Cavities describe resonant structures where optical fields experience constructive enhancement at resonant frequencies. These structures are among the most fundamental and prevalent devices in optics. Typically of macroscopic size, they have served as tools in fundamental investigations of physics (e.g., for nonlinear mode investigations (Haken and Sauermann 1963), mode-locking dynamics (Kelly 1989), and quantum optics (Pysher et al. 2011)), as well as the basis for countless photonics applications. Among those, the most outstanding has been the development of the laser (Schawlow and Townes 1958), but also include telecommunications (Kaminov et al. 2013), spectroscopy (Parson 2015), and life sciences (Schindl et al. 2000) to name a few.