Quantum electron dynamics in graphitic systems
Graphitic nanostructures are among the most promising versatile materials for applications in photonics and electronics. Graphene, the last carbon allotrope to have been discovered, has indeed showed a large number of remarkable fundamental effects, mostly related to its unusual band structure, which gives rise to several unique electronic and optical properties. Single-layer graphene (SLG) is also gradually emerging as a prominent platform for ultrafast photonics and optoelectronics [Nature, 490, 192, 2012], because of its ultrawideband frequency tunability [Nature Photonics 4, 611 - 622 (2010)] and its extremely fast response to optical excitation. Femtosecond laser spectroscopy has revealed the production of ultrafast (few tens of fs) non-equilibrium electronic distribution in SLG [Phys Rev Lett., 108, 167401, 2012], which then relax on the ultrafast timescale via electron-electron and electron-phonon interactions. While the slower electron-phonon thermalization process has been largely investigated, the mechanisms involved in the initial ultrafast electronic excitations are still waiting a detailed description. In the last decade, the birth of a new research field, namely “attosecond science”, has produced a revolution in the ultrafast atomic/molecular science, opening a promising way for the study of such ultrafast processes. The generation of ultrashort extreme-ultraviolet (EUV) sources, characterized by attosecond-duration, makes it possible to observe and control electronic dynamics in atomic and molecular systems. [Ann. Rev. Phys. Chem., 63, 447, 2012], [Chem. Phys. Lett., 463, 11, 2008]. Pump-probe spectroscopy is the preferred technique to study such dynamical behavior: the dynamics triggered by the pump pulse can be monitored by the time-dependent interaction of the perturbed system with the delayed probe pulse, that can be measured in terms of the absorption changes or of the kinetic energy spectrum of photoemitted electrons. The time resolution of these experiments is mainly limited by the duration of the pulses, which in the EUV spectral range can be of the order of a few hundred attoseconds [Nature Photonics 5, 655–663 (2011)]. In parallel, time-dependent electron dynamics simulations, having a natural time-scale in the range of attoseconds, can provide information on the evolution of the electron wavepackets generated by the pump field. Exploiting the theoretical time-scale advantage offered by time-dependent electron dynamics, a microscopic “on-the-fly”description of ultrafast electronic response in photoexcited systems with sub-atto-second resolution can be provided, which can be directly compared to experimental observations.
The work will be aimed at monitoring, by means of an in-house developed quantum dynamics code, the time-dependent dynamics of wavepackets generated in Single and multiple layer graphene by an external ultrashort light field. During the simulations, the electronic response of the system will be followed in terms of a large number of observables, i.e., time-dependent electron density distribution, crystal orbital occupancies, dipole moment changes and relative absorption spectra. The approach was previously successfully applied to investigate dynamics of photo-induced dissociation, non linear optical properties, on the fly electron transfer pathways, and, in particular, to study a similar photo-induced electronic response occurring in excited graphite.[see for instance, JACS, 132,12166,2010].
In the present work, the same approach will be applied in a SLG sheet studied by two-color pump-probe spectroscopy, with the aim of gaining information on electronic system reorganization as a function of pump-probe delay time at different k points.