Direct laser writing

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A way to structure photosensitive materials in three dimensions is called 'Direct Laser Writing' (DLW). Let's assume you take a laser operating at a wavelength where the exposed photo-resist would usually be completely transparent due to the non-existence of one-photon absorption processes. In this case the chemical property of the photosensitive material can not be altered unless one focuses ultrashort laser pulses hard into the material. By that the likeliness of multi-photon absorption is strongly increased in the very focal volume. Hence, a chemical modification of this area occurs, which in a subsequent baking process leads to a local polymerization (in the case of using the photoresist SU-8)^[1]. By scanning the photoresist relative to the fixed focal position one can write arbitrary 3D structures like photonic crystals into the photosensitive material (e. g. SU-8, Ormocere, PDMS, chalcogenide glass), which is quite similar to the rapid prototyping technique.

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