Surfactant-Induced Interfacial Anisotropy Governs Polarization-Selective Whispering Gallery Mode Excitation in Dye-Doped Trapped Microdroplets.

Surfactant-stabilized liquid microdroplets are soft photonic cavities whose optical response is strongly influenced by the molecular organization of their liquid-liquid interface. In this work, we investigate polarization-dependent whispering-gallery-mode (WGM) excitation in Rhodamine 640-doped benzyl-alcohol microdroplets stabilized with a custom fluorocarbon surfactant. The surfactant layer promotes interfacial ordering of Rhodamine 640 transition dipoles, creating an anisotropic boundary environment that preferentially enhances TE-like WGM emission under vertically polarized excitation. Compared with horizontally polarized excitation at matched pump energy, vertically polarized excitation produces WGM peaks up to approximately 1 order of magnitude stronger. Fourier-transform analysis confirms that the dominant resonances circulate along the droplet equator, with extracted path lengths consistent with the optical perimeter for both polarization states. Representative analytical labeling further supports the assignment of interleaved TE-like and TM-like resonance families within the dominant first-radial-order branch. Control measurements on surfactant-depleted droplets and nematic liquid-crystal droplets support an interfacial origin of the observed polarization selectivity. These results show that surfactant-mediated interfacial ordering provides a practical route for tuning polarization-dependent light-matter coupling in soft WGM microcavities, with potential relevance for droplet-based microlasers and polarization-sensitive sensing.