For example, the thermal tuning technique has been applied to adjust the resonator frequency in parity-time (PT) symmetric resonator systems 19, 55, 56 and to accurately measure the optothermal properties of resonators 57, 58. In addition, by making use of the thermal effects, innovative photonic techniques have been developed. Therefore, various thermal-stability techniques have been developed to suppress thermal noise in WGM microresonator applications, such as sensing and metrology, where thermally induced signal fluctuations are undesired. Taking WGM sensing as an example, the resonance shifts induced by a target of interest 34 are typically mixed with the thermally induced mode shift. Specifically, temperature fluctuations affect the material refractive index and/or size of the resonator, both of which modify the mode distributions and shift the resonance frequencies of WGMs. For example, thermo-optic nonlinear dynamics 25, 26, 27, 28 and thermal instability 29, 30 have been observed in various applications. Consequently, thermal effects and the associated dynamics are ubiquitous in WGM microresonators. The capability of WGM microresonators to trap light in a highly confined volume for a long period of time significantly enhances light−matter interactions and enables a high-power build-up 23, 24. In the last two decades, whispering-gallery-mode (WGM) microresonators 1 have enabled numerous advances in fundamental science and technology, including optomechanics, non-Hermitian physics, communications, frequency combs, high-performance sensors, and cavity quantum electrodynamics (QED) 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22. Finally, we review some techniques employed to achieve thermal stability in a high-quality-factor resonator system. The thermal locking technique and thermal imaging mechanisms are discussed briefly. By tuning the temperature of the environment, the resonant mode frequency will shift, which can also be used for thermal sensing/tuning applications. With the help of the thermal bistability effect, optothermal spectroscopy and optical nonreciprocity have been demonstrated. In this review, we discuss the mechanisms of laser-field-induced thermal nonlinear effects, including thermal bistability and thermal oscillation. Thermal behaviors induced by power build-up in the resonators or environmental perturbations are ubiquitous in high-quality-factor whispering-gallery-mode resonators and have played an important role in their operation for various applications. They have been demonstrated as a diverse platform for a wide range of applications in photonics, such as nonlinear optics, optomechanics, quantum optics, and information processing. Optical whispering-gallery-mode microresonators with ultrahigh quality factors and small mode volumes have played an important role in modern physics.
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