Thermal properties of a photonic resonator, determined by both intrinsic properties of materials and the geometry and structure of the resonator, play important roles in various applications including radiation detection, biosensing, and microlaser. In this work, we propose and demonstrate a method to measure the thermal relaxation time and thermal conductance of an optical microresonator. The method utilizes the optothermal effect of two nearby optical modes in the transmission spectrum of the same resonator to extract the thermal properties of the resonator. We show that the thermal relaxation time, as well as thermal conductance, can be tailored by changing the geometric parameters of the resonator. Furthermore, we provide an analytical model that can be used to estimate the thermal relaxation time of a microtoroid resonator given its geometric parameters. The experimental results agree well with the analytical predictions. Our method can be exploited to characterize and optimize the thermal properties of other types of optical microresonators.
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