88 research outputs found
Molecular orbital tomography beyond the plane wave approximation
The use of plane wave approximation in molecular orbital tomography via
high-order harmonic generation has been questioned since it was proposed, owing
to the fact that it ignores the essential property of the continuum wave
function. To address this problem, we develop a theory to retrieve the valence
molecular orbital directly utilizing molecular continuum wave function which
takes into account the influence of the parent ion field on the continuum
electrons. By transforming this wave function into momentum space, we show that
the mapping from the relevant molecular orbital to the high-order harmonic
spectra is still invertible. As an example, the highest orbital of
is successfully reconstructed and it shows good agreement with
the \emph{ab initio} orbital. Our work clarifies the long-standing controversy
and strengthens the theoretical basis of molecular orbital tomography
Probing rotational wave-packet dynamics with the structural minimum in high-order harmonic spectra
We investigate the alignment-dependent high-order harmonic spectrum generated
from nonadiabatically aligned molecules around the first half rotational
revival. It is found that the evolution of the molecular alignment is encoded
in the structural minima. To reveal the relation between the molecular
alignment and the structural minimum in the high-order harmonic spectrum, we
perform an analysis based on the two-center interference model. Our analysis
shows that the structural minimum position depends linearly on the inverse of
the alignment parameter . This linear relation indicates the
possibility of probing the rotational wave-packet dynamics by measuring the
spectral minima
Interference of high-order harmonics generated from molecules at different alignment angles
We theoretically investigate the interference effect of high-order harmonics
generated from molecules at different alignment angles. It is shown that the
interference of the harmonic emissions from molecules aligned at different
angles can significantly modulate the spectra and result in the anomalous
harmonic cutoffs observed in a recent experiment [ Nature Phys. 7, 822 (2011)
]. The shift of the spectral minimum position with decreasing the degree of
alignment is also explained by the interference effect of the harmonic
emissions.Comment: 6 pages,5 figures,journa
The selection rules of high harmonic generation: the symmetries of molecules and laser fields
The selection rules of high harmonic generation (HHG) are investigated using
three-dimensional time-dependent density functional theory (TDDFT). From the
harmonic spectra obtained with various real molecules and different forms of
laser fields, several factors that contribute to selection rules are revealed.
Extending the targets to stereoscopic molecules, it is shown that the allowed
harmonics are dependent on the symmetries of the projections of molecules. For
laser fields, the symmetries contributing to the selection rules are discussed
according to Lissajous figures and their dynamical directivities. All the
phenomena are explained by the symmetry of the full time-dependent Hamiltonian
under a combined transformation. We present a systematic study on the selection
rules and propose an intuitive method for the judgment of allowed harmonic
orders, which can be extended to more complex molecules and various forms of
laser pulses
Time-dependent population imaging for solid high harmonic generation
We propose an intuitive method, called time-dependent population imaging
(TDPI), to map the dynamical processes of high harmonic generation (HHG) in
solids by solving the time-dependent Schr\"{o}dinger equation (TDSE). It is
shown that the real-time dynamical characteristics of HHG in solids, such as
the instantaneous photon energies of emitted harmonics, can be read directly
from the energy-resolved population oscillations of electrons in the TDPIs.
Meanwhile, the short and long trajectories of solid HHG are illustrated clearly
from TDPI. By using the TDPI, we also investigate the effects of
carrier-envelope phase (CEP) in few-cycle pulses and intuitively demonstrate
the HHG dynamics driven by two-color fields. Our results show that the TDPI
provides a powerful tool to study the ultrafast dynamics in strong fields for
various laser-solid configurations and gain an insight into HHG processes in
solids
Reciprocal-space-trajectory perspective on high harmonic generation in solids
We revisit the mechanism of high harmonic generation (HHG) from solids by
comparing HHG in laser fields with different ellipticities but constant maximum
amplitude. It is shown that the cutoff of HHG is strongly extended in a
circularly polarized field. Moreover, the harmonic yield with large ellipticity
is comparable to or even higher than that in the linearly polarized field. To
understand the underlying physics, we develop a reciprocal-space-trajectory
method, which explains HHG in solids by a trajectory ensemble from different
ionization times and different initial states in the reciprocal space. We show
that the cutoff extension is related to an additional pre-acceleration step
prior to ionization, which has been overlooked in solids. By analyzing the
trajectories and the time-frequency spectrogram, we show that the HHG in solids
cannot be interpreted in terms of the classical re-collision picture alone.
Instead, the radiation should be described by the electronhole interband
polarization, which leads to the unusual ellipticity dependence. We propose a
new four-step model to understand the mechanism of HHG in solids.Comment: arXiv admin note: text overlap with arXiv:1805.1237
Quenching effect in below-threshold high harmonic generation
We theoretically demonstrate the quenching effect in below-threshold high
harmonic generation (HHG) by using the time-dependent density-functional theory
(TDDFT) and solving the time-dependent Schr\"{o}dinger equation (TDSE). It is
shown that the HHG is substantially suppressed in particular harmonic orders in
the below-threshold region when multi-electron interaction comes into play. The
position of the suppression is determined by the energy gap between the highest
occupied orbital and the higher-lying orbital of the target. We show that the
quenching effect is due to a new class of multi-electron dynamics involving
electron-electron energy transfer, which is analog to the fluorescence
quenching owing to the energy transfer between molecules in fluorescent
material. This work reveals the important role of the multi-electron
interaction on HHG especially in the below-threshold region.Comment: 5 figure
Contour Loss for Instance Segmentation via k-step Distance Transformation Image
Instance segmentation aims to locate targets in the image and segment each
target area at pixel level, which is one of the most important tasks in
computer vision. Mask R-CNN is a classic method of instance segmentation, but
we find that its predicted masks are unclear and inaccurate near contours. To
cope with this problem, we draw on the idea of contour matching based on
distance transformation image and propose a novel loss function, called contour
loss. Contour loss is designed to specifically optimize the contour parts of
the predicted masks, thus can assure more accurate instance segmentation. In
order to make the proposed contour loss to be jointly trained under modern
neural network frameworks, we design a differentiable k-step distance
transformation image calculation module, which can approximately compute
truncated distance transformation images of the predicted mask and
corresponding ground-truth mask online. The proposed contour loss can be
integrated into existing instance segmentation methods such as Mask R-CNN, and
combined with their original loss functions without modification of the
inference network structures, thus has strong versatility. Experimental results
on COCO show that contour loss is effective, which can further improve instance
segmentation performances.Comment: Under review in IET Computer Visio
Frequency shift in high order harmonic generation from isotopic molecules
We report the first experimental observation of frequency shift in high order
harmonic generation (HHG) from isotopic molecules H2 and D2 . It is found that
harmonics generated from the isotopic molecules exhibit obvious spectral red
shift with respect to those from Ar atom. The red shift is further demonstrated
to arise from the laser-driven nuclear motion in isotopic molecules. By
utilizing the red shift observed in experiment, we successfully retrieve the
nuclear vibrations in H2 and D2, which agree well with the theoretical
calculations from the time-dependent Schrodinger equation (TDSE) with
Non-Born-Oppenheimer approximation. Moreover, we demonstrate that the frequency
shift can be manipulated by changing the laser chirp.Comment: 5pages, 5figure
High harmonic generation with circularly polarized fields in solid: a quantum trajectory perspective
We investigate the high harmonic generation (HHG) in solids driven by laser
fields with different ellipticities. The HHG spectra show a two-plateau
structure within the energy gap between the valence band and the first
conduction band. These two plateaus depend distinctly on the laser ellipticity.
The first plateau is decreased while the second plateau is enhanced with
increasing the laser ellipticity. To understand these phenomena, we develop an
intuitive Reciprocal-Space- Trajectory (RST) method, with which HHG in solids
is explained by a trajectory-ensemble from different initial states and
different ionization times in the reciprocal space. In the framework of RST, we
can not only quantitatively reproduce the HHG spectra, but also well understand
the underlying physics of these phenomena, providing a deep insight into the
mechanism of HHG in solids
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