Graphene is emerging as a viable alternative to conventional optoelectronic, plasmonic, and nanophotonic materials. The interaction of light with carriers creates an out-of-equilibrium distribution, which relaxes on an ultrafast timescale to a hot Fe
rmi-Dirac distribution, that subsequently cools via phonon emission. Here we combine pump-probe spectroscopy, featuring extreme temporal resolution and broad spectral coverage, with a microscopic theory based on the quantum Boltzmann equation, to investigate electron-electron collisions in graphene during the very early stages of relaxation. We identify the fundamental physical mechanisms controlling the ultrafast dynamics in graphene, in particular the significant role of ultrafast collinear scattering, enabling Auger processes, including charge multiplication, key to improving photovoltage generation and photodetectors.
Two-pulse correlation is employed to investigate the temporal dynamics of both two-photon photoluminescence (2PPL) and four-photon photoluminescence (4PPL) in resonant and nonresonant nanoantennas excited at a wavelength of 800 nm. Our data are consi
stent with the same two-step model being the cause of both 4PPL and 2PPL, implying that the first excitation step in 4PPL is a three-photon sp->sp direct interband transition. Considering energy and parity conservation, we also explain why 4PPL behavior is favored over three-and five-photon photoluminescence in the power range below the damage threshold of our antennas. Since sizeable 4PPL requires larger peak intensities of the local field, we are able to select either 2PPL or 4PPL in the same gold nanoantennas by choosing a suitable laser pulse duration. We thus provide a first consistent model for the understanding of multiphoton photoluminescence generation in gold nanoantennas, opening new perspectives for applications ranging from the characterization of plasmonic resonances to biomedical imaging.
The competition between electron localization and de-localization in Mott insulators underpins the physics of strongly-correlated electron systems. Photo-excitation, which re-distributes charge between sites, can control this many-body process on the
ultrafast timescale. To date, time-resolved studies have been performed in solids in which other degrees of freedom, such as lattice, spin, or orbital excitations come into play. However, the underlying quantum dynamics of bare electronic excitations has remained out of reach. Quantum many-body dynamics have only been detected in the controlled environment of optical lattices where the dynamics are slower and lattice excitations are absent. By using nearly-single-cycle near-IR pulses, we have measured coherent electronic excitations in the organic salt ET-F2TCNQ, a prototypical one-dimensional Mott Insulator. After photo-excitation, a new resonance appears on the low-energy side of the Mott gap, which oscillates at 25 THz. Time-dependent simulations of the Mott-Hubbard Hamiltonian reproduce the oscillations, showing that electronic delocalization occurs through quantum interference between bound and ionized holon-doublon pairs.
