Luminescent Capillary-Based Whispering Gallery Mode Sensors: Crossing the Lasing Threshold

Silicon nanocrystals (Si NCs) present many advantages for sensor applications, including relatively bright and stable luminescence, low-to-negligible toxicity, and high physical and chemical robustness. In this work, the efforts in creating capillary-based sensors from smooth films of oxide-embedded SiNCs are summarized, and their responses to dye-doped polymer films are compared. The main method is to form sub-micrometer luminescent coatings on microcapillary channel walls. The coating must have a high index of refraction so that it can support the luminescence whispering gallery modes (WGMs) which propagate through the film and extend into the channel medium. Using Si NCs, a general refractometric sensing and the detection of layer-by-layer polyelectrolyte deposition on the capillary channel are demonstrated. The Si NC sensors are exceptionally robust and can be cleaned and re-used multiple times. The main limitation of the method currently involves the relatively slow detection (requiring typically more than 20s per collected luminescence spectrum) due to the low light levels associated with the Si NC luminescence. Finally, our most recent work is discussed, which aims to extend the luminescent capillary sensor into the lasing regime.