Where does Brillouin collide?

In optics Brillouin scattering is the inelastic scattering of light by acoustic waves (thermal or driven sound-wave density ripples) in a material, so that the scattered light is shifted in frequency by the acoustic frequency as photons exchange energy and momentum with phonons. Léon Brillouin's 1922 Diffusion de la lumière et des rayons X par un corps transparent homogène predicted this frequency-shifted scattering from the thermal elastic waves in a transparent body. The referent is a photon-phonon interaction, the basis of stimulated Brillouin scattering, Brillouin fibre sensors, and slow-light devices, and it is a wholly different thing from a region of reciprocal space or a magnetisation curve that happen to carry the same surname.[1]

Unit

The Brillouin frequency shift between incident and scattered light, set by the acoustic velocity

Recovery means

Not applicable

Measured by

Measured by the Brillouin frequency shift and linewidth in a scattering spectrum, and by the stimulated-scattering gain

In statistical mechanics the Brillouin function is the exact expression for the average magnetisation of an ensemble of non-interacting magnetic moments of a given angular-momentum quantum number as a function of applied field and temperature, replacing the classical Langevin curve with the quantised result. Brillouin's 1927 Les moments de rotation et le magnétisme dans la mécanique ondulatoire derived it from the quantised orientations of angular momentum in wave mechanics. The referent is a specific saturation curve used to fit paramagnetic data and extract effective moments, and it is unrelated to a light-scattering process or a Bragg-reflection boundary in a crystal apart from the common name.[2]

Unit

The reduced magnetisation as a function of the ratio of magnetic energy to thermal energy

Recovery means

Not applicable

Measured by

Fitted to magnetisation-versus-field data to recover the angular-momentum quantum number and effective magnetic moment

In solid-state physics a Brillouin zone is the primitive cell of a crystal's reciprocal lattice, the region of wavevector space within which the distinct electron or phonon states of a periodic solid can be labelled, with its boundaries marking the wavevectors that satisfy the Bragg reflection condition and so open energy gaps. Brillouin's 1930 Les électrons libres dans les métaux et le rôle des réflexions de Bragg introduced these zones to explain how band gaps arise in metals. The referent is a geometric construction in reciprocal space underlying band structure, Fermi surfaces, and phonon dispersion, and it has nothing in common with inelastic light scattering or a paramagnet's magnetisation curve beyond the shared surname.[3]

Unit

A region of reciprocal (wavevector) space bounded by Bragg planes

Recovery means

Not applicable

Measured by

Mapped by diffraction to fix the reciprocal lattice, then used to plot band structure and phonon dispersion across the zone