Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging1, optical data storage2,3 and lithographic microfabrication4,5,6. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures7.
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We thank S. Fraser and the Biological Imaging Center, California Institute of Technology, for use of a two-photon scanning laser microscope system, and S. Thayumanavan for synthetic precursors used in this work. H.R. thanks the Alexander von Humboldt Stiftung for a postdoctoral fellowship through the Theodore Lynen Program. This work was supported by the Ballistic Missile Defense Initiative Organization (at the Jet Propulsion Laboratory) and by the Office of Naval Research (through CAMP at the University of Arizona), the NSF and the Air Force Office of Scientific Research (at the California Institute of Technology).
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Cumpston, B., Ananthavel, S., Barlow, S. et al. Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication. Nature 398, 51–54 (1999). https://doi.org/10.1038/17989
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