Refined orientation of the optical axes as a function of wavelength in three monoclinic double tungstate crystals KRE(WO_4)_2 (RE = Gd, Y or Lu)

We measured a rotation of the dielectric frame as a function of wavelength of about 2° in the spectral range of 0.4-1.6 μm in the monoclinic double tungstate crystals, KRE(WO4)2 where RE = Gd, Y or Lu. We also refined the Sellmeier equations of their three principal refractive indices. By taking into account these data, we calculated the real orientation of the optical axes as a function of wavelength which is of key importance for the design of conical refraction lasers based on monoclinic double tungstate crystals.