408 research outputs found
Continuous wave lasing between Landau levels in graphene
We predict the general feasibility and demonstrate the specific design of the
THz laser operating between Landau levels in graphene placed on a polar
substrate in a magnetic field of order 1 T. Steady state operation under a
continuous wave optical pumping is possible due to an interplay between Auger
and surface-phonon mediated relaxation of carriers. The scheme is scalable to
other materials with massless Dirac fermions, for example surface states in 3D
topological insulators such as BiSe or BiTe.Comment: 12 pages, 5 figure
Formation and dynamics of self-sustained neutron haloes in disk accreting sources
It has been recognized long ago that the presence of hot plasma in the inner
accretion disks around black holes could lead to the neutron production via
dissociation of helium nuclei. We show that, for a broad range of accretion
parameters, neutrons effectively decouple from protons and pile up in the inner
disk leading to the formation of self-sustained halo. This means that new
neutrons in the halo are supplied mainly by the splitting of helium nuclei in
their collisions with existing neutrons. Once formed, such a halo can exist
even if the proton temperature is much lower than the energy threshold of
helium dissociation. We show that neutron haloes can be the natural source of
relativistic electrons and positrons, providing characteristic comptonization
spectra and hard spectral tails observed in many black hole candidates, and
also giving rise to relativistic outflows. Deuterium gamma-ray line at 2.2 MeV
resulting from neutron capture is also expected at a level detectable by future
INTEGRAL mission. Furthermore, the presence of a neutron halo strongly affects
the dynamics of accretion and leads to the rich variety of transient dynamical
regimes.Comment: 10 pages, submitted to Astronomy and Astrophysic
Generation of entangled photons in graphene in a strong magnetic field
Entangled photon states attract tremendous interest as the most vivid
manifestation of nonlocality of quantum mechanics and also for emerging
applications in quantum information. Here we propose a mechanism of generation
of polarization-entangled photons, which is based on the nonlinear optical
interaction (four-wave mixing) in graphene placed in a magnetic field. Unique
properties of quantized electron states in a magnetized graphene and optical
selection rules near the Dirac point give rise to a giant optical nonlinearity
and a high rate of photon production in the mid/far-infrared range.Comment: 5 pages, 4 figure
Second-order nonlinear optical response of graphene
Although massless Dirac fermions in graphene constitute a centrosymmetric
medium for in-plane excitations, their second-order nonlinear optical response
is nonzero if the effects of spatial dispersion are taken into account. Here we
present a rigorous quantum-mechanical theory of the second-order nonlinear
response of graphene beyond the electric dipole approximation, which includes
both intraband and interband transitions. The resulting nonlinear
susceptibility tensor satisfies all symmetry and permutation properties, and
can be applied to all three-wave mixing processes. We obtain useful analytic
expressions in the limit of a degenerate electron distribution, which reveal
quite strong second-order nonlinearity at long wavelengths, Fermi-edge
resonances, and unusual polarization properties.Comment: The new version makes the bold font vector notation uniform
throughout the paper and corrects a misprint in Eq. (34): replaces v_F^2 with
v_F^3 in the common facto
Efficient nonlinear generation of THz plasmons in graphene and topological insulators
Surface plasmons in graphene may provide an attractive alternative to
noble-metal plasmons due to their tighter confinement, peculiar dispersion, and
longer propagation distance. We present theoretical studies of the nonlinear
difference frequency generation of terahertz surface plasmon modes supported by
two-dimensional layers of massless Dirac electrons, which includes graphene and
surface states in topological insulators. Our results demonstrate strong
enhancement of the DFG efficiency near the plasmon resonance and the
feasibility of phase-matched nonlinear generation of plasmons over a broad
range of frequencies.Comment: 5 pages, 4 figure
Nonlinear optics of graphene in a strong magnetic field
Graphene placed in a magnetic field possesses an extremely high
mid/far-infrared optical nonlinearity originating from its unusual band
structure and selection rules for the optical transitions near the Dirac point.
Here we study the linear and nonlinear optical response of graphene in strong
magnetic and optical fields using quantum- mechanical density-matrix formalism.
We calculate the power of coherent terahertz radiation generated as a result of
four-wave mixing in graphene. We show that even one monolayer of graphene gives
rise to appreciable nonlinear frequency conversion efficiency and Raman gain
for modest intensities of incident infrared radiation.Comment: 16 pages, 6 figure
Luminous Blue Variables & Mass Loss near the Eddington Limit
During the course of their evolution, massive stars lose a substantial
fraction of their initial mass, both through steady winds and through
relatively brief eruptions during their Luminous Blue Variable (LBV) phase.
This talk reviews the dynamical driving of this mass loss, contrasting the
line-driving of steady winds to the potential role of continuum driving for
eruptions during LBV episodes when the star exceeds the Eddington limit. A key
theme is to emphasize the inherent limits that self-shadowing places on
line-driven mass loss rates, whereas continuum driving can in principle drive
mass up to the "photon-tiring" limit, for which the energy to lift the wind
becomes equal to the stellar luminosity. We review how the "porosity" of a
highly clumped atmosphere can regulate continuum-driven mass loss, but also
discuss recent time-dependent simulations of how base mass flux that exceeds
the tiring limit can lead to flow stagnation and a complex, time-dependent
combination of inflow and outflow regions. A general result is thus that
porosity-mediated continuum driving in super-Eddington phases can explain the
large, near tiring-limit mass loss inferred for LBV giant eruptions.Comment: Conference proceedings, Massive Stars as Cosmic Engines, IAU Symp
250, ed. F. Bresolin, P. A. Crowther, & J. Puls (Cambridge Univ. Press
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