**1. Introduction**

142 Photonic Crystals – Innovative Systems, Lasers and Waveguides

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In nature glass is formed as a result of rapid cooling and solidification of rock melts of volcanic origin. That hints at the existence of glass to have dated back since the dawn of creation. Man has quite soon borrowed the pattern to synthesize glass for their needs: in the production of glass beads and glazes for ceramic pots and vases (3500 BC), hollow glass items (1500 BC) - these developing further through the discovery of glassblowing (between 27 BC and AD 14), leading to fabrication of cast glass windows by Romans (around AD 100) and glass sheets (11th century) (http://glassonline.com/infoserv/history.html). Starting its applications as a non-transparent material for decoration purposes, glass has made a breakthrough to become used in a wide range of fields - from housing construction through optics (for lenses and protective coatings for mirrors), information processing (optical fibres and, in general, waveguides) and storage to photonics. The ingredients of the first accidentally man-synthesized glass were nitrate and sand, the latter being mostly composed of silica (SiO2), usually in the form of quartz. On its part, quartz is the second most abundant mineral in the Earth's continental crust and is thus justifiable as a basic material in glassproduction. Its high glass-transition temperature, however, places some limitations for its applications and other substances, such as sodium carbonate, aluminium, boron, calcium, cerium, magnesium, lead, thorium oxides are added to facilitate the processing and modulate the optical properties, e.g. refractive index. Alternatively, there are non-silica containing glasses as: borate, fluoride, phosphate and chalcogenide. Namely "chalcogenides" stay in the scope of the present chapter to be revealed as superb materials for photonics and in particular – photonic crystals (PhC). It would be traced in Section 3 what are the properties that we strive to derive out from chalcogenides in order to conform to the theoretical considerations. In the following lines, however we will see what chalcogenides originally offer.
