No abstract

A two-photon-activatable photoacid generator, based on a bis[(diarylamino) styryl]benzene core with covalently attached sulfonium moieties, has been synthesized. The photoacid generator has both a large two-photon absorption cross section (delta = 690 x 10(-50) centimeter(4) second per photon) and a high quantum yield for the photochemical generation of acid (phiH+ = 0.5). Under near-infrared laser irradiation, the molecule produces acid after two-photon excitation and initiates the polymerization of epoxides at an incident intensity that is one to two orders of magnitude lower than that needed for conventional ultraviolet-sensitive initiators. This photoacid generator was used in conjunction with a positive-tone chemically amplified resist for the fabrication of a three-dimensional (3D) microchannel structure.

3D free‐standing and embedded metallic structures with a height of 100 μm (see Figure and also cover) have been microfabricated and characterized. Polymer nanocomposites containing metal nanoparticles, a metal salt, and an appropriate photoreducing dye are found to be efficient precursors for direct laser writing of continuous metal structures. The authors offer a versatile new approach to the 2D and 3D patterning of metals on different length scales.

Degenerate four-wave mixing measurements, using 45 ps pulses at 1064 nm, have been used to determine the magnitude of the third-order optical susceptibility tensor for thin films of a conjugated porphyrin polymer. The time dependence of the signals indicates that the dominant response is fast relative to the duration of the optical pulses. It is shown that a response on this time scale cannot be consistent with a mechanism in which resonant absorption is significant, and therefore that the primary component of the susceptibility must correspond to an instantaneous electronic polarization. The microscopic polarizability per macrocycle of the polymer is approximately 3 orders of magnitude greater than that of the monomera result that indicates the role of inter-macrocycle conjugation in the nonlinearity. This appears to be the largest one-photon−off-resonance third-order optical susceptibility reported for any organic material.

The radical photoinitiation characteristics of a range of D-it-D chromophores (where D is an electron-donor and 1t is a biphenyl, stilbene, or bis(styryl)benzene conjugated bridge) under two-photon excitation are reported. Photo-crosslinkable resins were formulated with these initiators and were used to fabricate a variety of complex three-dimensional structures by two-photon induced polymerization (TPIP). These structures illustrate that TPIP can be a highly versatile technique for the rapid single-step fabrication of complex microstructures and devices. The new two-photon resins were found to be as much as 50 times more sensitive than resins containing conventional UV initiators. The increased two-photon sensitivity is attributed to the large two-photon absorption cross-sections and the efficient electron-transfer mediated initiation of the polymerization by the D-it-D chromophores. A three-dimensional test-structure was used to evaluate how varying the intensity and the exposure times affects the shape and dimensions of the polymerization volume element (voxel). Conditions were found for which the voxel could be as small as -200 nm in width and -700 nm in length, even though the wavelength of the two-photon excitation radiation was 775 nm.