  In general, a high-Q microresonator can accommodate abundant whispering
gallery modes (WGMs) with the mode number increasing with the dimensional sizes
of the microresonator. Removing the unnecessary modes while reorganizing the
remaining modes is of vital importance, which, however, has been proved
challenging and usually results in a tradeoff with the Q of the microresonator.
Here, we reveal an effective and controllable mode trimming and clustering
mechanism underlying the generation of polygon and star modes in weakly
perturbed tapered fiber-coupled lithium niobate whispering gallery
microresonators. Experimentally, various polygon and star modes are observed in
sequence within a single microresonator by tuning the excitation wavelength or
varying the coupling position between a tapered fiber and the circular
microresonator, which can be well reproduced with our theoretical model. The
finding offers a ubiquitous solution for a broad range of applications
requiring elaborate selection and organization of the high-Q WGMs.
