178 research outputs found
Attosecond Transient Absorption in Gases and High Harmonic Generation in Solids
We present a theoretical study of attosecond transient absorption in gases and high harmonic generation in solids. In this thesis, different features in the attosecond transient absorption laser-dressed spectrum are studied, including features near bright (dipole allowed) atomic states and features that appear approximately one or two laser photons away from the bright states due to the laser-induced coupling to dark (non-dipole allowed) states. The calculations are carried out using both numerical and analytic solutions to the time-dependent Schr\ odinger equation (TDSE). From these solutions, several models of time-dependent absorption and electron dynamics at the femtosecond time scale are presented. We discuss transient absorption in two different regimes: a resonant regime when the dressing field resonantly couples bright and dark states; and a non-resonant regime when the dressed states are detuned. Moreover, a Floquet picture of the transient absorption process is presented, in which the different features in the absorption spectrum are explained as the Floquet states induced by the strong probe pulse. We demonstrate that this Floquet picture applies even though dressing field lasts only a few cycles. We also present a theoretical study of high harmonic generation (HHG) by Bloch electrons in a model transparent solid. This model applies to the recent experiments in ZnO. We solve the TDSE using a velocity gauge numerical method and the resulting harmonic spectrum exhibits a plateau due to the coupling of the valence band to the first conduction band. The energy cutoff of the plateau scales linearly with field strength, which agrees with the ZnO experiment. To facilitate the analysis of the time-frequency characteristics of the emitted harmonics, the TDSE is also solved in the so-called Houston basis which allows the separation of the interband and intraband contributions to the time-dependent current. The interband and intraband transitions are shown to correspond to diabatic and adiabatic dynamics of the system, respectively. The contributions from interband and intraband display very different time-frequency characteristics, which can potentially be used as an experimental signature of the intraband and interband dynamics
Graph Harmony: Denoising and Nuclear-Norm Wasserstein Adaptation for Enhanced Domain Transfer in Graph-Structured Data
Graph-structured data can be found in numerous domains, yet the scarcity of
labeled instances hinders its effective utilization of deep learning in many
scenarios. Traditional unsupervised domain adaptation (UDA) strategies for
graphs primarily hinge on adversarial learning and pseudo-labeling. These
approaches fail to effectively leverage graph discriminative features, leading
to class mismatching and unreliable label quality. To navigate these obstacles,
we develop the Denoising and Nuclear-Norm Wasserstein Adaptation Network
(DNAN). DNAN employs the Nuclear-norm Wasserstein discrepancy (NWD), which can
simultaneously achieve domain alignment and class distinguishment. DANA also
integrates a denoising mechanism via a variational graph autoencoder that
mitigates data noise. This denoising mechanism helps capture essential features
of both source and target domains, improving the robustness of the domain
adaptation process. Our comprehensive experiments demonstrate that DNAN
outperforms state-of-the-art methods on standard UDA benchmarks for graph
classification
Quantum interference in attosecond transient absorption of laser-dressed helium atoms
We calculate the transient absorption of an isolated attosecond pulse by
helium atoms subject to a delayed infrared (\ir) laser pulse. With the central
frequency of the broad attosecond spectrum near the ionization threshold, the
absorption spectrum is strongly modulated at the sub-\ir-cycle level. Given
that the absorption spectrum results from a time-integrated measurement, we
investigate the extent to which the delay-dependence of the absorption yields
information about the attosecond dynamics of the atom-field energy exchange. We
find two configurations in which this is possible. The first involves multi
photon transitions between bound states that result in interference between
different excitation pathways. The other involves the modification of the bound
state absorption lines by the IR field, which we find can result in a sub-cycle
time dependence only when ionization limits the duration of the strong field
interaction
3D Point Cloud Completion with Geometric-Aware Adversarial Augmentation
With the popularity of 3D sensors in self-driving and other robotics
applications, extensive research has focused on designing novel neural network
architectures for accurate 3D point cloud completion. However, unlike in point
cloud classification and reconstruction, the role of adversarial samples in3D
point cloud completion has seldom been explored. In this work, we show that
training with adversarial samples can improve the performance of neural
networks on 3D point cloud completion tasks. We propose a novel approach to
generate adversarial samples that benefit both the performance of clean and
adversarial samples. In contrast to the PGD-k attack, our method generates
adversarial samples that keep the geometric features in clean samples and
contain few outliers. In particular, we use principal directions to constrain
the adversarial perturbations for each input point. The gradient components in
the mean direction of principal directions are taken as adversarial
perturbations. In addition, we also investigate the effect of using the minimum
curvature direction. Besides, we adopt attack strength accumulation and
auxiliary Batch Normalization layers method to speed up the training process
and alleviate the distribution mismatch between clean and adversarial samples.
Experimental results show that training with the adversarial samples crafted by
our method effectively enhances the performance of PCN on the ShapeNet dataset.Comment: 11 page, 5 figure
High harmonic generation from Bloch electrons in solids
We study the generation of high harmonic radiation by Bloch electrons in a
model transparent solid driven by a strong mid-infrared laser field. We solve
the single-electron time-dependent Schr\"odinger equation (TDSE) using a
velocity-gauge method [New J. Phys. 15, 013006 (2013)] that is numerically
stable as the laser intensity and number of energy bands are increased. The
resulting harmonic spectrum exhibits a primary plateau due to the coupling of
the valence band to the first conduction band, with a cutoff energy that scales
linearly with field strength and laser wavelength. We also find a weaker second
plateau due to coupling to higher-lying conduction bands, with a cutoff that is
also approximately linear in the field strength. To facilitate the analysis of
the time-frequency characteristics of the emitted harmonics, we also solve the
TDSE in a time-dependent basis set, the Houston states [Phys. Rev. B 33, 5494
(1986)], which allows us to separate inter-band and intra-band contributions to
the time-dependent current. We find that the inter-band and intra-band
contributions display very different time-frequency characteristics. We show
that solutions in these two bases are equivalent under an unitary
transformation but that, unlike the velocity gauge method, the Houston state
treatment is numerically unstable when more than a few low lying energy bands
are used
The emerging roles of PHOSPHO1 and its regulated phospholipid homeostasis in metabolic disorders
Emerging evidence suggests that phosphoethanolamine/phosphocholine phosphatase 1 (PHOSPHO1), a specific phosphoethanolamine and phosphocholine phosphatase, is involved in energy metabolism. In this review, we describe the structure and regulation of PHOSPHO1, as well as current knowledge about the role of PHOSPHO1 and its related phospholipid metabolites in regulating energy metabolism. We also examine mechanistic evidence of PHOSPHO1- and phospholipid-mediated regulation of mitochondrial and lipid droplets functions in the context of metabolic homeostasis, which could be potentially targeted for treating metabolic disorders
Spatiotemporal filtering of high harmonics in solids
We study the macroscopic spatial and temporal properties of harmonic radiation generated by a model solid in the interaction with an intense, focused laser beam. We show that different temporal contributions to the harmonic yield can be separated in the spatial domain because they lead to radiation with different divergences, similar to what is observed in gas-phase harmonic generation. We show that applying a spatial filter in the far field results in a temporal separation of the two contributions upon refocusing, which yields spatially collimated harmonics, a spectrum with well-resolved peaks, and a subcycle time profile of the harmonic radiation with only one burst per half-cycle
Multilevel perspective on high-order harmonic generation in solids
We investigate high-order harmonic generation in a solid, modeled as a multilevel system dressed by a strong infrared laser field. We show that the cutoff energies and the relative strengths of the multiple plateaus that emerge in the harmonic spectrum can be understood both qualitatively and quantitatively by considering a combination of adiabatic and diabatic processes driven by the strong field. Such a model was recently used to interpret the multiple plateaus exhibited in harmonic spectra generated by solid argon and krypton [G. Ndabashimiye, Nature 534, 520 (2016)NATUAS0028-083610.1038/nature17660]. We also show that when the multilevel system originates from the Bloch state at the Γ point of the band structure, the laser-dressed states are equivalent to the Houston states [J. B. Krieger and G. J. Iafrate, Phys. Rev. B 33, 5494 (1986)PRBMDO1098-012110.1103/PhysRevB.33.5494] and will therefore map out the band structure away from the Γ point as the laser field increases. This leads to a semiclassical three-step picture in momentum space that describes the high-order harmonic generation process in a solid
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