<title>Three-dimensional microfabrication using two-photon-activated chemistry</title>

Abstract

Photochemical reactions which can be activated by the simultaneous absorption of two photons provide a means for singlestep fabrication of complex three-dimensional microstructures. These types of structures are needed for a wide range 1 applications, including microfluidics, electrooptics, and micro-electromechanical systems (MEMS). We have shown that chromophores can be engineered to have both large two-photon absorptivities as well as an efficient means for activating chemical processes, such as radical polymerization, subsequent to the photoexcitation. Chromophores designed following this strategy two-photon-activate the radical polymerization of acrylates at lower incident laser powers than conventional UV initiators. Efficient two-photon photopolymer resins based on these chromophores were used in the fabrication of complex microarchitectures, such as photonic bandgap structures and tapered waveguides. We have devised a strategy which allows this approach to be extended to other chemical systems.

Two new asymmetrical two-photon photopolymerization initiators: Synthesis, characterization and nonlinear optical properties

“…Some approaches to improving the photosensitivity of the molecule that have been reported are based on the idea of increasing their two-photon absorption cross-sections [17] or the chromophore number density without causing aggregation. In exploring strong TPA compounds, Albota donor/acceptor strength, and molecular symmetry [18], and Reinhardt et al have emphasized the asymmetric intramolecular charge transfer ones and stressed the planarity of p-center, donor strength, and molecular asymmetry [19].…”

High-dynamic-range cationic two-photon photopolymerization

“…where ∆ 12 denotes the difference in the underlying function's values for two chosen experimental points J (1) and J (2) . J 2th needs to be excluded from Eq.…”

“…Single-photon sensitization of cationic polymerization of epoxides with thioxanthones (TX) and onium salts is known to be due to an electron transfer from the triplet state of TX to the cation of the onium salt: The extended conjugated electronic structure of the thioxanthone molecule (see Fig. 1) allows the possibility of low excitation energy via a two-photon absorption process at (1) λ max = 375 nm (two-photon equivalent being (2) λ max = 750 nm; data of λ max are listed for methanol).…”

“…(16) is to plot the data on a logarithmic scale to see whether it yields a straight line. If it does, then the exponent m can be derived from the tilt of this experimental line, which in turn is calculated by substituting the data for two experimental points J (1) and J (2) into Eq. (16):…”

“…1,2 The subject has not yet been studied in depth, though two-photon initiation was demonstrated, and several basic parameters of the process were presented. Interest in this process is not only because it is a feasible alternative to its free-radical counterpart in 3D microlithography, but also due to specific unique features of its origination from photoinduced acid generation as a basis for the process itself.…”