Ultrahigh Purcell Factor, Improved Sensitivity, and Enhanced Optical Force in Dielectric Bowtie Whispering-Gallery-Mode Resonators

We propose and theoretically investigate an all-dielectric bowtie whisperinggallery-mode (WGM) resonator which consists of two tip-to-tip coupled semiconductor nanorings with triangular cross section separated by low refractive index material gap. Mode splitting of symmetric WGM and antisymmetric WGM is observed and analyzed. Due to extremely field enhancements by the “slot” and “tip” effects, strong localization of light in the gap of symmetric WGMs leads to ultrasmall modal volume of 0.3μm³. This value is two orders of magnitude smaller than that in toroidal microresonator which is the typical WGM microcavity with tight mode confinement. Importantly, large amount of light confined in the gap in bowtie WGM resonator not only suppresses the radiation loss (radiation-loss-related quality factor is above 108), but also makes it as an ideal platform for Purcell effect enhancement, ultrasensitive sensing, and optical trapping of nanoparticles. Calculation results show that Purcell factor can reach to 108 at room temperature when assuming quantum dots or atoms placed in the gap. Refractive index sensitivity is improved to 700 nm/RIU as compared with conventional slot waveguide with rectangular cross section. The optical gradient force is greatly enhanced and allows efficient trapping of single nanoscale particle with diameter of 5 nm even at a relatively large gap of 100 nm.