18,069 research outputs found
Tuning High-Harmonic Generation by Controlled Deposition of Ultrathin Ionic Layers on Metal Surfaces
High harmonic generation (HHG) from semiconductors and insulators has become
a very active area of research due to its great potential for developing
compact HHG devices. Here we show that by growing monolayers (ML) of insulators
on single-crystal metal surfaces, one can tune the harmonic spectrum by just
varying the thickness of the ultrathin layer, not the laser properties. This is
shown from numerical solutions of the time-dependent Schr\"odinger equation for
ML NaCl/Cu(111) systems () based on realistic potentials available
in the literature. Remarkably, the harmonic cutoff increases linearly with
and as much as an order of magnitude when going from 1 to 30, while
keeping the laser intensity low and the wavelength in the near-infrared range.
Furthermore, the degree of control that can be achieved in this way is much
higher than by varying the laser intensity. The origin of this behavior is the
reduction of electronic "friction" when moving from the essentially discrete
energy spectrum associated with a few-ML system to the continuous energy
spectrum (bands) inherent to an extended periodic system.Comment: 6 pages, 4 figure
High harmonic generation in crystals using Maximally Localized Wannier functions
In this work, the nonlinear optical response, and in particular, the high
harmonic generation of semiconductors is addressed by using the Wannier gauge.
One of the main problems in the time evolution of the Semiconductor Bloch
equations resides in the fact that the dipole couplings between different bands
can diverge and have a random phase along the reciprocal space and this leads
to numerical instability. To address this problem, we propose the use of the
Maximally Localized Wannier functions that provide a framework to map ab-initio
calculations to an effective tight-binding Hamiltonian with great accuracy. We
show that working in the Wannier gauge, the basis set in which the Bloch
functions are constructed directly from the Wannier functions, the dipole
couplings become smooth along the reciprocal space thus avoiding the problem of
random phases. High harmonic generation spectrum is computed for a 2D monolayer
of hBN as a numerical demonstration
Molecular environment, reverberation, and radiation from the pulsar wind nebula in CTA 1
We estimate the molecular mass around CTA 1 using data from Planck and the
Harvard CO survey. We observe that the molecular mass in the vicinity of the
complex is not enough to explain the TeV emission observed by VERITAS, even
under favorable assumptions for the cosmic-ray acceleration properties of the
supernova remnant. This supports the idea that the TeV emission comes from the
PWN. Here, we model the spectrum of the PWN at possible different stages of its
evolution, including both the dynamics of the PWN and the SNR and their
interaction via the reverse shock. We have included in the model the energy
lost via radiation by particles and the particles escape when computing the
pressure produced by the gas. This leads to an evolving energy partition, since
for the same instantaneous sharing of the injection of energy provided by the
rotational power, the field and the particles are affected differently by
radiation and losses. We present the model, and study in detail how the
spectrum of a canonical isolated PWN is affected during compression and
re-expansion and how this may impact on the CTA 1 case. By exploring the
phase-space of parameters that lead to radii in agreement with those observed,
we then analyze different situations that might represent the current stage of
the CTA 1 PWN, and discuss caveats and requirements of each one.Comment: 13 pages, 8 figures, accepted for publication in MNRA
On a general implementation of - and -adaptive curl-conforming finite elements
Edge (or N\'ed\'elec) finite elements are theoretically sound and widely used
by the computational electromagnetics community. However, its implementation,
specially for high order methods, is not trivial, since it involves many
technicalities that are not properly described in the literature. To fill this
gap, we provide a comprehensive description of a general implementation of edge
elements of first kind within the scientific software project FEMPAR. We cover
into detail how to implement arbitrary order (i.e., -adaptive) elements on
hexahedral and tetrahedral meshes. First, we set the three classical
ingredients of the finite element definition by Ciarlet, both in the reference
and the physical space: cell topologies, polynomial spaces and moments. With
these ingredients, shape functions are automatically implemented by defining a
judiciously chosen polynomial pre-basis that spans the local finite element
space combined with a change of basis to automatically obtain a canonical basis
with respect to the moments at hand. Next, we discuss global finite element
spaces putting emphasis on the construction of global shape functions through
oriented meshes, appropriate geometrical mappings, and equivalence classes of
moments, in order to preserve the inter-element continuity of tangential
components of the magnetic field. Finally, we extend the proposed methodology
to generate global curl-conforming spaces on non-conforming hierarchically
refined (i.e., -adaptive) meshes with arbitrary order finite elements.
Numerical results include experimental convergence rates to test the proposed
implementation
- …