title: Ultrafast Lateral Photo-Dember Effect in Graphene Induced by Nonequilibrium Hot Carrier Dynamics.
abstract: The photo-Dember effect arises from the asymmetric diffusivity of photoexcited electrons and holes, which creates a transient spatial charge distribution and hence the buildup of a voltage. Conventionally, a strong photo-Dember effect is only observed in semiconductors with a large asymmetry between the electron and hole mobilities, such as in GaAs or InAs, and is considered negligible in graphene due to its electron-hole symmetry. Here, we report the observation of a strong lateral photo-Dember effect induced by nonequilibrium hot carrier dynamics when exciting a graphene-metal interface with a femtosecond laser. Scanning photocurrent measurements reveal the extraction of photoexcited hot carriers is driven by the transient photo-Dember field, and the polarity of the photocurrent is determined by the device's mobility asymmetry. Furthermore, ultrafast pump-probe measurements indicate the magnitude of photocurrent is related to the hot carrier cooling rate. Our simulations also suggest that the lateral photo-Dember effect originates from graphene's 2D nature combined with its unique electrical and optical properties. Taken together, these results not only reveal a new ultrafast photocurrent generation mechanism in graphene but also suggest new types of terahertz sources based on 2D nanomaterials.
authors.affiliation: †Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States.
authors.forename: Chang-Hua
authors.initials: CH
authors.lastname: Liu
authors.affiliation: ‡Center for Photonics and Multiscale Nanomaterials, University of Michigan, Ann Arbor, Michigan 48109, United States.|§Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States.
authors.forename: You-Chia
authors.initials: YC
authors.lastname: Chang
authors.affiliation: †Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States.
authors.forename: Seunghyun
authors.initials: S
authors.lastname: Lee
authors.affiliation: ∥Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China.
authors.forename: Yaozhong
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: ∥Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China.
authors.forename: Yafei
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: †Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States.|‡Center for Photonics and Multiscale Nanomaterials, University of Michigan, Ann Arbor, Michigan 48109, United States.
authors.forename: Theodore B
authors.initials: TB
authors.lastname: Norris
authors.affiliation: †Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States.|‡Center for Photonics and Multiscale Nanomaterials, University of Michigan, Ann Arbor, Michigan 48109, United States.
authors.forename: Zhaohui
authors.initials: Z
authors.lastname: Zhong
keywords: Graphene; hot carriers; photo-Dember effect; photocurrent