Two-Photon Polymerization of Organically Modified Ceramics Used in 3D Microfabrication
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be playing a role in the proposed stickslip mechanism. TIM PALUCKA
Two-Photon Polymerization of Organically Modified Ceramics Used in 3D Microfabrication Two-photon polymerization (2PP) of photosensitive acrylates and epoxies has been used in recent years to fabricate threedimensional (3D) microstructures such as photonic crystals and mechanical devices. Femtosecond laser pulses, which initiate the 2PP, can achieve submicrometer resolution when tightly focused into the volume of liquid resin. B.N. Chichkov of Laser Zentrum Hannover, R. Houbertz of the Fraunhofer-Institut für Silicatforschung in Würzburg, and co-workers have extended this technique to a commercial multifunctional inorganic–organic hybrid polymer that has properties advantageous for photonic applications. They have applied this technique to the fabrication of microstructures and photonic crystals with a structure size down to 200 nm and a periodicity of 450 nm. As reported in the March 1 issue of Optics Letters, the material employed for 2PP was ORMOCER, an acronym for organically modified ceramic, developed at the Fraunhofer-Institut für Silicatforschung. The researchers recognized that ORMOCERs, which are produced by a solgel method, have many properties important for photonics, such as an adjustable index of refraction in the range of 1.47–1.56, high optical transparency with low losses for both the resin and polymerized material, high chemical resistance, and exceptional thermal and mechanical properties. Specifically, ORMOCER-1 is designed for UV photolithography and contains the UVsensititve Irgacure 369 initiator. All ORMOCERs are transparent in the IR and especially at 780 nm, so that Ti-sapphire laser pulses can be focused into the volume of liquid resin. The research team used a Ti-sapphire oscillator with a repetition rate of 80 MHz, a pulse duration of 80 fs, and a laser wavelength of 780 nm. A mechanical shutter with a minimum switching time of 5 ms controlled the number of laser pulses and the radiation time. Femtosecond laser pulses were focused with a 100× oilimmersion-lens microscope objective with a numerical aperture of 1.4 and noil = 1.515 for index matching. The researchers said that resolution beyond the diffraction limit can be achieved because of the threshold behavior of the 2PP process. The researchers developed expressions for the diameter and length of the polymerized volume (volume pixel, or voxel) MRS BULLETIN/APRIL 2003
as functions of laser power and irradiation time. By curve-fitting the experimental data to these expressions, the researchers were able to determine the expressions’ unknown parameters, which in turn define the lateral and axial resolution of the objective (340 nm and 1.2 µm, respectively). The researchers demonstrated 3D microstructuring with femtosecond lasers and fabrication by 2PP by presenting scanning electron micrographs of a micrometer-scale statue of the Venus de Milo. The researchers scanned the laser beam with an x–y galvo scanner and moved the sample in the z direction with a translation s
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