Control of photochromic molecules adsorbed to optical microfibres

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.<br /> 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.<br /> 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. <br /> 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. <br /> 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.