Frequency up-conversion in fluid and solid solutions of the oxazine dye, Rhodamine B

Yellow fluorescence observed from Rhodamine B doped poly(methylmethacrylate) and poly(styrene) wave-guides when guiding 632.8 nm light from a helium–neon laser, is attributed to a single-photon absorption into the first excited singlet state of the dye. The excitation wavelength is 2280 cm–1(0.28 eV) lower in energy than the absorption maximum and can be said to fall outside the absorption band. Fluorescence is observed as a result of inhomogeneous line broadening and the thermal population of the higher vibrational energy states in the ground state. Absorption is not directly observed at the excitation wavelength, but is observed in fluorescence due to the high fluorescence quantum yield of the Rhodamine B.