Optimization of Whispering Gallery Mode Microsphere Resonators and Their Application in Biomolecule Sensing

Abstract

Optical Whispering gallery mode (WGM) resonators have attracted attention due to their label-free and sensitive detection capabilities for sensing. Light is confined and continuously recirculated within the cavity via total internal reflection at the resonant wavelength. The long recirculation time significantly enhances the interaction of light with sample, enabling improved sensitivity. Among various WGM resonators with different geometries, microsphere resonators fabricated with heating via surface tension can achieve ultrahigh quality factors and small mode volumes, making them good candidates for WGM sensing applications. The sensing performance of a microsphere resonator is greatly related to its size. Therefore, the fabrication method is optimized to form silica microspheres with smooth surfaces and different sizes (15𝜇𝑚~165𝜇𝑚 in diameter). The size effects on their sensitivities and quality factors are studied. Silica microspheres with optimal sizes (~45𝜇𝑚 in diameter), which achieve high sensitivities and large quality factors, are selected for sensing applications. In addition, the thermal effect on WGM resonators is also explored for silica microspheres and barium titanate microspheres. WGM resonant wavelength shift versus temperature change is measured in a range of 21°C to 45°C. Red shifts with increasing temperature are observed in all size microsphere resonators, which match the theoretical analysis. Silica microsphere resonators with the optimal sizes are utilized in a WGM biomolecule sensing application. Cholera toxin specifically binds to GM1, which can be detected by the WGM resonant wavelength shift. A DOPC/GM1 bilayer is transferred onto the surface of a silica microsphere via the Langmuir-Blodgett technique. When cholera toxin molecules bind to GM1, the refractive index increases, leading to the red shift of the resonant wavelength. By changing the GM1 concentration in the DOPC/GM1 bilayer and measuring the total WGM shift, we find the maximum amount of GM1 that can be coated onto a silica microsphere’s surface to bind cholera toxin molecules. Overall, the cavity size effect and thermal influence on WGM silica microsphere resonators are studied and optimized to achieve high sensitivities and high quality factors. As label-free optical detectors, silica microsphere resonators’ application in biomolecule sensing is demonstrated.