Visible Light Emission From Dye Molecular Grains via Infrared Excitation Based on the Nonadiabatic Transition Induced by the Optical Near Field

We observed light emission in the visible wavelength range (lambda = 600-690 nm) from aggregated 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) dye molecule grains excited by infrared light (lambda(ex) = 805 nm). The domains of visible light emission were localized at the surface and edges of the dye grains, where the optical near field was strengthened. The emitted visible light intensity decayed exponentially according to the time constants tau(1) = 0.45 ns and tau(2) = 1.37 ns, which were equivalent to those of conventional fluorescence excited by visible light at lambda(ex) = 402 nm. The emitted light intensity increased with the infrared excitation intensity, in agreement with the theoretical results of the exciton-phonon polariton model. This confirmed that the visible light emission originated from the nonadiabatic transition process due to optical near-field features. The frequency upconversion efficiency for excitation from infrared (lambda(ex) = 805 nm) to visible (lambda = 600-690 nm) in the film of the DCM molecular grains was experimentally estimated to be higher than that of the second harmonic generation (SHG) from a potassium dihydrogen phosphate (KDP) crystal. In particular, it was higher when the fundamental light power density was lower than 100 W/cm(2). Visible light emission from the grains of the rhodamine 6G (N-{2-[2-(2-aminoethoxy)ethoxy]ethyl} rhodamine 6G-amide bis[trifluoroacetate]) dye molecule was also observed in the infrared light (lambda(ex) = 805 nm). Our results demonstrated the universality of the nonadiabatic transition process.