Enhancement of 2-photon absorption of a dye in a polymer microsphere based on an optical cavity effect

We examined optical cavity effects on the 2-photon absorption efficiency of a dye (Rohdamine B; RhB) doped in polymer microspheres (ion-exchanging resin and poly(methyl methacrylate) (PMMA)). When 1064 nm pulsed-laser light was irradiated on single polymer microspheres doped with RhB under an optical microscope, we clearly observed dye fluorescence from individual microspheres, although the dye was transparent at 1064 nm. We confirmed that the fluorescence intensity was proportional to the square of the laser power for excitation. In contrast, such fluorescence was not observed from RhB doped in a PMMA film, in which the enhancement of a light electric field by an optical cavity effect was never expected. Theoretical calculations indicated that the microsphere possessed several peak values of the quality factor (Q = 10(2)-10(5)) at around certain particle diameters under 1064 nm irradiation. This means that the electromagnetic field of incident light is enhanced through light confinement in a microsphere. Based on these results, we conclude that the 2-photon absorption probability of RhB would be considerably enhanced by the optical cavity effects of the microsphere.