Investigation of the Effect of Notch and Particle Geometries on Performance of WGM Microsphere Sensor

Optical microsphere resonators that are based on whispering gallery modes (WGMs) are emerging as highly sensitive devices for label-free detection of nanoparticles. In this paper, the change in resonant spectrum due to the presence of a notch on the surface of the microsphere resonator is investigated. The notch causes strong coupling between the WGMs field and nanoparticle; also, a broad range of nanoparticles will be detected with the change of the notch size. When a nanoparticle having a diameter close to the notch size enters the notch, the detection limit will occur, and consequently, the nanoparticle will not be detected. To overcome this detection limit, a chalcogenide glass (As
₂
S
₃
) layer is used at the bottom of the notch. This layer causes a shift of the symmetric mode to the left of the asymmetric mode. Therefore, the detection ranges of nanoparticles compared with the notch without layer are increased. At a frequency close to 106.7 THz, high-Q modes, up to 7.6×10
³
can be excited. The results have been achieved with a numerical simulation and finite-difference method.