Controlling spontaneous emission with metasurfaces

Designing a light source involves not only generating photons but also controlling their properties such as the emission frequency, direction and polarization, to meet the diverse needs of light-based technologies. In the case of a monomode laser, the cavity with gain serves both to emit photons and to control the beam properties defined by the mode: the stimulated emission process ensures that all the light is emitted in a single mode. In the spontaneous emission regime, all the modes can be populated resulting in broadband, isotropic, and unpolarized emission. It is, thus, necessary to use other components, such as lenses, filters, and polarizers, to shape a light beam produced by incandescent emitters or LEDs. In this perspective paper, we discuss light-emitting metasurfaces, which are platforms that combine light emission and beam-shaping in a single thin device. We discuss the difference between diffractive metasurfaces used to control incident coherent beams and light-emitting metasurfaces, based on spontaneous, hence incoherent emission. We show that the funneling of energy from emitters into a few selected modes, which is done by stimulated emission in lasers, can be achieved for spontaneous emission by harnessing the local density of states in light-emitting metasurfaces.