Micro-scale, whispering gallery mode (WGM) resonators are important devices for trapping, slowing down, and intensifying light in opto-electronic applications. Of the many available silica-based devices, surface nanoscale axial photonics (SNAP) resonators are a new generation of WGM microresonators, consisting of an optical fiber with nanoscale variations of its effective radius. They have received great attention owing to their advantages over conventional silica-based devices, such as flexibility and ultra-low losses, which lead to ultra-high quality factors. They have demonstrated great potential as photonic micro-devices in switching, slowing light, filtering, lasing, and sensing with high precision, as well as for research on Anderson ...