Dynamic Responses of Cylindrical Whispering Gallery Mode Revealed by Axial Liquid Disturbance

Hongchun Zhao,Yihui Wu,Yue Wang

doi:10.1109/jphot.2020.2998866

Abstract

Cylindrical whispering gallery mode (WGM) microresonators have achieved much attention in the sensing field due to high sensitivity, compatibility and integration. The excitation of spiral modes in such a resonator is unavoidable, causing the axial field distribution, and the injected analytes will undoubtedly interact with the axial mode field, resulting in unclear spectral responses. Here, these responses of the cylindrical WGM are investigated experimentally and theoretically for the first time. Using an optical fiber as the resonator, the experiment is performed by introducing the liquid into a part of the evanescent field along the axis. The perturbation is attributed to the modification of the resonator's profile, leading to an increase of the effective slope. We find the experimental demonstrations of the short-wavelength oscillations of spectral line shapes and the blue shifts of resonant wavelengths. Theoretical studies are consistent with the experimental phenomena. The research in the responses to the axial perturbation provides valuable insights into the effect of spiral modes on cylindrical WGMs and a potentially powerful tool for real-time biochemical analysis.

Highlights

We find the experimental demonstrations of the short-wavelength oscillations of spectral line shapes and the blue shifts of resonant wavelengths
Whispering gallery mode (WGM) optical microresonators based on cylindrical geometry including solid cylinders and hollow tubes are very attractive for sensing applications due to their high sensitivity, compatibility with microfluidics and integration on a chip [1]–[3]
Some approaches are adopted to reduce the effect of spiral modes on the in-plane whispering gallery mode (WGM), such as by establishing a curve-fitting equation to extract the resonant wavelengths [11] or by coupling with a wider tapered fiber to decrease the appearance of spiral modes [12]

Summary

Introduction

Whispering gallery mode (WGM) optical microresonators based on cylindrical geometry including solid cylinders and hollow tubes are very attractive for sensing applications due to their high sensitivity, compatibility with microfluidics and integration on a chip [1]–[3]. It is generally considered that the resonance mode excited in the cylindrical microresonator is identical as that in an optical ring microresonator, that is, the in-plane WGM with zero axial wavevector components propagating along the circumferential direction [7]–[9]. Most studies pay more attention to the sensing performance of the in-plane WGM They believe that the coupling of the in-plane WGM with the excited spiral modes broadens its line shape, causing the inaccurate identification of the resonant wavelength and the loss of sensitivity. The interaction between the analyte and the axial mode field will inevitably cause the perturbation of the cylindrical WGM, the influence on the spectral responses remains unclear as well. The interaction mechanism clarified here provides solutions to better performance of cylindrical resonators, and the research provides a basis for the further development of cylindrical microresonators in the sensing and other fields

Experiment Setup and Spectral Responses

Analysis Model and Discussion

Findings

Conclusion