Brillouin Spectroscopy
![An illustration of an example Brillouin and Raman spectrum. In practice the distinction between the two depends on which frequencies we choose to sample. Brillouin scattering generally lies within the GHz frequency regime.](/Images/godic/202501/01/Spectroscopy4915.jpg")
![Illustration of the geometric relationships between longitudinal, L, and transverse, T, acoustic waves.](/Images/godic/202501/01/Wave_Polarization4915.jpg")
![Relationships between elastic constants and X for cubic systems depending upon the direction of propagation of the phonon, q, and the eigenvector of the phonon, U, where L = longitudinal and T = transverse acoustic waves (Hayes & Loudon, 1978).](/Images/godic/202501/01/Cubic_Elastic_Constants4915.JPG")
Brillouin spectroscopy is an empirical technique which allows for the determination of elastic moduli for a given material. The technique uses inelastic scattering of light when it encounters acoustic phonons in a crystal, a process known as Brillouin scattering, to determine phonon energies and therefore interatomic potentials of a material (Polian, 2003). The scattering occurs when an electromagnetic wave interacts with a density wave, photon-phonon scattering.