Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum-and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap, since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a non-direct gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states.

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Carrier multiplication is a many-particle process giving rise to the generation of multiple electron-hole pairs. This process holds the potential to increase the power conversion efficiency of photovoltaic devices. In graphene, carrier multiplication has been theoretically predicted and recently experimentally observed. However, due to the absence of a bandgap and competing phonon-induced electron-hole recombination, the extraction of charge carriers remains a substantial challenge. Here we present a new strategy to benefit from the gained charge carriers by introducing a Landau quantization that offers a tunable bandgap. Based on microscopic calculations within the framework of the density matrix formalism, we report a significant carrier multiplication in graphene under Landau quantization. Our calculations reveal a high tunability of the effect via externally accessible pump fluence, temperature and the strength of the magnetic field.

Abstract:In an external magnetic field, the energy of massless charge carriers in graphene is quantized into non-equidistant degenerate Landau levels including a zero-energy level. This extraordinary electronic dispersion gives rise to a fundamentally new dynamics of optically excited carriers. Here, we review the state of the art of the relaxation dynamics in Landau-quantized graphene focusing on microscopic insights into possible many-particle relaxation channels. We investigate optical excitation into a non equilibrium distribution followed by ultrafast carriercarrier and carrier-phonon scattering processes. We reveal that surprisingly the Auger scattering dominates the relaxation dynamics in spite of the non-equidistant Landau quantization in graphene. Furthermore, we demonstrate how technologically relevant carrier multiplication can be achieved and discuss the possibility of optical gain in Landau-quantized graphene. The provided microscopic view on elementary many-particle processes can guide future experimental studies aiming at the design of novel graphene-based optoelectronic devices, such as highly efficient photodetectors, solar cells, and spectrally broad Landau level lasers. MotivationWith an ever-growing impact of technology on everyday life, the importance of semiconductor physics has constantly increased since the information revolution. Of particular interest is the field of optoelectronics enabling key technologies for modern communication. The fast techno-*Corresponding Author: Ermin Malic: Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden, E-mail: ermin.malic@chalmers.se Florian Wendler: Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden, E-mail: florian.wendler@tu-berlin.de Andreas Knorr: Institute of Theoretical Physics, Nonlinear Optics and Quantum Electronics, Technical University Berlin, Germany logical progress is accompanied by the demand for materials with new optical and electronic properties. One of the most promising materials in this regard is graphene [1][2][3][4], which was first grown epitaxially on top of SiC [5], but it was not until Novoselov and Geim prepared samples by mechanical exfoliation and demonstrated a graphenebased field-effect transistor [6] in 2004 that it received wide-spread attention.Graphene as a single layer of carbon atoms is the thinnest known two-dimensional material [3,7]. It exhibits extraordinary optical, electronic, thermal, mechanical, and chemical properties [3]. In particular, its linear electronic dispersion in the low-energy regime near the Dirac points in the Brillouin zone is most remarkable. Here, electrons move as if they were massless, just like photons in quantum electrodynamics providing the possibility to test the predictions of relativistic quantum mechanics, such as Klein tunneling [8,9], in a small-scale table top experiment.Besides the fascinating fundamental physics that can be explored in graphene, several optoelectronic applications were proposed [1], ranging fr...

Terahertz (THz) technology has attracted enormous interest with conceivable applications ranging from basic science to advanced technology. One of the main challenges remains the realization of a well controlled and easily tunable THz source. Here, we predict the occurrence of a long-lived population inversion in Landau-quantized graphene (i.e. graphene in an external magnetic field) suggesting the design of tunable THz Landau level lasers. The unconventional non-equidistant quantization in graphene offers optimal conditions to overcome the counteracting Coulomb- and phonon-assisted scattering channels. In addition to the tunability of the laser frequency, we show that also the polarization of the emitted light can be controlled. Based on our microscopic insights into the underlying many-particle mechanisms, we propose two different experimentally realizable schemes to design tunable graphene-based THz Landau level lasers.