Angular-momentum coupling of optical whispering-gallery modes to liquid droplet microresonators

Starting from a recently developed model based on the generalized Lorenz-Mie theory, we simulate and experimentally investigate the resonance spectrum and the scattered light field of whispering-gallery modes (WGMs) in a liquid droplet excited with a free-space visible laser beam. Depending on the illumination configuration and the spectral region, we observe WGMs with diverse quality factors and coupling efficiencies. Through imaging of scattering patterns, we show a robust agreement between theory and experiment. Our study provides an insight of the free-space optical coupling phenomenon, opening pathways for a controlled excitation of ultrahigh-quality WGMs in a plethora of liquid and solid optical microresonators for sensing and imaging applications.