Upconversion of He-Ne laser light in xanthene dye-doped polymer waveguides

We report on generation of yellow light emission (peak intensity at 577 nm) from a low power red He-Ne laser beam (1 mW, 632.8 nm) propagating in dye-doped planar polymer waveguides. Upconversion of He-Ne light (higher energy photons created by lower energy photons) was observed in planar slab waveguides made of poly(N-vinylpyrrolidone) (PVP) doped with Phloxine B. The films viewed from the side show a characteristic streak of yellow light. Monitoring it with a CCD camera we find that logarithm of intensity decays linearly with propagation distance, the slope giving the total loss factor. The loss factor at 632.8 nm depended linearly on the dye concentration providing the decadic absorption coefficient of 54 cm^-1 for neat Phloxine B. The upconverted yellow luminescence can be regarded as a convenient indicator of waveguide losses. Monitoring fluorescence instead of the scattered guided light provides much more noise-free waveguide loss data. We measured spectral and temporal response of the upconverted fluorescence as well as the dependence of luminescence on the intensity of the He-Ne beam to learn about the nature of the observed emission. The spectra excited with He-Ne laser as well as with 488 nm from cw Ar ion laser were similar. The lifetime of the luminescence in films of Phloxine B and Ethyl Eosin in PVP was of the order of 1 ms. The intensity of upconverted emission was found to vary linearly with the input intensity. We postulate that the mechanism of upconversion in the materials involves the first triplet state of the dye and generation of delayed fluorescence by these states.