Energy Transfer-Assisted Whispering Gallery Mode Lasing in Conjugated Polymer/Europium Hybrid Microsphere Resonators

Lanthanide metal complexes display luminescence with narrow bandwidth. Here, we present coupling of the luminescence from europium ion (Eu³⁺ ) with whispering gallery modes (WGMs) in conjugated polymer microsphere resonators. Self-assembly of fluorene-terpyridine alternating copolymer, coordinated by Eu³⁺ (F8tpy-Eu³⁺ ), forms well-defined microspheres with an average diameter of 3.2 μm. Upon focused laser excitation, a microsphere of F8tpy copolymer displays WGM photoluminescence (PL) at a wide spectral range from 420 to 680 nm. In contrast, F8tpy-Eu³⁺ hybrid microspheres exhibit sharp WGM PL at a narrow spectral range of 615-630 nm, which is characteristic of luminescence from Eu³⁺ . The PL behavior indicates that photoinduced energy transfer from F8tpy to Eu³⁺ occurs efficiently. Furthermore, the intensity of the PL peak increases nonlinearly upon strong pumping, indicating that a lasing action appears with the threshold of 1.85 mJ cm^-2 . These results will pave the way for developing microlasers and photonic devices from soft organic materials.