Characterization of cobalt doped ZnSe and ZnS crystals as saturable absorbers for alexandrite lasers

Abstract

Cobalt doped ZnSe and ZnS crystals have been studied to determine their effectiveness for passive Q-switching for 700-800nm spectral range (Alexandrite laser). Samples were prepared using Bridgeman technique for single-step growth of Co doped crystals as well as after growth thermal diffusion of Co in undoped crystals. ZnS:Co:Cr crystals, which have been produced using the Bridgeman technique, show maximum initial absorption coefficients of 17 cm^-1 at 725nm. Experimental results are reported on effective thermal diffusion of Co²⁺ in ZnSe and ZnS polycrystals and thermal diffusion constants of cobalt ions in ZnSe and ZnS are estimated. The nonlinear saturation properties of cobalt doped ZnSe and ZnS crystals have been investigated experimentally. The induced transparency measurements were performed using electro-optically Q-switched, alexandrite laser radiation at 731, 741, and 778 nm with a pulse duration of about 70 ns. The induced transmission measurements were analyzed using a four-level absorber model and the absorption cross sections have been estimated at both 731nm and 741nm to be 9.5 × 10^-18 cm² and 8.2 × 10^-18 cm², respectively. Absorption cross sections calculated from saturation measurements at ⁴A₂→⁴T₁(4P) transition are in agreement with results earlier reported for mid-infrared spectral region ⁴A₂→⁴T₂ of Co²⁺ ions. The described Co-doped crystals are very promising as passive Q-switches for alexandrite laser resonators. Co²⁺ centers feature high cross section of saturation and their absorption bands are nicely matched to the spectral emission of the tunable alexandrite laser. An efficient ZnS:Co:Cr passive Q-switching of the alexandrite laser cavity was realized with output energy of 15 mJ and 50 ns pulse duration.