technology. Moreover, we show a nonlinear coupling rate of seed and idler larger than the energy dissipation rate in the resonator, indicating a strong coupling regime. Consequently we observe a Rabi-like splitting, for which we provide a detailed theoretical description. This yields new insight into the dynamics of ultrastrong effective nonlinear interactions in microresonators, and access to novel phenomena and applications in classical and quantum nonlinear optics.