WGM lasing in irregular cavities with arbitrary boundaries
Because of its limited light field mode and high Q value, the whispering-gallery-mode (WGM) cavity has been widely studied. In this study, we propose a simple, rapid, low-cost and no-manufacturing technology method that we call the drip-coating method to obtain an irregular cavity with arbitrary boundaries. By using polyvinyl alcohol (PVA) solution doped with rhodamine 6G, the irregular cavity with arbitrary boundaries was drip-coated on a high-reflection mirror, forming a WGM laser. The sample was pumped with a 532 nm pulsed laser, and the single-mode WGM and multi-WGM lasing were obtained. All WGMs are the vertical oscillation modes, which originate from both the total internal reflection of the PVA/air interface and vertical reflection of the PVA/mirror interface. The other boundaries of the cavity were not involved in the reflection and could have any shape. The mechanism of producing single-mode lasing is due to the action of the loss–gain cavity. Multi-WGM lasing is attributed to at least two groups of different WGMs existing in an irregular cavity. This can be confirmed by using a microsphere model and intensity correlation method. These results may provide an alternative for the design of WGM lasers.