96 research outputs found
Electrically pumped graphene-based Landau-level laser
Graphene exhibits a nonequidistant Landau quantization with tunable Landau-level (LL) transitions in the technologically desired terahertz spectral range. Here, we present a strategy for an electrically driven terahertz laser based on Landau-quantized graphene as the gain medium. Performing microscopic modeling of the coupled electron, phonon, and photon dynamics in such a laser, we reveal that an inter-LL population inversion can be achieved resulting in the emission of coherent terahertz radiation. The presented paper provides a concrete recipe for the experimental realization of tunable graphene-based terahertz laser systems
Effect of nonhydrostatic pressure on the superconducting kagome metal CsVSb
High-pressure single-crystal x-ray diffraction experiments reveal that the
superconducting kagome metal CsVSb transforms from hexagonal ()
to monoclinic () symmetry above 10 GPa if nonhydrostatic pressure
conditions are created in a diamond anvil cell with silicon oil as
pressure-transmitting medium. This is contrary to the behavior of CsVSb
under quasi-hydrostatic conditions in neon, with the hexagonal symmetry
retained up to at least 20 GPa. Monoclinic distortion leaves the kagome planes
almost unchanged but deforms honeycomb nets of the Sb atoms. Using ab initio
density-functional calculations, we show that this distortion facilitates the
pressure-induced shift of van Hove singularities away from the Fermi level and
assists in the Fermi surface reconstruction caused by the formation of
interlayer Sb-Sb bonds, thus adding a structural transition component to the
reentrant behavior of CsVSb.Comment: 5 pages, 4 figures, 2 table
Ultrafast Relaxation Dynamics of Spin-Density Wave Order in BaFeAs under High Pressures
BaFeAs is the parent compound for a family of iron-based
high-temperature superconductors as well as a prototypical example of the
spin-density wave (SDW) system. In this study, we perform an optical pump-probe
study of this compound to systematically investigate the SDW order across the
pressure-temperature phase diagram. The suppression of the SDW order by
pressure manifests itself by the increase of relaxation time together with the
decrease of the pump-probe signal and the pump energy necessary for complete
vaporization of the SDW condensate. We have found that the pressure-driven
suppression of the SDW order at low temperature occurs gradually in contrast to
the thermally-induced SDW transition. Our results suggest that the
pressure-driven quantum phase transition in BaFeAs (and probably other
iron pnictides) is continuous and it is caused by the gradual worsening of the
Fermi-surface nesting conditions
Role of Transient Reflection in Graphene Nonlinear Infrared Optics
International audienceUnderstanding the optical response of graphene at terahertz frequencies is of critical importance for designing graphene-based devices that operate in this frequency range. Here we present a tera-hertz pump-probe measurement that simultaneously measures both the transmitted and reflected probe radiation from multilayer epitaxial graphene, allowing for an unambiguous determination of the pump-induced absorption change in the graphene layers. The photon energy in the experiment (30 meV) is on the order of the doping level in the graphene which enables the exploration of the transition from interband to intraband processes, depending on the amount of pump-induced heating. Our findings establish the presence of a large, photoinduced reflection that contributes to the change in sign of the relative transmitted terahertz radiation, which can be purely positive or predominantly negative depending on the pump fluence, while the change in absorption is found negative at all fluences. We develop a straightforward theory that confirms the sign reversible nature of the relative transmitted terahertz radiation through the graphene multilayer and determine that this behavior originates from either an absorption-bleached or reflection-dominated regime. The theoretical results are incorporated into a model utilizing an energy balance equation that reproduces the measured pump-probe data. These findings, which extend to mid-and far infrared frequencies, illuminate the importance of considering reflection in graphene-light interactions and have implications for the design of future terahertz photonic components
Anisotropy of excitation and relaxation of photogenerated Dirac electrons in graphene
We investigate the polarization dependence of the carrier excitation and
relaxation in epitaxial multilayer graphene. Degenerate pump-probe experiments
with a temporal resolution of 30 fs are performed for different rotation angles
of the pump-pulse polarization with respect to the polarization of the probe
pulse. A pronounced dependence of the pump-induced transmission on this angle
is found. It reflects a strong anisotropy of the pump-induced occupation of
photogenerated carriers in momentum space even though the band structure is
isotropic. Within 150 fs after excitation an isotropic carrier distribution is
established. Our observations imply the predominant role of collinear
scattering preserving the initially optically generated anisotropy in the
carrier distribution. The experiments are well described by microscopic time-,
momentum, and angle-resolved modelling, which allows us to unambiguously
identify non-collinear carrier-phonon scattering to be the main relaxation
mechanism giving rise to an isotropic distribution in the first hundred fs
after optical excitation.Comment: Submitted to Nano Letter
Four-Wave Mixing in Landau-Quantized Graphene
International audienceFor Landau-quantized graphene, featuring an energy spectrum consisting of a series of nonequidis-tant Landau levels, theory predicts a giant resonantly-enhanced optical nonlinearity. We verify the nonlinearity in a degenerate time-integrated four-wave mixing (FWM) experiment in the mid-infrared spectral range, involving the Landau levels LL −1 , LL 0 and LL 1. A rapid dephasing of the optically induced microscopic polarization on a timescale shorter than the pulse duration (∼4 ps) is observed, while a complementary pump-probe experiment under the same experimental conditions reveals a much longer lifetime of the induced population. The FWM signal shows the expected field dependence with respect to lowest order perturbation theory for low fields. Saturation sets in for fields above ∼ 6 kV/cm. Furthermore, the resonant behavior and the order of magnitude of the third-order susceptibility are in agreement with theoretical calculations
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