Lasers with ultra-low threshold and compact size are highly desirable components in photonic integrated circuits, aiming at the application of optical communications, chip-scale solid-state LIDAR and quantum information. The general approach to realizing such lasers is to effectively trap light and boost light-matter interaction by embedding gain materials into few- or sub-wavelength scale optical cavities with high quality (Q) factor and/or small mode volume (V-mode).
