Field Enhancements Of Whispering Gallery Modes In High-refractive-index Subwavelength Dielectric Resonators

Trapping of light in localized modes is extremely important for various applications in optics and photonics.For many optical devices,it becomes critical to localize electromagnetic fields in small subwavelength volumes.Plasmonic structures based on metals allow subwavelength localization of light by means of surface plasmon polaritons.However,metals impose inevitable losses and heating,which limit the device performance and efficiency.In contrast to the plasma,the unique features of dielectric nanoparticles with high refractive index are low absorption loss and large electromagnetic field enhancement,which became an important alternative to the different metal components of plasma devices.This has a wide range of applications and prospects.Besides,high refractive index dielectric nanostructures can cause the strong coupling and interference between optical modes.However,the electromagnetic field produced by the dielectric resonators is small,so much effort has been made to obtain a larger field enhancement in a single dielectric cavity.It is well known that the whispering gallery modes is formed by the full reflection of localized photons in the cavity at the boundary constantly,and thus localized photons for a long time,which provides a large electromagnetic field enhancement.Despite the known of pattern distribution,few study is focusing on the value of the electromagnetic field enhancement.Therefore,the main research content of this paper is as follows:(1)we show that whispering gallery modes of subwavelength dielectric resonators can exhibit ultrahigh electric and magnetic field enhancements due to their high-quality factors.The resonators are dielectric disks.The field enhancements are dependent on the height of the disk,the refractive index n and the azimuthal mode index.For a disk dimer of n=3.5,the electric field enhancement reaches～10~3 in the small gap.The Purcell factor of an electric and magnetic dipole emitter in the dimer can reach～10~5 and～10~4,respectively.We also consider some experimentally feasible configurations,where the strong optical response of the whispering gallery mode can still be maintained.(2)We utilize low-order Mie modes in high refractive index dielectric nanostructures to excite whispering gallery mode,which can be realized experimentally.It is found that the whispering gallery mode in the disk can be excited by near-field coupling via the low-order Mie modes in the nanorod.We construct dimers by placing nanorods on the nanodisk.The whispering gallery mode with the azimuthal mode m=6 is studied by changing the parameters of the rod,so that the low-order Mie mode in the rod is coupled to the whispering gallery mode in the disk.It can be found that in this case,the field enhancement of whispering gallery modes can reach about 70%of the original single disk.