Efficient Micro-Lasers Based on Highly Doped Monoclinic Double Tungstates

We demonstrate the suitability of monoclinic double tungstates (MDTs), KRE(WO₄)₂, where RE = Gd, Y or Lu, doped with Nd³⁺, Yb³⁺, Tm³⁺, or Ho³⁺ ions and co-doped with Yb³⁺-Tm³⁺, or Tm³⁺-Ho³⁺ ion couples, for highly efficient micro-lasers at <inline-formula> <tex-math notation="LaTeX">$\sim 1~\mu \text{m}$ </tex-math></inline-formula> and at <inline-formula> <tex-math notation="LaTeX">$\sim 2~\mu \text{m}$ </tex-math></inline-formula>. This is facilitated by the use of high rare-earth doping levels (up to 15 at.% for Tm, 10 at.% for Nd, and 25 at.% for Yb) and a special crystal cut along the <inline-formula> <tex-math notation="LaTeX">$\text{N}_{\mathrm {g}}$ </tex-math></inline-formula>-axis providing the thermal guiding. Record slope efficiencies for bulk MDT lasers are achieved for each studied ion. A 15 at.% Tm:KLu(WO₄)₂ laser generated 785 mW at 1957<inline-formula> <tex-math notation="LaTeX">$\ldots$ </tex-math></inline-formula>1965 nm with a slope efficiency <inline-formula> <tex-math notation="LaTeX">$\eta =77$ </tex-math></inline-formula>%. The quantum efficiency for Tm³⁺ ions amounted to <inline-formula> <tex-math notation="LaTeX">$\eta _{\mathrm {q}} = 1.98\pm 0.02$ </tex-math></inline-formula>. With a 0.9 mm-thick 25 at.% Yb:KLuW micro-laser, <inline-formula> <tex-math notation="LaTeX">$\eta =91$ </tex-math></inline-formula>% is achieved, approaching the theoretical limit set by the Stokes shift.