Wavelength dependence of the optical axis in double tungstate crystals

We demonstrate experimental results on the angular dependence of the optical axis in biaxial materials from 430 nm to 1580 nm. Using pure KGd(WO 4 ) 2 (KGW) and Neodymium doped KGW crystals of around 1 cm in length. The variation of the angle is about 2.4° over the above wavelength range. Within the investigated spectral domain, a rotation of the index ellipsoid along the crystallographic b-axis is not observed, the variation occurs only in one plane. Moreover, no significant influence of doping of the crystal was observed. Comparison with literature refractive index data results in a difference of up to 0.5° with our data based on an arbitrary reference chosen at 560 nm, showing the importance of this measurement for conical refraction applications.