Microcavity-assisted optical resonators are one of the important building blocks for all-optical signal processing. These microresonators have been explored for a variety of applications like filters, modulators, sensors, lasers, optical memories, etc. When the microcavity is 'on resonance' the electric field intensity in the cavity is enhanced, and so are the corresponding nonlinear optical effects. One such case is that of nonlinearity-assisted optical bistability, on which for example optical memory applications are based. In this Spotlighted paper, the authors give a detailed account of the application of silica toroid microresonators as a bistable memory using the Kerr nonlinearity effect.

Earlier attempts of realizing optical bistability with optical resonators relied on the thermo-optic effect or the carrier-plasma effect in semiconductors, which are typically accompanied by absorption losses. Such losses can be avoided by employing instead the optical Kerr nonlinearity effect, which has a nonresonant electronic origin and is therefore not linked to absorption. But realizing Kerr bistability has so far been a challenge mainly due to accompanying inherent thermo-optic and carrier effects. By systematically controlling the coupling between the microcavity and the adjacent waveguides, the authors show how to engineer Kerr bistabilility in the microcavities.

The authors discuss a model which gives insight into the time-scales associated with the photon lifetime in the microcavity in the presence of thermo-optic and Kerr nonlinearities. This model reveals that the thermo-optic effect is dominant only after a certain time interval, which is a function of the thermal generation rate caused by absorption in the cavity. Prior to this, the Kerr nonlineairty dominates. With their model the authors show that in this Kerr-nonlinearity-dominated regime the total photon lifetime in the cavity can be controlled by the coupling rate, which is the key result of this work.

The authors substantiate the key findings with a well-discussed analytical model and numerical simulations. They discuss both ideal and realistic settings from the points of view of the speed of memory operations and the power consumption necessary to access the whole range of refractive indices over which the Kerr bistability in the microcavities takes place. This work opens new avenues for nonlinear optics with high Q microresonators towards the development of novel applications like quantum information processing.

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