IMPURITY CENTERS FOR TUNABLE LASERS IN THE ULTRAVIOLET AND VISIBLE REGIONS

In these last few years much effort has been devoted to the search for new materials for solid state tunable lasers. Many crystals have been investigated with defects and/or substitutional impurities, which offer luminescence characteristics suitable for realizing lasers. After a short theoretical treatment of the main parameters of four-level vibronic lasers, a review is presented of active media, which have demonstrated lasing action in the UV-visible region at room temperature. Among them increasing interest has been devoted to two classes of Cr3+-doped crystals operating between 700 and 800 nm (oxide garnets and beryls) and to sapphire (tunability range 660-1000 nm). In the UV, Ce-doped LaF3 and LiYF4 cover the range 280-330 nm, while Nd-doped LaF3 is active in the vacuum UV, around 170 nm. Experimental data on spectroscopic features and lasing properties are presented. Alexandrite and sapphire are the best materials for the visible/IR region, while in the UV/visible similar materials do not exist and this region is not well covered. A survey is given of the promising active media. Finally, some of the experimental techniques employed to measure the characteristic laser parameters are presented.