Amplification properties of polycrystalline Fe:ZnSe crystals for high power femtosecond mid-IR laser systems

Abstract In this Letter, a multi-pass amplifier based on polycrystalline Fe:ZnSe element grown by the chemical vapor deposition (CVD) method and doped through diffusion technique is realized with output energy of more than 3 mJ at 4.5 μm. The crystal used in the experiment has external doping layers with peak Fe ions concentration of (1-2)х1019 сm−3 located on the surface and Fe ions diffusion depth of hundreds of microns. The crystal demonstrates small-signal gain per pass of 5.7 (corresponding to effective gain of 7.1 cm−1 taking into account 8 mm total length of the crystal) for short pump pulse (40-ns) under the optimal pump fluence of around 0.8 J/cm2 and upper-level lifetime of 670 ± 10 ns under moderate cooling to 7-8 °C, which makes such a crystal very attractive for mid-IR femtosecond multi-pass amplifiers and generators. Moreover, properties of the polycrystalline Fe:ZnSe elements grown by CVD and doped through diffusion and CVD methods with higher Fe ions concentration are also studied at different temperatures and compared with a single crystal element.