Nonlinearly enhanced refractive index sensing in coupled optical microresonators

We study the use of nonlinear self-phase modulation (SPM) in coupled optical microresonators for ultrasensitive refractive index sensing. SPM leads to a positive feedback enhancement of the resonance frequency shift caused by a perturbation of the index of refraction in a resonator. Moreover, the use of two resonators coupled through input and output waveguides leads to a further improvement in the sensitivity owing to constructive interference and feedback via the waveguides. For parameters based on Si microresonators, the sensitivity is more than 10(2) larger than a single resonator without SPM leading to a minimum detectable index change of 10(-11) RIU with resonator Q-factors of ~10(4). We show that the nonlinearly enhanced system is robust with respect to laser noise when operated at input powers below the onset of bistability.