Multiphoton-excited fluorescence microscopy and modification of dye-doped polystyrene microspheres and polymer dispersed liquid crystals

This thesis reports the development of a multiphoton microscope and its application to the modification and characterization of dye-doped polystyrene microspheres and polymer dispersed liquid crystals (PDLCs). Multiphoton microscopy (MPM), which has been around since 1990, relies on the simultaneous absorption of multiple photons to produce fluorescence emission or to modify materials. Multiphoton absorption was predicted by Maria Goppert-Mayer in 1931. However, its application in microscopy had to wait for the development of ultrafast laser sources. Ultrafast lasers are capable of providing the intense coherent light required for multiphoton absorption. Multiphoton microscopy is used extensively in the biological sciences. Several applications in the materials sciences have also been found for MPM.; This dissertation has as its underlying theme, the use of multiphoton absorption techniques for the microscopic characterization and modification of dye-doped polystyrene microspheres and polymer dispersed liquid crystals. Polymeric microspheres are of current interest because of their potential as photonic bandgap materials and as lasing devices. PDLCs have great potential for use in electro-optical applications. PDLC materials are comprised of micron-sized droplets of liquid crystal dispersed in a polymeric matrix.; MPM was used to study some of the droplet textures that are encountered in PDLC films. Droplet textures correspond to the arrangement of liquid crystal molecules in the droplet. The electro-optical performance of the droplet is related to its texture. The MPM images of PDLC samples are generated from the three-photon excited excimer fluorescence of the cyanobiphenyl components from which the liquid crystal mixture is composed.; Dye-doped polystyrene microsphere arrays are used in this study to demonstrate their potential for optical data storage. The dye in individual microspheres is photobleached using the tightly focused output of a femtosecond laser. Each bleached sphere correspond to one bit of data storage. Readout is accomplished by imaging using laser powers at which photobleaching is negligible. Many of the exciting features of MPM, including depth profiling, background noise rejection and confinement of the illumination volume are demonstrated in the work presented in this thesis.