5 at% Tm-doped NaGd(MoO<sub>4</sub>)<sub>2</sub> laser crystal operated in CW conditions provided up to 641 mW of output power at λ ≈ 1910 nm with a slope efficiency of 50.8% and a pump power laser threshold of 166 mW. 10 at% Tm-doped
Li<sub>3</sub>Ba<sub>2</sub>Lu<sub>3</sub>(MoO<sub>4</sub>)<sub>8</sub> laser operated in quasi-CW conditions provided up to 510 mW of output power at λ ≈ 1950 nm with a
slope efficiency of 71.4% and a pump power laser threshold of 125 mW. Both crystals were grown by the Top Seeded
Solution Growth method at about two hundreds degrees below their melting points. The structural disorder of these
crystals confers inhomogenous broadening to the Tm<sup>3+</sup> electronic transitions. Slightly broader laser tuning range and
laser emission bandwidths are observed in the
Li<sub>3</sub>Ba<sub>2</sub>Lu<sub>3</sub>(MoO<sub>4</sub>)<sub>8</sub> crystal despite of the lower expected degree of
crystalline disorder. The crystals are promising for the development of mode locked ultrafast (fs) lasers with emission
close to λ = 2 μm.
Among the crystalline rare earth laser hosts the isotropic sesquioxides Sc<sub>2</sub>O<sub>3</sub>, Y<sub>2</sub>O<sub>3</sub>, and Lu<sub>2</sub>O<sub>3</sub> (cubic bixbyite structure) are known for their superior thermo-mechanical properties. Their thermal conductivity considerably exceeds that of Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> (YAG). Their low phonon energy ensures large energy storage times by minimizing non-radiative relaxation processes. Yb-doped sesquioxides exhibit somewhat broader absorption and emission bandwidths than Yb:YAG which is advantageous for uncritical diode laser pumping and short pulse generation. The splitting of the lower Yb<sup>3+</sup> manifold is also larger which is important in the quasi-four-level operation scheme. Solid solutions with the isostructural Yb<sub>2</sub>O<sub>3</sub> are possible but the observed strong lifetime quenching makes the sesquioxide hosts more suitable for laser geometries that profit from relatively low Yb concentrations. Lu<sub>2</sub>O<sub>3</sub> is the host whose thermal conductivity is least affected by Yb-doping. The high melting point (above 2400°C) makes it difficult to grow the sesquioxides from the melt. Recently, the use of the heat-exchanger-method (HEM) allowed to considerably enhance the optical quality of the grown crystals and the available single crystal size. Here we review the properties and present laser results obtained recently with Yb-doped sesquioxide crystals in the continuous-wave (cw) and mode-locked (picosecond and femtosecond) regimes using both Ti:sapphire and diode-laser pumping. In the cw regime optical-to-optical efficiency of 62.2% and slope efficiency of 72.7% were reached with Yb:Sc<sub>2</sub>O<sub>3</sub> operating at 1041.6 nm. Passive mode-locking of both Yb:Sc<sub>2</sub>O<sub>3</sub> and Yb:Lu<sub>2</sub>O<sub>3</sub> was achieved by semiconductor saturable absorber mirrors. Pulse durations of the order of 200 fs were obtained with intracavity dispersion compensation.