Refractive indices determination of a small-size nonlinear biaxial crystal by use of double-refraction measurements with a laser beam

We show that it is possible to determine the magnitude of the three principal refractive indices of a biaxial crystal from the measurement of the double-refraction effect at the exit of the crystal cut as a millimetric sphere and illuminated with a laser beam. By this way, we determined the indices of a monoclinic crystal, Ca4YO(BO3)3, at λ=0.6328 μm with an accuracy of 6×10−3. The sphere method, which we implemented several years ago, is now self-sufficient for a complete characterization of the linear and nonlinear optical properties of any new biaxial crystal.