Investigation Of Whispering Gallery Mode Splitting Properties Based On A Coupled Microspheres

Whispering Gallery Mode(WGM)optical microcavity has many optical properties,such as high Q value,small mode volume,etc.,it is widely used in sensing field,and the researchers proposed to use coupled WGM microcavity to improve the sensing performance.Most of the current research is to immerse the coupled microcavity in the liquid to be measured or inject the liquid sample into the hollow microcavities.However,for coupled WGM microcavities,the electromagnetic(EM)field tends to concentrate on the gap of the contact part of the two microcavities,indicating the coupling position is sensitive to environmental changes.Therefore,in this paper,a method of adding a liquid to the coupling position of the spherical microcavities is proposed to form a bi-spheres structure with microdroplets to explore the optical properties and sensing application performance.The work is as follows:Firstly,silica microspheres were prepared by heating a single mode fiber(SMF),and the dye/ polymethyl methacrylate(PMMA)coating was covered on microspheres to form active microcavities,and a 532 nm pump pulsed laser was used to excite the WGM laser of the microspheres,and microspheres with the same resonance wavelength were selected through the spectra to ensure they have same diameter.A stable WGM splitting spectrum were collected by during the closing process of the microspheres,and after adding deionized water to the gap using a microfluidic injection device,the splitting size was increased,which indicates the presence of the droplet enhanced the interaction of the two WGM resonances;it was tested with different concentrations of ethanol solution,and it was found there is a linear relationship between the mode splitting size and concentration,thus indicating this structure has a sensing ability for substance detection.Next,we investigated the sensing properties of the droplet-microspheres structure.Since the dye of the active microspheres and PMMA both have a short lifetime when they are irradiated by a pulsed laser,we switch to using tapered fiber coupling to excite the WGM resonance of the passive microspheres.While collecting the transmission spectrum in real time,we added droplets of deionized water to the coupling position of the bi-spheres.During the 9-hour evaporation process,we monitored the spectrum and found that the spectrum splitting both and the resonance intensity gradually increased,and the spectrum gradually moved to the air.The variation of the spectra is a recovery trend,which shows that the sensor structure can be changed to detect the liquid inventory in the gap.In order to test its response to the change of the refractive index of the droplet,we added oily liquids with different refractive indexes and found that the size of the WGM splitting has a linear relationship with it.Therefore,the bispheres structure has a strong application prospect in the field of high-sensitivity microfluidic sensors.