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The Distribution and Evolution of Refractive Index in a Polystyrene Whispering Gallery Microcavity during Glass Transition

There is limited knowledge on the evolution of physical and chemical parameters near the surface of polymer materials at the microscopic level during glass transition, owing to the complexity of this process. For example, the distribution and evolution of the refractive index near the surface of polymer materials during glass transition are thus far unknown; however, this information is necessary for the design of optical micro‐elements based on polymer materials. Herein, a strategy is proposed to achieve the expression of the refractive index as a function of depth and temperature inside a polystyrene microsphere. The difference in the wavelength shifts between two whispering gallery modes with different radial orders is employed to detect the evolution of the refractive index during the glass transition. A model is established based on experimental observations and further confirmed by numerical simulations. It is believed that the results obtained can assist with the design of polymer optical micro‐elements and systems as well as sensing applications of whispering gallery microcavities.