Multiphoton-Excited Fluorescence Imaging and Photochemical Modification of Dye-Doped Polystyrene Microsphere Arrays

The use of nonlinear optical methods for thin-film polymeric materials modification and characterization is explored. Ordered 3-dimensional (3-D) dye-doped polystyrene microsphere arrays are photobleached and imaged in these studies. Efficient, irreversible photochemical bleaching of the dye within individual 0.5 and 1 μm diameter microspheres occurs when 810 nm light from a mode-locked Ti:sapphire laser is focused to an ∼400 nm diameter spot within the spheres. Photobleaching is shown to result from three-photon absorption and may involve ionization of the dye. The three-photon-induced photochemistry is dramatically more efficient than that resulting from single-photon excitation. Imaging of the unbleached and bleached arrays is accomplished by monitoring the two-photon-excited fluorescence from the dye. Both the nonlinear photobleaching and imaging methods provide inherent depth-discriminating capabilities, allowing for high-resolution 3-D control of the volume modified and imaged. The results suggest that the methods and materials employed here may have important optical data storage applications. The capabilities of these methods are demonstrated by bleaching individual spheres in 3-D arrays, without affecting neighboring spheres. Optical data storage densities as high as 1013 bits/cm3 are readily achievable. Unique photobleaching patterns observed within the spheres are explained by the radiation distribution within individual microspheres under focused-beam illumination.