Fig. 1. Schematic of light trapping inside a WGM microcavity by frustrated total internal reflection.

Fig. 2. Different methods of exciting WGMs.(a) Prism coupling. (b) Polished fiber coupling. (c) Tapered fiber coupling.

Fig. 3. (a) Layout of the WGM sensing set-up. (b), (c) Transient interactions of single zinc or mercury ions with the NRs with the corresponding spectrum shifts. Figure reproduced from ref.⁵, Springer Nature.

Fig. 4. The illustration of the coupled liquid-core laser.

Fig. 5. (a) Scanning electron micrograph of a polystyrene bead coated with ELNPs. (b) Left: wide-field image of a lasing microsphere. Right: simulated field distributions in the x–y plane. (c) Simulated NIR spectra of WGMs supported by a 5-µm polystyrene microsphere. Figure reproduced from ref.⁶⁶, Springer Nature.

Fig. 6. (a) Schematic of an all-optical tunable microlaser. (b) Fabrication process of the erbium-doped hybrid microbottle cavity coated with iron oxide nanoparticles. Figure reproduced from ref.⁶⁸, American Chemical Society.

Fig. 7. Illustrations of aqueous QDs (a) in solution inside an OFRR and (b) immobilized as a single layer on the inner surface of an OFRR. (c) Illustration of the experimental setup using confocal geometry. Figure reproduced from ref.⁸, American Chemical Society.

Fig. 8. Diagram and operation of the dual-microcavity narrowlinewidth laser.Figure reproduced with permission from ref.⁷, The Optical Society.

Table 1. Sensitivities of temperature sensors based on WGM microresonator.