Photoexcitation cascade and multiple hot carrier generation in graphene

For many optoelectronic applications, such as photodetection and light harvesting, it is highly desirable to identify materials in which an absorbed photon is efficiently converted to electronic excitations. The unique properties of graphene, such as its gapless band structure, flat absorption spectrum and strong electron-electron interactions, make it a highly promising material for efficient broadband photon-electron conversion [1]. Indeed, recent theoretical work has anticipated that in graphene multiple electron-hole pairs can be created from a single absorbed photon during energy relaxation of the primary photoexcited e-h pair [2]. A photoexcited carrier relaxes initially trough two competing pathways: carrier-carrier scattering and optical phonon emission. In the former process the energy of photoexcited carriers remains in the electron system, being transferred to secondary electrons that gain energy (become hot), whereas in the phonon emission process the energy is lost to the lattice as heat. While recent experiments have shown that photoexcitation of graphene can generate hot carriers [3], it remains unknown how efficient this process is with respect to optical phonon emission.