Single whispering-gallery mode lasing in polymer bottle microresonators via spatial pump engineering

Single-mode lasing in whispering-gallery mode (WGM) microresonators is challenging to achieve. In bottle microresonators, the highly non-degenerated WGMs are spatially well-separated along the long-axis direction and provide mode-selection capability. In this work, by engineering the pump intensity to modify the spatial gain profiles of bottle microresonators, we demonstrate a simple and general approach to realizing single-mode WGM lasing in polymer bottle microresonators. The pump intensity is engineered into an interference distribution on the bottle microresonator surface. By tuning the spacing between axial positions of the interference pump patterns, the mode intensity profiles of single-bottle WGMs can be spatially overlapped with the interference stripes, intrinsically enabling single-mode lasing and selection. Attractive advantages of the system, including high side-mode suppression factors >20 dB, large spectral tunability >8 nm, low-lasing threshold and reversible control, are presented. Our demonstrated approach may have a variety of promising applications, ranging from tunable single-mode lasing and sensing to nonlinear optics. By engineering the spatial profile of a pump beam, scientists in China have made a bottle-shaped microresonator lase in a single mode. Whispering-gallery-mode microresonators, which rely on light circulating in a closed path around a disk, sphere, toroid or bottle, are popular due to their small size and high quality factor. However, they tend to be multimodal when lasing due to the lack of a mode-selection element. Fuxing Gu at the University of Shanghai for Science and Technology and co-workers overcame this limitation by using a pump beam with a striped interference pattern to excite a single transverse mode of a small polymer bottle microresonator. The result was a miniature single-mode laser with a low lasing threshold and high suppression of unwanted side modes.