High-intensity brillouin scattering from parametrically excited phonons in FeBO3

The saturation of the ferromagnetic resonance in FeBO₃ is due to the parametric decay into short wavelength phonons of half the exciting frequency. These phonons are observed using the technique of inelastic light scattering. Their intensity is so high (10⁵ above thermal level) that the scattered light can be observed visually. Light scattering and parametric decay selection rules result in well defined scattering angles. The analysis of these directions allows the evaluation of anisotropic sound velocities.