New Photopolymers Based on Two-Photon Absorbing Chromophores and Application to Three-Dimensional Microfabrication and O
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volumes that can be produced and the ability to excite in the near-infrared where many organic compounds have no absorption. Low absorption minimizes unwanted photochemical processes and optical fatigue upon optical readout. For instance, two-photon excitation allows one to polymerize very small volumes near the focus of a laser beam creating a change in refractive index. This index change allows for data storage densities of 1012 bits/cm 3 [1]. The greatest advantage in the use of twophoton excitation comes from an extremely small excitation depth in the longitudinal direction. Two-photon absorption falls off as z 4 in this direction since the process depends on the square of the excitation intensity. Confinement is smaller than that achieved by single-photon excitation in the lateral dimension as well, but only by a factor of 1/4I2. The exceptional volumetric confinement afforded by two-photon excitation also presents unique opportunities in three-dimensional microlithography or microfabrication. This technique has been previously presented by Webb and coworkers [2, 3]. The Webb group has demonstrated 0.5 gim trenches with an aspect ratio of 8 in polyimide films as well as trenches with an undercut profile. They have also used two-photon polymerization of acrylate systems to write letters with linewidths on the order of 1 lim. Maruoetal. have fabricated a helical structure with a diameter of 6 gtm and a pitch of 10 tim [4]. Unfortunately, little work has been done to synthesize, by design, molecules with large two-photon absorptivities. Photopolymerization by the two-photon process, as
217 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
discussed above, has generally used commercially available one-photon photopolymer systems, with no effort to optimize the materials system. Recognizing that the class of bis-donor-substituted molecules with extended conjugation should have strong twophoton absorption, our group has synthesized and characterized many compounds of this type. Structure/property studies have allowed for a greater understanding of the chemical properties leading to strong two-photon absorption. Also, control of the chemical structure of these compounds affords wavelength tunability of the two-photon absorption maximum. Compounds with absorption maxima ranging from 530 nm to 980 nm with two-photon absorptivities, 8, of up to 1500x10-50 cm 4 s/photon have been characterized. This value of 8 is unprecedentedly high. Many of these chromophores are also good electron donors and can initiate radical polymerization of acrylic monomers by photoinduced electron transfer. Excited-state charge transfer rates have been determined for a series of bis-donor diphenyl polyenes and triarylamines. In order to explore the
efficiency of polymerization initiated by these molecules, rates of polymerization have
also been measured. Photopolymer systems incorporating these chromophores have been developed and used to demonstrate 3D microfabrication and optical data storage potential. EXPERIMENTAL Two
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