Localized optical manipulation in optical ring resonators.

We propose a tunable optical trapping system for nanoparticles based on generating standing wave by coupling two coherent beams into a ring resonator in opposite directions, respectively. The distributions of the mode field excited in three types of the ring-resonators-based trapping systems (microring, microdisk and slot ring) and the corresponding optical forces on the nanoparticles are calculated numerically. By the stability analysis in all directions, the smallest size of the particles could be stably trapped under the Brownian motion in the microring resonator is 61.2 nm when the input power is 10 mW, and the azimuthal orientations of the trapped particles are depended on the phase difference between the two input beams. On the other hand, the appearance of high order radial modes in the microdisk resonator enables a tunable radial trapping. To improve the trapping capability for the smaller particles, we utilize the slot ring resonator to make full use of the optical power and the trapping size could be minimized to ~29 nm when the input power is also set as 10 mW.