Three-dimensional nanostructuring of hybrid materials by two-photon polymerization
Two-photon polymerization (2PP) of photosensitive inorganic-organic hybrid polymers (ORMOCERs, developed at the Frauenhofer Institut für Silicatforschung) is demonstrated as a very promising approach for the fabrication of complicated three-dimensional micro- and nanostructures. These materials are produced by sol-gel synthesis with molecular level mixing of different components. It is remarkable that properties of the hybrid polymers can be tuned from those that are characteristic for organic polymers to those that are similar to inorganic glasses. They have negative resist behaviour and can be used as storage-stable, liquid photo-polymerizable resins. When Ti: sapphire femtosecond laser pulses are tightly focused into the volume of this resin (which is transparent in the infrared) they can initiate twophoton polymerization process transferring liquid into solid state. This process is confined to a highly localized area at the focal point due to the quadratic dependenc e of the two-photon absorption rate on the laser intensity. When the laser focus is moved through the resin in three dimensions, the polymerization occurs along the trace of the focus. This allows to fabricate any computer-generated 3D structure by direct laser "recording" into the volume of the ORMOCER. The non-irradiated liquid resin can be dissolved in alcohol leaving the polymerized copy of the computer model. Compared to conventional photo-lithography which is a planar processing, two-photon polymerization is a real three-dimensional volume microfabrication technique. This technology can be used for rapid prototyping and low-cost fabrication of artificial micro- and nanostructured components which are required for different applications in optics, medicine, and biology. Numerous examples such as photonic crystals, micromechanical and microoptical devices are presented.