Abstract:
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 of the quenching phenomena a simple model has been developed and used to analyze and to simulate the complex experimental observations.
To investigate the possible influence of the degree of disorder on the bimolecular quenching of trapped excitons, PL efficiency experiments have been performed on Alq3 films that have been grown at different growth conditions. The investigations include films with varying thickness, films that were grown at various growth rates and on different substrates as well as annealed Alq3 films. Changes of bimolecular annihilation effects due to these modifications have been qualitatively explained.
