Amplified Spontaneous Emission from Dye-Doped Polymer Micro-Ring Cavities

We have found that some of the self-organized sub-micrometer particles of organic dye didn´t emit near-field fluorescence under near-field excitation whereas some other particles did¹. Polarized evanescent field excitation showed that their fluorescence depended on the polarization direction of the excitation and therefore suggesting that dye molecules were oriented within every particle². This could explain that particles wouldn´t fluoresce when the excitation near-field polarization was perpendicular to the dye´s transition moment. However there is another possibility that dyes were in an optically inactive state possibly due to oxidation or chemical reaction under humidity. To remove this possibility we have prepared dye specimen embedded in a transparent polymer film that sealed dye molecules from atmospheric oxygen and water. We have incidentally found that thus formed dye-doped thin film showed a characteristic fluorescence peak, possibly an amplified spontaneous emission (ASE) judging from the very narrow peak width compared with the fluorescence of solution specimen, which was observed when we prepared the film around the perimeter of a multimode silica fiber. The peak position showed clear dependence on the excitation wavelength, whose mechanism will be discussed in detail. The dye-doped film showed a metallic reflection, i. e. a uniform optical reflection over wide visible wavelength, which suggests that some sort of dye crystals may grow inside the film.