The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all of these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy- and momentum-dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme-ultraviolet pulses to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. Whereas excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find any indication of carrier multiplication, questioning the applicability of graphene for light harvesting.

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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We have observed the production of multi-keV electrons through the irradiation of Xe clusters by an intense, near infrared, femtosecond laser pulse. We find the electron kinetic energy distribution consists of two features, a "warm" peak of between 0.1 and 1 keV and a "hot" peak of energy between 2 and 3 keV. These measurements are consistent with a picture of rapid electron collisional heating in the cluster and exhibit good agreement with numerical modeling of the electron energy distribution.[S0031-9007(96)01420-2] PACS numbers: 36.40.Vz, 33.80.Rv, 36.40.Gk Though the nature of intense, short pulse laser interactions with single atoms and solid targets has been the subject of extensive experimental and theoretical investigation over the last 15 years [1], the dynamics of intense laser interactions with large molecules and atomic clusters has scarcely been studied during this time. The production of highly charged ions from individual atoms through multiphoton [2] and tunnel ionization [3] in a strong field has been thoroughly researched, as have the energy distributions of the electrons produced during these interactions [above threshold ionization (ATI)] [4][5][6]. Concurrently, the production of hot, high density plasmas by the intense irradiation of a solid by a short pulse laser has also been the subject of detailed studies [7,8]. Experiments on individual atoms have confirmed that the majority of electrons released by single atoms in a laser field of intensity ,10 16 W ͞cm 2 typically exhibits kinetic energies of ,100 eV [6]. Interactions with solids, on the other hand, have been shown to be much more efficient at coupling laser energy into electron energy. The electron temperature in these experiments is, however, usually clamped at a few hundred eV due to the conduction of the laser energy into surrounding cold, bulk material [9].Only recently has the nature of intense laser interactions with van der Waals bonded atomic clusters of 20-100 Å been addressed in experiments. These experiments have suggested that the laser-cluster interaction is much more energetic than that of isolated atoms, producing bright xray emission (100-5000 eV photons) when a low density gas containing clusters is illuminated [10][11][12]. The interactions also appear to be quite different than those of laser solid target interactions since a cluster, though like a solid, having high local density and therefore a high collision frequency, is unlike a solid because it is an isolated system, much smaller than a laser wavelength. Consequently, the laser interacts uniformly with all the atoms, much more like the interaction of a laser with a low density gas. Recent experiments by Ditmire et al. have indicated that the electrons in a cluster undergo rapid collisional heating for the short time ͑,1 ps͒ before the cluster disassembles in the laser [11]. These measurements indicated indirect evi-dence for keV electron production in the cluster through time resolved x-ray spectroscopic data. In fact, irradiation of Xe clusters at intensi...

We use time- and angle-resolved photoemission spectroscopy with sub-30-fs extreme-ultraviolet pulses to map the time- and momentum-dependent electronic structure of photoexcited 1T-TaS(2). This compound is a two-dimensional Mott insulator with charge-density wave ordering. Charge order, evidenced by splitting between occupied subbands at the Brillouin zone boundary, melts well before the lattice responds. This challenges the view of a charge-density wave caused by electron-phonon coupling and Fermi-surface nesting alone, and suggests that electronic correlations play a key role in driving charge order.