PhD, University of Cincinnati, 2012, Arts and Sciences: Physics

In this thesis, linear and non-linear optical properties of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and of tris(8-hydroxyquinolinato)aluminum (Alq3)films have been investigated using various experimental methods. The films were grown using the technique of organic molecular beam deposition (OMBD). The dispersion of the in-plane and normal refractive index in PTCDA waveguides has been studied using the m-line technique. The PTCDA waveguides were grown on Pyrex substrates. TE and TM mode coupling at excitation wavelengths ranging from 633 to 910 nm has been accomplished using a Rutile prism. The derived refractive index values are in good agreement with existing ellipsometric data, which emphasizes the high structural quality of our waveguides. The nonlinear absorption (two-photon absorption) of PTCDA and Alq3 films has been investigated using the z-scan technique. The films were grown on Pyrex substrates and a high repetition rate (80 MHz) laser was used as excitation source. Various methods have been utilized to minimize laser induced damaging of the soft organic films and to improve the signal-to-noise ratio. In addition, nonlinear fluorescence (two-photon induced fluorescence) measurements have been performed on Alq3 films to further investigate nonlinear absorption processes in this material. The singlet-singlet annihilation (nonlinear bimolecular quenching) of excitons in disordered quasi-amorphous Alq3 films has been investigated using both time-resolved and continuous wave (cw) spectroscopic techniques at temperatures ranging from 15 K to 300 K. A significant decrease of the PL efficiency with increasing exciton density (excitation intensity), especially at low temperatures, has been observed which is attributed to bimolecular quenching of excitons that are funneled into low-energy traps. This effect is different from the known diffusion based singlet-singlet annihilation in Alq3. To explain both the intensity and temperature dependence (open full item for complete abstract)

Committee: Hans Peter Wagner PhD (Committee Chair); Young Kim PhD (Committee Member); Leigh Smith PhD (Committee Member); L.C.R. Wijewardhana PhD (Committee Member) Subjects: Physics

PhD, University of Cincinnati, 2008, Arts and Sciences : Physics

This thesis contains optical investigations on 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA) and tris (8-hydroxy) quinoline aluminum (Alq3) films as well as on PTCDA/Alq3 multilayers and co-deposited films. The organic layers are grown on Si and Pyrex substrates using the technique of organic molecular beam deposition (OMBD). In temperature dependent (10 - 300 K) transmission measurements the exciton absorption in PTCDA⁄Alq3 multilayers and co-deposited films is shifted to higher energies with respect to the pure PTCDA films. In comparison with a recently developed theoretical model for α PTCDA the observed energy shift is explained by Coulomb screening. In strain dependent photoluminescence (PL) measurements of PTCDA film uniaxial pressure is applied along the molecular stacking direction using a specially designed pressure cell. With increasing pressure exciton emission channels are shifted to lower energy and the integrated PL intensity is quenched. When the pressure is released, the PL spectrum and the total PL intensity partially recover, indicating a reversibility of the strain effects to a large extent. The experimental results are compared with recent total energy calculations. The assignment of different emission channels in PTCDA single crystals and polycrystalline films is validated by photoluminescence excitation studies. From the excitation energy dependence of the spectral position of different emission bands the transition energies of free and self trapped excitons are deduced. The obtained transition energies are compared with theoretically predicted values. The anisotropy of the refractive index in PTCDA waveguides is investigated using the m-line technique. The observed effective refractive index values of TE and TM modes are used to determine the waveguide thickness as well as the inplane and perpendicular bulk refractive index values. The recombination dynamics of excitonic states in thin Alq3 films is studied using temperature and time (open full item for complete abstract)

Committee: Hans-Peter Wagner PhD (Committee Chair); Kay Kinoshita PhD (Committee Member); Michael Ma PhD (Committee Member); Leigh M Smith PhD (Committee Member) Subjects: Molecules; Optics; Physics