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

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.