We report that the optical frame orientation is wavelength independent over the entire transmission range of the nonlinear monoclinic crystal (YCOB). We used a new method based on internal conical refraction associated with x-ray diffraction on a single crystal cut as a sphere. Direct phase-matching-angle measurements of second-harmonic generation were performed in the principal planes of the spherical crystal for fundamental wavelengths up to 3.5 µm, and three absorption peaks were measured above 2.4 µm. By fitting all data simultaneously, we found new dispersion equations of the refractive indices of YCOB.
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