A single crystal of NaGd(WO 4) 2 with disordered structure was grown by the Czochralski method with 5 mol % of Tm 3+-doping in the melt. This crystal host belongs to the I 4 tetragonal space group. The Tm 3+ optical absorption and emission spectroscopy revealed strong polarization dependence in accordance with the S 4 local site symmetry of the uniaxial host. The Stark energy levels of Tm 3+ in this host were determined from 5 K optical absorption and photoluminescence measurements. The 300 K lifetime of the upper laser level for the 3 F 4 → 3 H 6 transition amounts to 1.35±0.20 ms. The maximum ground state absorption cross sections for the 3 H 4 level of interest for diode pumping near 795 nm are 2.9×10-20 and 1.18×10-20 cm 2 for the σ and π polarization, respectively. The maximum emission cross sections amount to 10.6×10-21 cm 2 at 1796 nm and 9.5×10-21 cm 2 at 1847 nm, for the σ and π polarization, respectively. Laser operation in the cw regime has been achieved at room temperature under Ti:sapphire and diode laser pumping using an uncoated sample. A maximum output power of ≈300 mW was obtained at 1925 nm for the σ polarization under Ti:sapphire laser pumping. Slightly higher powers were obtained with diode pumping but the threshold pump power also increased. In this case the laser naturally selected the σ polarization. The optical bands of Tm 3+ in this crystal have inhomogeneous contribution and their FWHM reaches, e.g. for the 3 H 6 → 3 F 4 transition at 5 K, about 60 cm-1. This is related to Na and Gd (or Tm) local disorder between the two nonequivalent lattice sites of the host. The full cw laser tunability range at 300 K, from 1813 to 2025 nm, corresponds to ≈17 THz, and is one of the broadest achieved with a Tm 3+-doped crystalline material, despite the relatively low efficiency obtained with this first sample of Tm:NaGd(WO 4) 2 .

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Continuous wave and femtosecond mode-locked laser operation of Yb 3+ in the tetragonal NaLu͑WO 4 ͒ 2 crystal host is demonstrated by pumping with a Ti:sapphire laser. Pumping with 1.8 W at 974 nm, a maximum output power of 650 mW was achieved at 1029.6 nm. The slope efficiency was in excess of 60%. The laser performance was similar for the two polarizations. By inserting a birefringent filter the output wavelength was tunable from 1010 to 1055 nm. Pulses as short as 90 fs with an average power of 50 mW were generated by passive mode locking at a repetition rate of 95 MHz. These attractive laser properties of NaLu 1−x Yb x ͑WO 4 ͒ 2 are related to the inhomogeneous broadening of the Yb 3+ spectral features resulting from the local disorder of the host crystal. We report the spectroscopic properties of Yb 3+ in the 5 -300 K temperature range and the optical properties of the host at room temperature.

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