The photoresponse of materials is governed by energy relaxation pathways of photo-excited electron-hole pairs. In graphene, due to frequent electron-electron collision and weak electron-lattice coupling, a novel transport regime is reached in which the photo-generated carrier population can remain hot while the lattice stays cool. In this talk, I will show that light is converted to electrical currents through a hot-carrier assisted thermoelectric effect in intrinsic graphene[1]. The thermal energy slowly leaks to the lattice via two distinct processes of electron-phonon coupling that can be tuned by temperature and charge density[2]. We also implemented a scheme to control the initial stage of photo-excited carriers before they collide with ambient carriers (less than ten femtoseconds) to form a hot Fermi-Dirac distribution, which is realized in a graphene-boron nitride-graphene heterostructure[3]. 

      The weak electron-phonon coupling and frequent electron-electron scattering revealed above strongly alter the nature of particle and energy transport, leading to a collision dominant fluid behavior for electrons. In the last part of the talk, I will discuss our observation of highly-ordered photocurrent patterns at the charge neutral point of graphene, which is likely related to the ballistic thermal transport of hot electron-hole plasma in the hydrodynamic regime[4]. 

[1] N. Gabor, J. Song, Q. Ma, N. Nair, T. Taychatanapat, K. Watanabe, T. Taniguchi, L. Levitov    and P. Jarillo-Herrero, Science 334, 648 (2011).

[2] Q. Ma, N. Gabor, T. Andersen, N. Nair, K. Watanabe, T. Taniguchi and P. Jarillo-Herrero, Phys. Rev. Lett. 112, 247401 (2014).

[3] Q. Ma, T. Andersen, N. Nair, N. Gabor, M. Massicotte, C. Lui, A. Young, W. Fang, J. Kong, K. Watanabe, T. Taniguchi, N. Gedik, F. Koppens and P. Jarillo-Herrero, Nature Physics (2016) doi:10.1038/nphys3620.

[4] Q. Ma, et al., “Geometric Edge Photocurrent Generation and Hydrodynamics in Graphene.”    In preparation (2016)