New Color Center Lasers Based on Molecule-Doped Alkali Halides

Abstract

Several types of F aggregate centers in alkali halide crystals have been developed in the last decade into active materials for tunable IR lasers. The ionic structures and electron distributions in ground and excited state of the more important centers, useful both for cw and mode-locked laser operation, are illustrated in Fig. 1. The centers permitting long-term stable laser operation (at cryogenic temperatures) can be categorized into three groups:

• (1) F centers attached to either Li⁺ impurities (F_A(II) centers) or Na⁺ pairs (F_B(II) centers) in the hosts KC1 and RbC1. Relaxing into an ionic saddlepoint configuration after optical excitation the centers have a strongly Stokes shifted emission with ~50% quantum efficiency. When pumped by ion lasers in the visible they provide tunable laser operation over the 2.2 to 3.4 μm region [1,2] with output power levels in the 10–100 mW range. The laser crystals are produced by simple additive coloration, can be handled at room temperature and show no output power fading effects up to ~2 W pump power levels.

• (2) F centers attached to Tl⁺ impurities (F_A(Tl) or T1°(1) centers), produced in several host lattices. Due to the large electron affinity of the Tl⁺ impurity the F center electron is localized partly (~50%) at the Tl⁺ site. The low-energy optical transitions of the centers in the IR produce a small Stokes shift and a nearly full quantum efficiency of the emission. Pumped by the 1.06 μm Nd:YAG laser line, F_A(Tl) systems in the hosts KC1 and KBr provide tunable laser operation over the 1.4 to 1.7 in range [3]. In KC1 cw output power levels in excess of 1 W have been achieved at ~5 W pump power. Laser crystal preparation of these systems requires coloration by e⁻-beam exposure and is thus restricted to facilities with a suitable e⁻-accelerator.

• (3) F ⁺₂ renters attached to Li⁺ or Na⁺ cationic impurities ((F ⁺₂ )_A centers). These F ⁺₂ -like systems retain the basic optical properties of the laser-active pure F ⁺₂ center (an electron shared by two <110> neighboring anion vacancies), i.e. they have essentially an (H ⁺₂ -molecule-like) ls σ_g↔2p σ_u transition [4] in the infrared with small Stokes shift and full emission quantum efficiency. Color center lasers based on the pure F ⁺₂ center are not stable due to pump laser-induced center reorien$ations which lead to migration and aggregation into other defects. In the (F ⁺₂ )_A center systems the reorientations are still possible but are limited to sites near the “anchoring” cation impurity. Stable (F ⁺₂ )_A center laser operation has been realized in the 1.6 to 2.5 μm range [5], with output powers up to several hundred mW in one case, KCl:Li⁺. The centers can be produced by simple additive coloration, however it is difficult to produce these centers in high concentrations in larger host ion lattices. Since (F ⁺₂ )_A centers are positively charged they require stable electron traps for their production. The latter are assumed to be provided by F_A centers [5]. In order to maintain a high concentration of (F ⁺₂ )_A centers during laser operation, continuous irradiation of the crystal with near UV light is necessary.