A Nonlinear Microresonator Refractive Index Sensor

We propose an ultrasensitive refractive index (RI) sensor based on the nonlinear index of refraction in a microring resonator coupled to waveguides in an add-drop configuration. The nonreciprocity caused by the difference between the nonlinear self-phase and cross-phase modulation leads to a bifurcation of the optical intensities for two counterpropagating modes. The magnitude of the splitting depends sensitively on the resonance frequency of the resonator, and hence, the effective index of refraction. This nonlinear system is up to two orders of magnitude more sensitive to RI changes than the theoretical sensitivity of a linear microresonator RI sensor. We discuss potential material implementations of the microresonator and the detection limit for these materials.