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Tracking electron dynamics in graphene and transition metal dichalcogenide

Date Do, 24.11.2016 - Do, 24.11.2016
Time 16:30
Speaker Dr. Céphise Cacho Artemis Central Laser Facility, Rutherford Appleton Laboratory, United Kingdom
Location EPFL CH G1 495, Lausanne
Program Abstract
Novel quantum materials such as graphene and transition metal dichalcogenides (TMDC) are attracting vast interest particularly for their application in spintronic and optoelectronic devices. Their properties are intrinsically governed by the large momentum electrons (at the Brillouin zone K-point). In order to eject such electrons in vacuum and observe their dynamics, a high energy (>20 eV) photon source is required as well as ultrashort pulse duration. High Harmonic Generation source [1] combined to an Angle-Resolved PhotoEmission Spectroscopy (ARPES) end-station is a powerful tool to observe such electron dynamics. After an introduction on ARPES and experimental concepts, I will present an overview of few recent time-resolved ARPES studies performed at the Artemis facility. A quasi-free-standing monolayer of graphene (MLG) is optically excited in a direct interband regime. The measurement of the electronic temperature reveals the role of the supercollisions during the cooling process of the electrons [2]. Over ~100 fs, a population inversion across the Dirac point is observed [3] supporting the potential of graphene for THz amplification applications. Single-layers of MoS2 [4-5], WS2 [6] and bulk WSe2 [7] were resonantly pumped across the band gap at the K point. Measurements of the valence and conduction bands populations give access to the direct quasiparticle band gap [4]. Depending on the screening effects a strong band gap renormalization can be induced [5] by optically excited free charge carriers.
Control of spin- and valley-quantum numbers in valence and conduction bands is achieved [6-7] by using circularly polarized optical excitation.
1. F. Frassetto et al., Optics Express 19, 19169 (2011)
2. J. Johannsen et al., Phys Rev Lett 111, 027403 (2013)
3. I. Gierz et al., Nature Materials 12, 1119 (2013)
4. A. Grubišić Čabo et al., Nano Lett. 15, 5883 (2015)
5. S. Ulstrup et al., ACS Nano 10, 6315 (2016)
6. S. Ulstrup et al., arXiv:1608.06023 (2016)
7. R. Bertoni et al., arXiv:1606.03218 (2016)
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