Unfolding a design rule for microparticle buffering and dropping in microring-resonator-based add-drop devices

We propose an intuitive and quantitative design rule to determine the microparticle transport processes, including buffering and dropping, on microring-resonator-based add-drop devices at cavity resonances in an integrated optofluidic chip. The design rule uses the splitting ratio, S, of the optical-field intensity at the microring feedback-arc just after the output-coupling region to that at the drop-waveguide as a figure-of-merit for particle transport to determine between particle buffering (S > 1) and dropping (S < 1). The particle transport, however, becomes probabilistic in the case that S is close to 1. The S factor thus provides a clearer physical criterion for determining the particle transport processes compared to the cavity quality (Q) factor. We experimentally investigate this design rule on four different devices with different design parameters on a silicon nitride-on-silica substrate, and show that the particle transport behaviours of 2.2 μm- and 0.8 μm-sized polystyrene particles are consistent with the S values extracted from the transmission spectra. Our numerical simulations of the four devices suggest that the S values extracted from the simulated transmission spectra are consistent with those extracted from the simulated mode-field intensity distributions. We calculate the optical force field using Maxwell stress tensor and an effective microdisk model to relate the S values to the particle transport processes. We further experimentally demonstrate the viability of the design rule by switching between deterministic particle buffering and probabilistic particle transport processes by switching the polarization modes.