Hot carriers in graphene

E-mail: christophe.voisin@lpa.ens. fr and bernard.placais@lpa.ens. fr Hot carrier effects are key issues in the operation of electronics and optoelectronic devices. They can also be regarded as an insightful probe of interactions in condensed matter. In this context they have been extensively investigated both in metals [1] and semiconductors [2]. Depending on the excitation energy, electron or hole relaxation mainly involves non-radiative electron–electron and electron-phonon collisions. Electron–electron interactions dominate at high energy and amount to redistribute the electrical or optical power within the electron gas and build up a hot carrier population. Electron-phonon intervenes on a longer time scale to equilibrate the electron and phonon bath temperatures and to cool the hot carriers. Both processes are highly sensitive to the nature of electronic excitations due to dispersion relation and dimensionality effects which constrain the available phase space for interactions. The advent of graphene, a decade ago, has shed a new light in this field, offering genuine 2D materials where both electrons and phonons are ultimately confined at the atomic scale. Furthermore, the nature of the carriers is new: in monolayer graphene one deals with massless Dirac fermions characterized by a linear dispersion with a large Fermi velocity, whereas multilayer graphene supports low-mass carriers but still with a gapless spectrum. Furthermore graphene is ambipolar and widely tunable with an electron-hole symmetry inherited from the crystal structure. Dirac Fermion physics is served in graphene by robust material parameters providing large energy scales suitable for physical characterization and valuable for applications. Put together, these symmetry and material properties have promoted graphene as a remarkable platform for hot carrier physics, attracting large interest from both optics and transport communities. Transport tools are appropriate to investigate, with enhanced energy resolution, the gradual building-up of a hot carrier population from a degenerate ground state. Probes are e.g. the conductance, the magneto-conductance or the current noise. At the other end of the energy spectrum, electrical heating can eventually be used to push the carrier temperature high enough to radiate light in the optical spectrum. It was soon realized that in graphene [3, 4] and carbon nanotubes [5–7], the electronic temperature can rise very high due to the combined effects of low dimensionality and weak acoustic phonon scattering. A broad temperature window is thus accessible to investigate the various phonon cooling mechanisms at work in graphene [8]; in particular the exotic regime of so-called ‘supercollisions’ could be predicted and observed in graphene [9–11]. The situation is different for electron–electron collisions which preserve the total electron gas momentum and therefore leave the electrical current unchanged. Transport alone is not a direct probe of these interactions and there is need for complementary optoelectronic characterizations. Hot carrier effects are also at the heart of the opto-electronic properties of graphene with the particularity that photo-carriers are generally excited at high