Near-field gap effects on small microcavity whispering-gallery mode resonators

The near-field gap effects are investigated in planar dielectric microdisc and waveguide coupling structures, emphasizing miniaturization of integrated sensor systems. The simulation results show that the resonance frequency is not obviously affected by the gap dimension when the gap between a microcavity and its coupler is larger than 300 nm. However, the resonance frequency shifts observably with a further decreasing gap to the nanometre level. This shift is generally larger than the cavity resonance linewidth in the 10 µm diameter microdisc system, but is comparable to the cavity resonance linewidth in the 2 µm diameter microdisc system. With increasing gap, the cavity Q increases exponentially until it is saturated at a limit Q factor. An optimal gap dimension exists for maximum light energy transfer and storage. The concept of optimum gap is introduced and defined at the gap dimension where half-maximum energy storage capability is achieved; meanwhile, the cavity Q is high and the resonance frequency remains stable.