24 research outputs found
Graphene as Gain Medium for Broadband Lasers
In contrast to conventional structures, efficient non-radiative carrier
recombination counteracts the appearance of optical gain in graphene. Based on
a microscopic and fully quantum-mechanical study of the coupled carrier,
phonon, and photon dynamics in graphene, we present a strategy to obtain a
long-lived gain: Integrating graphene into a photonic crystal nanocavity and
applying a high-dielectric substrate gives rise to pronounced coherent light
emission suggesting the design of graphene-based laser devices covering a broad
spectral range
Impact of doping on the carrier dynamics in graphene
We present a microscopic study on the impact of doping on the carrier
dynamics in graphene, in particular focusing on its influence on the
technologically relevant carrier multiplication in realistic, doped graphene
samples. Treating the time- and momentum-resolved carrier-light,
carrier-carrier, and carrier-phonon interactions on the same microscopic
footing, the appearance of Auger-induced carrier multiplication up to a Fermi
level of 300 meV is revealed. Furthermore, we show that doping favors the
so-called hot carrier multiplication occurring within one band. Our results are
directly compared to recent time-resolved ARPES measurements and exhibit an
excellent agreement on the temporal evolution of the hot carrier multiplication
for n- and p-doped graphene. The gained insights shed light on the ultrafast
carrier dynamics in realistic, doped graphene sample
Stacking-dependent energetics and electronic structure of ultrathin polymorphic V2VI3 topological insulator nanofilms
Topological insulators represent a paradigm shift in surface physics. The most extensively studied Bi2Se3-type topological insulators exhibit layered structures, wherein neighboring layers are weakly bonded by van der Waals interactions. Using first-principles density-functional theory calculations, we investigate the impact of the stacking sequence on the energetics and band structure properties of three polymorphs of Bi2Se3, Bi2Te3, and Sb2Te3. Considering their ultrathin films up to 6 nm as a function of its layer thickness, the overall dispersion of the band structure is found to be insensitive to the stacking sequence, while the band gap is highly sensitive, which may also affect the critical thickness for the onset of the topologically nontrivial phase. Our calculations are consistent with both experimental and theoretical results, where available. We further investigate tribological layer slippage, where we find a relatively low energy barrier between two of the considered structures. Both the stacking-dependent band gap and low slippage energy barriers suggest that polymorphic stacking modification may offer an alternative route for controlling the properties of this new state of matter