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 hetero structure[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

简介：

Qiong Ma graduated from the University of Science and Technology of China (USTC) in 2010 and pursued PhD under the supervision of Prof. Pablo Jarillo-Herrero in the Department of Physics at MIT. Her research is to explore fundamental physical processes of light-matter interaction in two-dimensional materials, including graphene, boron nitride, transition metal dichalcogenides and heterostructures formed from these materials.

时间：2016年5月6日（星期五）上午10：30

地点：中科院物理研究所M236会议室

联系人：金奎娟研究员（Tel：82648099）

邀请人：陆凌副研究员（Tel：82649203）

主办单位：中国科学院光物理重点实验室