## Purcell effect Beyond "large" cavities like Fabry-Perot cavity and ring resonator, we can extend the definition of quality factor $Q$ and mode volume $\tilde{V}$ to nano-resonators, like quantum dots or dye molecules. Purcell effect provides a comprehensive framework for quantifying the modification of spontaneous emission rates for an emitter placed within a resonator, highlighting how resonator properties such as quality factor and mode volume can enhance or suppress emission.