Justin Song, MIT
Monday, January 13, 2014 - 4:00pm to 5:00pm
When light is absorbed in a material, electron-hole pairs can be created far above the system's Fermi surface. The way in which the absorbed energy gets distributed amongst a system's available degrees of freedom determines its response and plays a crucial role in its ability to handle, convert, and utilize energy. I will describe how absorbed light energy in graphene is redistributed. In particular, I will illustrate how the excited electron distribution looks like from short time scales (tens of femtoseconds) to long time scales (tens of picoseconds). As I will show, energy redistribution occurs in two distinct stages: i) a fast energy cascade of photoexcited electrons that is dominated by electron-electron scattering, and ii) a slower relaxation of an elevated electron temperature to lattice degrees of freedom. In both stages, graphene's material properties provide interesting twists to the story. In the former, fast electron-electron scattering allows ambient electrons in doped graphene to capture a large fraction of the absorbed light energy (in contrast to traditional seminconductors). In the latter, a large optical phonon energy and a small Fermi surface allow disorder mediated phonon emission to dominate the cooling of a hot electron temperature. The character of these processes pave the way for graphene as a new opto-electronic material.