Precision fabrication of a microbubble resonator for mode control

Whispering gallery mode (WGM) microresonators benefit from having high Q-factors and small mode volumes, allowing high-performance sensing, lasing, frequency comb generation, and other nonlinear effect-based applications. Three-dimensional resonators provide the highest Q-factors by low-loss light confinement in all spatial dimensions. However, the addition of a third dimension, over chip-based resonators, also leads to the emergence of numerous higher-order modes, leading to a higher spectral mode density. Therefore, techniques allowing for selective mode excitation and filtering are needed. In this work, we integrate geometric filters onto the surface of a microbubble WGM resonator using focused ion beam (FIB) milling. By exploiting the limited number of modes after filtering, we isolate individual modes using a narrow piezo scan to observe the intensity distribution and to identify its axial order, p = l−|m|+1, where l and m are the polar and azimuthal mode numbers, respectively. Keywords: Whispering gallery modes, mode filtering, microbubble, mode selection, mode identification, mode