Mode Coupling of Whispering Gallery Modes Through Organic Semiconductor Thin Films

The mode coupling distance for the whispering gallery modes (WGMs) of microdisk resonators was investigated. Microdisk resonators with thicknesses in the 115-230 nm range and diameters of 15 +/- 3 mu m were fabricated from an organic semiconductor material, 4,4'-Hbis[(N-carbazole)styryl]biphenyl, via vacuum vapor deposition and photolithography without using a dry etching process. The microdisks were patterned two-dimensionally to form square-type unit cells. Using these square-type microdisk arrays, we observed periodic sharp peaks in the photoluminescence spectra. This spectral narrowing was the result of amplified spontaneous emission with a WGM resonance. Furthermore, fine structures from the mode coupling of the WGMs were observed when the distance between two adjacent microdisks was less than 8 +/- 1 mu m. This distance is 1 or 2 orders of magnitude longer than the mode coupling distance obtained through overlapping of evanescent waves, which is the method used in conventional WGM resonators. We consider that the long-range mode coupling originates from multiple reflections in the organic thin films on an offset area of the microdisks; an organic thin film area between the microdisks plays the role of a mode coupler in combining the WGM modes that are standing in the microdisks.