Whispering gallery modes in bowl-shaped stilbene microresonators

Abstract

Whispering gallery mode (WGM) resonance was investigated in bowl-shaped microresonators. These resonators were fabricated on 20-μm-diameter silica microspheres from the stilbene derivative 4,4′-bis[(N-carbazole)styryl]biphenyl. Photoluminescence (PL) spectra showed periodic peaks that resulted from amplified spontaneous emission (ASE) at the eigen-wavelength of the WGM. The ASE threshold (Ith) was dependent on the peak wavelength and had a minimum value of 4 μJ mm−2 at 482 nm. When the microbowls were fabricated on randomly stacked silica microspheres, no mode coupling was observed among the WGMs. This is a major difference from the behavior of microdisks, which exhibit long-range mode coupling. The microdisks and microbowls were both fabricated from the same materials, but the stilbene in the microdisks was deposited on substrates with surfaces that were decorated with silica microdisks. The main structural difference between the microbowls and the microdisks concerned the presence or lack of a planar stilbene thin film between the WGM resonators; the thin film mode is expected to act as the mode coupler between adjacent WGMs. Therefore, the lack of mode coupling among the microbowls supports our model that long-range mode coupling in the microdisks was realized by crosslinking of the WGMs via the thin film mode.