Growth, spectroscopy and laser operation of Ho:KY(WO₄)₂

Monoclinic Ho:KY(WO₄)₂ crystals doped with up to 7.5 at.% Ho are grown by the Top Seeded Solution Growth-Slow Cooling method. The evolution of their unit cell parameters in dependence on the Ho doping and temperature is studied. The polarized low-temperature (6 K) optical absorption of the Ho³⁺ ion is investigated in detail to determine the energy of the Stark sub-levels. Room-temperature absorption, stimulated-emission and gain cross-section spectra of Ho:KY(WO₄)₂ crystals are derived for polarizations parallel to the principal optical axes, E||N_p, N_m and N_g. The maximum absorption cross-section for the ⁵I₈→⁵I₇ transition is 1.60×10⁻²⁰ cm² at 1961.0 nm and the maximum stimulated-emission cross-section for the ⁵I₇→⁵I₈ transition is 2.65×10⁻²⁰ cm² at 2056.3 nm (for E||N_m). The radiative lifetime of the upper laser level of the Ho³⁺ ion (⁵I₇) amounts to 4.8 ms. Continuous-wave Ho³⁺ laser operation is achieved under in-band pumping by a Tm laser at 1946 nm. In the microchip configuration, the maximum output power reached 205 mW at 2105 nm with a slope efficiency as high as 85%.