2. Main crystalline matrices for hosting active ions

The possibilities are limited by the substitution between lattice cations and activator ions, which should preferably have the same charge and ion radii, and by the crystallogenesis of the laser single crystal. In addition, these materials must not only meet the conditions imposed by the optical properties suitable for lasers, i.e., a wide window of transparency between the fundamental ultraviolet absorption linked to the width of the band gap and the infrared absorption of the molecular groups making up the lattices. It is preferable for the doping to provide absorption and emission bands defined by energy levels located in the band gap, rather in the visible and near infrared, which are therefore not absorbed by the gratings themselves and which, moreover, do not favor absorption between excited states leading to energy delocalization in the conduction band (figure