Progress in the development of color centers for stable CW tunable laser operation

The term 'color center' is used for point-defects in insulating crystals, which introduce extrinsic electronic transitions into the forbidden gap, and, therefore, 'color' the normally transparent crystal. A wide variety of color center defects of the considered type are known. However, only three types of defects have so far been successfully operated as tunable lasers. All three types of defects are built from F centers (an electron in an anion-vacancy) in combination with other point defects. A brief introduction, survey, and critical assessment of existent color center laser systems are provided, and recently achieved advances in the field are discussed. Efforts concentrated on Tl(+) doped alkali-halides have led to the development of new near-IR laser systems of superior qualities. In recent experiments, the output power of the KCl:Tl(+) laser was improved to 750 mW in CW operation. Very recently, laser operation has been achieved in the RbCl:Tl(+) system.