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Two-photon polymerization is a non-linear optical process based on the simultaneous absorption of two photons in a photosensitive material (photoresist). This process changes the photosensitive material, i.e. it leads to a polymerization by activating so-called photo-initiators in the resist. These turn into radicals that polymerize the resist locally. In a subsequent step, the non-polymerized photoresist is washed out to uncover the structure. The material of the structures is not restricted to just polymers but can be converted for example into silicon via a secondary chemical process.

Two-photon absorption requires high intensities - these are provided by a tightly focused femtosecond laser beam. As two-photon absorption is proportional to the square of the intensity, it only takes place in the focus providing high spatial resolution. Accordingly, the resist polymerizes only in the ellipsoidal focus, termed “voxel” (abbr. for volume pixel). Scanning the laser through the resist in all three dimensions “writes” the desired structure voxel by voxel.
During two-photon polymerization the surrounding oxygen quenches the radicals to a certain extent. This results in feature sizes down to ~100 nm. Another advantage of 2-photon polymerization is that many polymers have next-to-none linear absorption in the near-infrared, allowing the laser to penetrate deeply into the material. These two aspects allow creating nano-structures that are otherwise not possible to produce.

Computer-aided exposure of a multitude of photoresists as well as established 3D casting techniques make direct laser writing an indispensable tool for a large variety of applications in life sciences (e.g. extra cellular matrices, lab-on-a-chip,…), (opto­-)electronics or photonics (e.g. photonic crystals).