Self-assembled conjugated polymer spheres as fluorescent microresonators

Confinement of light inside an active medium cavity can amplify emission. Whispering gallery mode (WGM) is one of mechanisms that amplifies light effectively by confining it inside high-refractive-index microstructures, where light propagates along the circumference of a sphere via total internal reflection. Here we show that isolated single microspheres of 2-10 μm diameter, formed from self-assembly of π-conjugated alternating copolymers, display WGM photoemission induced by laser pumping. The wavelengths of the emission peaks depend sensitively on the sphere size, position of the excitation spot and refractive index of each polymer. The Q-factor increases with increasing sphere diameter and displays a linear correlation with the reciprocal radius, indicating that the small curvature increases the efficacy of the total internal reflection. WGM photoemission from π-conjugated polymer microspheres is unprecedented and may be of high technological impact since the microspheres fulfill the role of fluorophores, high-refractive-index media and resonators simultaneously, in addition to their simple fabrication process.