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Mathematisch-Naturwissenschaftliche Fakultät - Jahrgang 2012


Titel Control of photochromic molecules adsorbed to optical microfibres
Autor Ulrich Wiedemann
Publikationsform Dissertation
Abstract The high light intensity in an optical microfibre and the resulting nonlinear effects were applied to develop a new method to precisely determine the microfibre diameter. The evanescent field of these optical microfibres was then used to control the internal state of surface-adsorbed photochromic molecules.
I start with a brief sketch of the mathematical description of light propagation in step-index optical fibres. From the results the main properties of optical microfibres are derived. Then, I describe the fabrication of optical microfibres with special requirements for the experiments presented later in the thesis.
A new technique to measure the submicrometre diameter of optical microfibres with an accuracy of better than 2 % is presented. This method is based on second- and third-harmonic generation. It is found that the fibre diameter can be unambiguously deduced from the peak wavelength of the harmonic light. High-resolution scanning electron microscope imaging is used to verify the results.
In the following, the experimental basics for the switching of photochromic molecules adsorbed to optical microfibres are described. I present the technique to deposit and detect the molecules and show their basic behaviour due to light exposure. The internal state of the molecules is measured via their state-dependent light absorption. Repeated switching between the states is achieved by exposure to the evanescent field of a few nanowatts of light guided in the microfibre.
The photochromic processes are then quantitatively analysed. Time-resolved photoswitching dynamics are measured and mathematically modelled with a rate equation model. By adjusting the microfibre evanescent field strength the dynamic equilibrium state of the molecules is controlled. I also study how many times the photochromic system can be switched before undergoing significant photochemical degradation.
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© Universitäts- und Landesbibliothek Bonn | Veröffentlicht: 17.02.2012