The ultrafast dynamics of excited carriers in graphene is closely linked to the Dirac spectrum and plays a central role for many electronic and optoelectronic applications. Harvesting energy from excited electron-hole pairs, for instance, is only possible if these pairs can be separated before they lose energy to vibrations, merely heating the lattice. Until now, the hot carrier dynamics in graphene could only be accessed indirectly. Here, we present a dynamical view on the Dirac cone by time- and angle-resolved photoemission spectroscopy. This allows us to show the quasi-instant thermalization of the electron gas to a temperature of ≈2000 K, to determine the time-resolved carrier density, and to disentangle the subsequent decay into excitations of optical phonons and acoustic phonons (directly and via supercollisions).

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The nature of the Dirac quasiparticles in topological insulators calls for a direct investigation of the electronphonon scattering at the surface. By comparing time-resolved ARPES measurements of the TI Bi2Se3 with different probing depths we show that the relaxation dynamics of the electronic temperature of the conduction band is much slower at the surface than in the bulk. This observation suggests that surface phonons are less effective in cooling the electron gas in the conduction band.The scientific and technological interest on topological insulators (TIs) stems from the unusual properties of their topologically protected metallic surface states, which exhibit a linear dispersion and a characteristic spin helicity [1][2][3][4][5][6][7]. For the Dirac quasiparticles elastic backscattering is forbidden by time-reversal symmetry, and transport is controlled by scattering events mediated by phonons. Attempts to measure the strength of the electron-phonon coupling in the representative TI Bi 2 Se 3 by angle-resolved photoelectron spectroscopy (ARPES) have produced somewhat conflicting results. The estimated values of the dimensionless coupling constant λ vary from small (λ ∼ 0.08) [8] to moderate (λ ∼ 0.25) [9]. Time-resolved ARPES (tr-ARPES) can tackle the problem in the time domain, complementary to the energy domain of conventional ARPES at equilibrium [10][11][12][13]. In pumpprobe tr-ARPES experiments, the electrons excited by a light pulse are described by an effective Fermi-Dirac (FD) distribution. The relaxation of the electronic temperature (T e ), as well as the variation of the chemical potential (µ) that reflects photo-doping of the conduction band, provide fundamental information on the de-excitation mechanisms, namely between the conduction band (CB) and the topologically protected surface state [12,13]. In a recent experiment on Bi 2 Se 3 , the contribution of various phonon modes to the electronic cooling has been addressed by comparing the relaxation dynamics of the FD distribution at various sample temperatures and for different charge densities [12].In this work we present a tr-ARPES investigation of the conduction band dynamics in Bi 2 Se 3 , where two different photon energies are exploited to vary the surface sensitivity. Standard tr-ARPES experiments, performed with laser-based sources at 6.2 eV photon energy, are rather bulk sensitive, due to the very low kinetic energy of the photo-electrons [14]. The comparison between more bulk sensitive (hν = 6.2 eV; UV) and more surface sensitive (hν = 17.5 eV; extreme UV, EUV) measurements reveals two different relaxation dynamics for T e in the conduction band. Namely, we observe a freezing of T e to an elevated value (∼ 600 K) at the surface but not in the bulk, suggesting a reduced efficiency of the phonons in the electronic cooling at the surface.A quantitative estimation of the photo-electron mean free path, l, as a function of the photo-electron kinetic energy is challenging. In particular, it is well established that at low kinetic energies ...

The emerging techniques of molecular spectroscopy by high order harmonic generation have hitherto been conducted only with Ti:Sapphire lasers which are restricted to molecules with high ionization potentials. In order to gain information on the molecular structure, a broad enough range of harmonics is required. This implies using high laser intensities which would saturate the ionization of most molecular systems of interest, e.g. organic molecules. Using a laser at 1300 nm, we are able to extend the technique to molecules with relatively low ionization potentials (approximately 11 eV), observing wide harmonic spectra reaching up to 60 eV. This energy range improves spatial resolution of the high harmonic spectroscopy to the point where interference minima in harmonic spectra of N(2)O and C(2)H(2) can be observed.