Growth, spectroscopy and 2 μm laser operation of monoclinic Tm3+:ZnWO4 crystal

We report on the crystal growth, polarized spectroscopy, and laser operation of monoclinic Tm³⁺, Na⁺(Li⁺) codoped zinc monotungstate (ZnWO₄) crystals. The Li⁺ codoping with an optimized Tm/Li ratio enables almost complete local charge compensation leading to better crystal quality, higher Tm segregation coefficient, and longer luminescence lifetime and ultimately the demonstration of laser operation. The modified Judd-Ofelt theory was employed to calculate the Tm³⁺ transition probabilities yielding a radiative lifetime of the ³F₄ state of 2.59 ms. The corresponding intensity parameters are Ω₂ = 5.194, Ω₄ = 0.658, Ω₆ = 0.471 [10⁻²⁰ cm²] and α = 0.110 [10⁻⁴ cm]. Tm,Li:ZnWO₄ features strongly polarized emission spectra extending beyond 2 μm owing to a large total Stark splitting of the ground-state, ΔE (³H₆) = 644 cm⁻¹. The stimulated-emission cross-section in this spectral range reaches 0.47 × 10⁻²⁰ cm² at 2015 nm for light polarization E || N_p. The continuous-wave Tm,Li:ZnWO₄ laser generated 282 mW at 1.98 μm with a slope efficiency of 14.7 %, and laser emission at 2.03 μm was also achieved.