Evanescent-field spectroscopy using structured optical fibers: detection of charge-transfer at the porphyrin-silica interface.

The fabrication of porphyrin thin films derived from dichloro[5,10,15,20-tetra(heptyl)porphyrinato]tin(IV) [Cl-Sn(THP)-Cl] in the holes of photonic crystal fibers over 90 cm in length is described. Evanescent field spectroscopy (EFS) is used to investigate the interfacial properties of the films, with the high surface optical intensity and the long path length combining to produce significant absorption. By comparison with results obtained for similar films formed from Cl-Sn(THP)-Cl inside fused-silica cuvettes and on glass slides, the film is shown to be chemisorbed as a surface Si-O-Sn(THP)-X (X = Cl or OH) species. In addition to the usual porphyrin Q and Soret bands, new absorptions in the in-fiber films are observed by EFS at 445 nm and between 660-930 nm. The 660-930 nm band is interpreted as a porphyrin to silicon charge-transfer transition and postulated to arise following chemisorption at mechanical-strain induced defect sites on the silica surface. Such defect sites are caused by the optical fiber production process and are less prevalent on other glass surfaces. EFS within optical fibers therefore offers new ways for understanding interface phenomena such as surface adsorbates on glass. Such understanding will benefit all devices that exploit interface phenomena, both in optical fibers and other integrated waveguide forms. They may be directly exploited to create ultrasensitive molecular detectors and could yield novel photonic devices.