Superradiance of optical phonons in two-dimensional materials

We study the superradiance of optical phonons during the two-to three-dimensional (2D-3D) crossover of the light-matter interaction in multilayers of atomic crystals. We show the emergence of a superradiant regime with a mode having a linewidth first increasing linearly with the number N of monolayers, and then decreasing as N-3 to zero because of the formation of stationary phonon polaritons. The linewidth culminates to values of the order of the longitudinal-transverse splitting. We estimate the extremum of the radiative efficiency for various 2D materials in the superradiant regime. We predict radiative efficiencies larger than 50% for optical phonons emitting between 6 and 165 mu m. Superradiance appears as a key resource for mid- and far-infrared optophononics and advanced thermal management using multilayers of 2D materials as the active medium.