He made contributions to quantum mechanics, radio wave propagation in the atmosphere, solid-state physics, and information theory.

Just a few months before Brillouin's arrival at LMU, Max von Laue had conducted his experiment showing X-ray diffraction in a crystal lattice.

[2] At the conclusion of the war, he returned to the University of Paris to continue his studies with Paul Langevin, and was awarded his Docteur ès science in 1920.

[4][5] After receipt of his doctorate, Brillouin became the scientific secretary of the reorganized Journal de Physique et le Radium.

During his work on the propagation of electron waves in a crystal lattice, he introduced the concept of Brillouin zones in 1930.

[4][5][10] Since Brillouin's study with Sommerfeld, he was interested and did pioneering work in the diffraction of electromagnetic radiation in a dispersive media.

[11] As a specialist in radio wave propagation, Brillouin was appointed director general of the French state-run agency, Radiodiffusion Nationale about a month before war with Germany, August 1939.