Multiphoton-Excited Fluorescence Imaging and Photochemical Modification of Dye-Doped Polystyrene Microsphere Arrays Gerald H. Springer and Daniel A. Higgins* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506-3701 Received November 10, 1999. Revised Manuscript Received March 7, 2000 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 demon- strated by bleaching individual spheres in 3-D arrays, without affecting neighboring spheres. Optical data storage densities as high as 10 13 bits/cm 3 are readily achievable. Unique photobleaching patterns observed within the spheres are explained by the radiation distribution within individual microspheres under focused-beam illumination. I. Introduction Ordered polymeric microsphere arrays find a range of possible applications in optical device technology. They are presently used in the fabrication of photonic band gap materials, 1 and may also have applications in organic light-emitting diodes. 2 The observation of stimulated emission 3 from novel whispering gallery modes has further increased interest in these materials. 4-8 Optical data storage in spheres and mi- crosphere arrays has also recently been demonstrated. In previous work by Denk et al., it was shown that relatively large (6 μm diameter) dye-doped spheres could be locally photobleached and reimaged using multi- photon-excitation methods. 9 More recently, Kumacheva and co-workers demonstrated that conventional linear optical methods could be used in a similar manner to modify 3-D microsphere arrays. 10 In this paper, it is demonstrated that well-ordered 3-D arrays of fluorescent microspheres may represent the best medium for microsphere-based data storage ap- plications. In these arrays, individual spheres represent the individual bits. Because of the well-ordered nature of the arrays, the individual bits are inherently addres- sable (i.e., the location of each bit is defined by the physical position of the sphere). As originally demon- strated by Denk and co-workers, 9 it is also shown that multiphoton-based optical techniques are ideal for inducing photochemistry within the spheres and for imaging the 3-D sphere arrays (representing writing and readout steps in the data storage process). Similar methods have also recently been employed to fabricate microminiature devices in photoresist materials. 11,12 Nonlinear optical methods provide several distinct advantages over alternative optical methods for materi- als modification and imaging. 13,14 Perhaps most impor- tantly, the use of multiphoton absorption confines the volume modified to the volume of the laser focus. 9,11,12,15,16 The volume in which fluorescence is excited in imaging experiments is similarly confined, providing enhanced * To whom correspondence should be addressed. (1) Yablonovitch, E. J. Mod. Opt. 1994, 41, 173. (2) Yamasaki, T.; Tsutsui, T. Appl. Phys. Lett. 1998, 72, 1957. (3) Tzeng, H.; Wall, K. F.; Long, M. B.; Chang, R. K. Opt. Lett. 1984, 9, 499. (4) Kuwata-Gonokami, M.; Takeda, K.; Yasuda, H.; Ema, K. Jpn. J. Appl. Phys. 1992, 31 Pt. 2, L99. (5) Kuwata-Gonokami, M.; Takeda, K. Opt. Mater. 1998, 9, 12. (6) Kamada, K.; Sasaki, K.; Masuhara, H. Chem. Phys. Lett. 1994, 229, 559. 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