Enhanced nanoparticle sensing by mode intensity in a non-reciprocally coupled microcavity

Optical microcavities operating at exceptional points have a strong mode splitting response to small perturbations such as nanoparticles. The detection limit is susceptible to mode linewidth so that small nanoparticles cannot induce a mode splitting in the transmission or reflection spectrum. Here, we propose a sensing mechanism to avoid the limitation of mode linewidth on the detection limit. We show that a microcavity with two Bragg gratings generates bright and dark modes due to completely non-reciprocal coupling. Since the two modes are not degenerate initially, utilizing the dark mode intensity as a readout scheme significantly reduces the detection limit for small perturbations. This work opens up the way toward a new class of ultrasensitive nanoparticle sensor.