“…an alkali metal or Ag, and Tl, RE=trivalent rare-earth metal, and M=Mo or W [15,16]. Low-temperature polymorphs of potassium and rare earth double tungstates, KRE(WO 4 ) 2 (or simply KREW, where RE=Y, Gd, and Lu) doped with Nd 3 þ , Ho 3 þ , Er 3 þ , Tm 3 þ , and Yb 3 þ ions are commonly used inorganic materials for high-power and short-pulse diode-pumped solid-state lasers, amplifiers, integrated optics and Raman frequency conversion [8][9][10][11][12][13][17][18][19][20][21][22]. High physical and optical anisotropy resulting from low symmetry structure, possibility of high doping levels due to close ionic radii without luminescence quenching and significant lattice distortions as well as a wide range of laser emission, i.e.…”