Photochemical Processes for Microstructuring

In the first case I, the energy of the laser photons, i.e. at short wavelengths Al, is larger than the effective band gap. This corresponds with the absorption edge of the glass, which results in a strong laser beam absorption. In 

The photon absorption is caused by different effects in glasses and is still not fully understood. Nevertheless, three main possibilities for the absorption are considered and explained for OH-group free and OH-group containing silica glass [138]. Deep UV (DUV) absorption can take place at defect centres, such as the E -centre, the non-bridging oxygen hole centre (NBOHC) and on peroxy radicals. 

The optical band gap for pure crystalline silica is 8.7eV (142 nm), whereas the band gap of a very pure silica glass is only 8.3eV (149nm). The reduction in the band gap is due to the weaker and varying Si-O-Si bonds (see Sect. 1.1.4). If this fact is superimposed by localised non-stoichiometric compositions and strained bonds, a weak absorption band ranging from 160 to 200 nm appears. 

Also the UV-light induced photoreduction in doped phosphate and fluoride-phosphate glasses [359] and the phase separation in highly tin-doped fibres and preforms during UV-eposure [63] were investigated. 

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