= Brillouin scattering
{c}
= Brillouin light scattering
{c}
{synonym}
<Brillouin scattering> is inelastic <scattering of light> involving <acoustic phonons> or the corresponding traveling elastic fluctuations. The scattered <photon> exchanges <energy> and <momentum> with the disturbance, producing a small shift from the incident <frequency>. For an isotropic medium with sound speed $v$, <refractive index> $n$, vacuum <wavelength> $\lambda_0$ and scattering angle $\theta$, the shift magnitude is approximately $\Delta\nu=(2nv/\lambda_0)\sin(\theta/2)$.
A narrow-line <laser> illuminates a sample, collection optics select a scattering angle, and a high-resolution <Fabry–Pérot interferometer> separates weak shifted components from strong nearly unshifted scattering. Tandem interferometers improve rejection of the central component. With known geometry and <refractive index>, the shift measures an acoustic speed and constrains elastic properties.<Brillouin Spectroscopy Lab by Carnegie Department of Energy Alliance Center>[Frequency-shift relation, tandem interferometer description and Figures 1–2]{ref}
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