FisMat2017 - Submission - View

Abstract's title: The ultrafast dynamics and THz photoconductivity of graphene at variable carrier density
Submitting author: Andrea Tomadin
Affiliation: Istituto Italiano di Tecnologia, Graphene Labs
Affiliation Address: Via Morego 30, 16163 Genova, Italy
Country: Italy
Oral presentation/Poster (Author's request): Oral presentation
Other authors and affiliations: S.M. Hornett (2), F.H.L. Koppens (3,4), E. Hendry (2), M. Polini (1), and K.J. Tielrooij (3) (1) Istituto Italiano di Tecnologia, Graphene Labs, Via Morego 30, 16163 Genova, Italy (2) School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK (3) ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Spain (4) ICREA - Institucio Catalana de Recerca i Estudis Avancats, 08010 Barcelona, Spain
The noteworthy optoelectronic properties of graphene stimulated an intense research activity in the past decade, among which it is of particular technological importance the design of photodetectors with breakthrough specifications [1].   Any progress in this direction relies on the understanding of fundamental physical processes such as photoexcitation and carrier relaxation (see e.g. [2,3]), and their impact on graphene's conductivity. In this work [4], we use optical-pump THz-probe spectroscopy to investigate the THz photoconductivity of a graphene system where the equilibrium carrier density is varied over a wide range by a drop cast ionic gate. The experimental results are interpreted using a theoretical framework which includes the calculations of the nonequilibrium carrier distribution following photoexcitation and of the THz conductivity of the photoexcited system. We find that, as the equilibrium carrier density increases, the photoconductivity changes sign from positive to negative and, at the same time, the dominant electronic heating channel changes from interband to intraband. We highlight the key role that, in this transition, is played by the temperature- and density-dependent electronic screening of the impurities which limit the dc conductivity. Our results are also an evidence of graphene supporting the so-called carrier multiplication process, which is of substantial technological relevance for the optimization of light-harvesting devices.

A.T. and M.P. are supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 696656 “Graphene Core1”.

[1] F.H.L. Koppens et al., Nature Nanotech. 9, 780 (2014).
[2] D. Brida et al., Nature Commun. 4, 1987 (2013).
[3] K.J. Tielrooij et al., Nature Nanotech. 10, 437 (2015).
[4] A. Tomadin et al., submitted.