Monoclinic Tm:MgWO4 crystal: Crystal-field analysis, tunable and vibronic laser demonstration

Highlights: • Thulium (Tm³⁺) doped monoclinic magnesium monotungstate crystal (Tm:MgWO₄). • Determination of the Stark splitting of Tm³⁺ multiplets and barycenter plot analysis. • Crystal-field theory modified for intermediate configuration interaction (ICI). • A unique feature of Tm:MgWO₄ to provide a naturally polarized emission above 2 μm. • First tunable (165 nm broad tuning range) and vibronic (at 2093 nm) Tm:MgWO₄ lasers. Tm³⁺-doped monoclinic magnesium monotungstate (MgWO₄) is a promising material for lasers emitting at above 2 μm. Stark splitting of ³H₆ to ¹G₄ Tm³⁺ multiplets in MgWO₄ is determined using low-temperature spectroscopy, calculated within crystal-field theory modified for the case of an intermediate configuration interaction (ICI) and analyzed in terms of the barycenter plot. Tm³⁺:MgWO₄ features a large Stark splitting of the ground-state (³H₆), 633 cm⁻¹, and high transition cross-sections for polarized light due to its low-symmetry structure. A unique feature of Tm³⁺:MgWO₄ to provide a naturally polarized emission at >2 μm, as well as its suitability for broadly tunable and mode-locked lasers in this spectral range are argued. The first tunable and vibronic Tm³⁺:MgWO₄ lasers are demonstrated, yielding a continuous tuning from 1897 to 2062 nm in the former case. A vibronic laser operated at even longer wavelengths up to 2093 nm due to electron-phonon coupling with low-energy phonons of the crystalline host. Moreover, the optical indicatrix axes of high-refractive-index biaxial MgWO₄ are assigned.