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Abstract's title: The role of electron-phonon scattering in the Raman spectrum of Graphene in presence of highly excited charge carriers
Submitting author: Alessandra Virga
Affiliation: IIT@Sapienza
Affiliation Address: Piazzale Aldo Moro, 5, 00185 Roma
Country: Italy
Oral presentation/Poster (Author's request): Oral presentation
Other authors and affiliations: C. Ferrante (Physics Department, University Rome “Sapienza”, Rome, Italy), L. Benfatto (ISC-CNR, Rome, Italy) , M. Martinati (Physics Department, University Rome “Sapienza”, Rome, Italy), Domenico De Fazio (Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 OFA, UK), U. Sassi (Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 OFA, UK), C. Fasolato (Physics Department, University Rome “Sapienza”, Rome, Italy), A.K. Ott (Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 OFA, UK), P. Postorino (Physics Department, University Rome “Sapienza”, Rome, Italy), D.Yoon (Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 OFA, UK), G. Cerullo ( IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, P.zza L. da Vinci 32, 20133 Milano, Italy), F. Mauri (Physics Department, University Rome “Sapienza”, Rome, Italy), A. C. Ferrari (Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 OFA, UK), T. Scopigno (Physics Department, University Rome “Sapienza”, Rome, Italy, Istituto Italiano di Tecnologia, Center for Life Nano Science @Sapienza, Rome, Italy)
Abstract

Electron-phonon scattering, along with anharmonicity,  has a pivotal role to enable the description of the equilibrium phonon properties in graphene [1] and Raman scattering is the main avenue to their characterization [2]. The interaction with light initially generates an out of equilibrium distribution of (hot) electrons with respect to the (cold) phonon bath. Relaxation to thermal equilibrium occurs within a few picoseconds through the fast electron-electron and electron-phonon non radiative recombination channels.  Hence, on the laboratory timescale, continuous wave laser sources commonly used for high resolution spontaneous Raman scattering probe a charge carriers-lattice system already thermally equilibrated.

Sub picosecond photoexcitation represents a way to impulsively localize energy into graphene’s electronic subsystem. While the response of hot charge carriers to such ultrafast perturbation has been thoroughly investigated [3,4], disclosing the behaviour of optical phonons under strongly out of equilibrium conditions remains a challenge. We perform spontaneous Raman measurements in graphene by using a 3-ps laser pulses, which is revealed to trade off between impulsive stimulation and the necessity of spectral resolution. Furthermore, we show how the Raman response of graphene can be detected in presence of an electronic subsystem temperature largely exceeding that of the phonon bath [5]. The energy and lifetime of both the G and 2D phonons as function of the carriers’ temperature in the range 1700-3100 K indicates a broadening of the Dirac cones. This is explained by modelling the electron-phonon scattering [6]  in a highly excited transient regime, which is critical for the emerging field of graphene-based nanophotonic and optoelectronic devices  operating at THz rates.

References

[1] N. Bonini, M. Lazzeri, N. Marzari, F. Mauri, “Phonon Anharmonicities in Graphite and Graphene”, Phys. Rev. Lett. 99, 176802  (2007).

[2]A. C. Ferrari, D. M. Basko, “Raman spectroscopy as a versatile tool for studying the properties of graphene”, Nat. Nanotechnol., 8, 235 (2013).

[3] C.H. Lui, K.F. Mak, J. Shan, T.F. Heinz, “Ultrafast Photoluminescence from Graphene”, Phys. Rev. Lett., 105, 127404 (2010).

[4] D. Brida et al.,“Ultrafast collinear scattering and carrier multiplication in Graphene” , Nat Commun, 4, 1987 (2013).

[5]C. Ferrante, A. Virga, L. Benfatto, M. Martinati, D. De Fazio, U. Sassi, C. Fasolato, A. K. Ott, P. Postorino, D.Yoon, G. Cerullo, F. Mauri, A. C. Ferrari, T. Scopigno, “Raman spectroscopy of graphene under ultrafast laser excitation”, arXiv:1704.00186

[6] T. Ando, “Anomaly of Optical Phonon in Monolayer Graphene”, Journal of the Physical Society of Japan, 75 (2006) 124701.