Highly sensitive detection of transient absorption in dye-doped ultrathin polymer films by the TiO2/K+ composite optical waveguide method upon pulsed laser excitation

A composite optical waveguide (OWG) composed of a 10-18 nm thick titanium dioxide (TiO2) film sputtered on a conventional K+-doped optical waveguide was first applied to detect transient absorption of organic dyes in ultrathin polymer films upon excitation with ns laser. The thickness of the TiO2 film considerably affected the relative sensitivity of the composite OWG. The composite OWG with 10 nm thick TiO2 gave much stronger transient absorption for 30-415 nm thick polymer films containing organic dyes than that with 18 nm TiO2. Transient absorption of phthalocyanine and spiropyran in 20-135 nm thick polymer films was detected 3-20 times more sensitively by the composite OWG with 10 nm TiO2 than the conventional K+-doped OWG which showed a 150-fold sensitivity as compared with the usual normal incidence method. The relative sensitivity of the composite waveguide was also affected by the thickness and refractive index of polymers.