Nanofiber-based spectroscopy of organic molecules
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This thesis reports on the experimental realization of nanofiber-based spectroscopy of organic molecules. The light guided by subwavelength diameter optical nanfibers exhibits a pronounced evanescent field surrounding the fiber which yields high excitation and emission collection efficiencies for molecules on or near the fiber surface.\r\nThe optical nanofibers used for the experiments presented in this thesis are realized as the\r\nsub-wavelength diameter waist of a tapered optical fiber (TOF). The efficient transfer of the\r\nlight from the nanofiber waist to the unprocessed part of the TOF depends critically on the\r\ngeometric shape of the TOF transitions which represent a nonuniformity of the TOF. This\r\nnonuniformity can cause losses due to coupling of the fundamental guided mode to other\r\nmodes which are not guided by the taper over its whole length. In order to quantify the loss\r\nfrom the fundamental mode due to tapering, I have solved the coupled local mode equations\r\nin the approximation of weak guidance for the three layer system consisting of fiber core and\r\ncladding as well as the surrounding vacuum or air, assuming the taper shape of the TOFs\r\nused for the experiments presented in this thesis. Moreover, I have empirically studied the\r\ninfluence of the TOF geometry on its transmission spectra and, based on the results, I have\r\ndesigned a nanofiber-waist TOF with broadband transmission for experiments with organic\r\nmolecules.\r\nAs an experimental demonstration of the high sensitivity of nanofiber-based surface spectroscopy, I have performed various absorption and fluorescence spectroscopy measurements on the model system 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA). The measured homogeneous and inhomogeneous broadening of the spectra due to the interaction of the dielectric surface of the nanofiber with the surface-adsorbed molecules agrees well with the values theoretically expected and typical for molecules on surfaces. Furthermore, the self-absorption effects due to reasorption of the emitted fluorescence light by circumjacent surface-adsorbed molecules distributed along the fiber waist have been analyzed and quantified. With time-resolved measurements, the reorganization of PTCDA molecules to crystalline films and excimers can be observed and shown to be strongly catalyzed by the presence of water on the nanofiber surface. Moreover, the formation of charge-transfer complexes due to the interaction with localized surface defects has been studied. The collection efficiency of the molecular emission by the guided fiber mode has been determined by interlaced measurements of absorption and fluorescence spectra to be about 10% in one direction of the fiber.\r\nThe high emission collection efficiency makes optical nanofibers a well-suited tool for experiments with dye molecules embedded in small organic crystals. As a first experimental realization of this approach, terrylene-doped para-terphenyl crystals attached to the nanofiber-waist of a TOF have been studied at cryogenic temperatures via fluorescence and fluorescence excitation spectroscopy. The statistical fine structure of the fluorescence excitation spectrum for a specific sample has been observed and used to give an estimate of down to 9 molecules with center frequencies within one homogeneous width of the laser wavelength on average for large detunings from resonance. The homogeneous linewidth of the transition could be estimated to be about 190MHz at 4.5K.