Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. Here, we investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS2 and MoSe2) through a study combining microscopic theory with spectroscopic measurements. We show that the excitonic coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. In particular, in WS2, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures.

The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton–exciton and exciton–phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors.

Nonequilibrium carrier dynamics in single exfoliated graphene layers on muscovite substrates are studied by ultrafast optical pump-probe spectroscopy and compared with microscopic theory. The very high 10-fs-time resolution allows for mapping the ultrafast carrier equilibration into a quasi-Fermi distribution and the subsequent slower relaxation stages. Coulomb-mediated carrier-carrier and carrier-optical phonon scattering are essential for forming hot separate Fermi distributions of electrons and holes which cool by intraband optical phonon emission. Carrier cooling and recombination are influenced by hot phonon effects.

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Graphene as a zero-bandgap semiconductor is an ideal model structure to study the carrier relaxation channels, which are inefficient in conventional semiconductors. In particular, it is of fundamental interest to address the question whether Auger-type processes significantly influence the carrier dynamics in graphene. These scattering channels bridge the valence and conduction band allowing carrier multiplication, a process that generates multiple charge carriers from the absorption of a single photon. This has been suggested in literature for improving the efficiency of solar cells. Here we show, based on microscopic calculations within the density matrix formalism, that Auger processes do play an unusually strong role for the relaxation dynamics of photoexcited charge carriers in graphene. We predict that a considerable carrier multiplication takes place, confirming the potential of graphene as a new material for high-efficiency photodevices.

We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 -250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical phonon frequency, however, owing to the presence of hot carriers, optical phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.PACS numbers: 78.67. Wj, 63.22.Rc, 78.47.jd Graphene, consisting of a single atomic layer of carbon atoms in a hexagonal lattice, exhibits a unique band structure with zero energy gap and linear energy dispersion, the so-called Dirac cone. The band structure gives rise to several remarkable properties, some of which are highly attractive for novel optoelectronic and photonic devices [1]. Of key importance are the dynamics of electronic relaxation, in particular due to the interaction with the phonon system. During the last years many insights into the relaxation dynamics have been obtained from single-color [2-5] and two-color [6-10] pump-probe experiments. Thermalization of the nonequilibrium electron distribution via electron-electron scattering on a sub-100 fs timescale and efficient scattering via optical phonons on a 100 fs -few ps timescale have been identified. Common to previous pump-probe experiments is an excitation energy of ∼1.5 eV, i.e. high above the Dirac point. Also most graphene photonic applications demonstrated so far involve near infrared or visible light [11,12]. The nature of graphene as a gapless material with constant absorption in the range, where the band structure is well described by Dirac cones, suggests to expand the studies into the mid infrared and terahertz range. In particular, it is crucial to obtain thorough insights into the relaxation dynamics in the range of the optical phonon energy and below, i.e., close to the Dirac point, where Coulomb as well as optical and acoustic phonon processes can be significant and were both interband and intraband processes are relevant. Here graphene serves as a model system to understand the relevance of electron-electron and electron-phonon interaction for both intraband and interband relaxation in materials with small or vanishing energy gap.In this Letter, we study the carrier dynamics close to the Dirac point by varying the excitation energy E by more than an order of magnitude (245 meV -10 meV). An optical phonon bottleneck is observed in the range E = 245 -30 meV, with decay times increasing from sub-ps to several 100 ps. Microscopic calculations based on the density matrix formalism explain these time constants by optical phonon scattering and additionally reveal contributions due to Coulomb-and acoustic phonon-induced processes. For E < 30 meV, a striking and unexpected sign reversal of the pump-probe signal is found. The in...