2,472 research outputs found
Spontaneous and Stimulated Raman Scattering near Metal Nanostructures in the Ultrafast, High-Intensity regime
The inclusion of atomic inversion in Raman scattering can significantly alter
field dynamics in plasmonic settings. Our calculations show that large local
fields and femtosecond pulses combine to yield: (i) population inversion within
hot spots; (ii) gain saturation; and (iii) conversion efficiencies
characterized by a switch-like transition to the stimulated regime that spans
twelve orders of magnitude. While in Raman scattering atomic inversion is
usually neglected, we demonstrate that in some circumstances full accounting of
the dynamics of the Bloch vector is required
An efficient and portable SIMD algorithm for charge/current deposition in Particle-In-Cell codes
In current computer architectures, data movement (from die to network) is by
far the most energy consuming part of an algorithm (10pJ/word on-die to
10,000pJ/word on the network). To increase memory locality at the hardware
level and reduce energy consumption related to data movement, future exascale
computers tend to use more and more cores on each compute nodes ("fat nodes")
that will have a reduced clock speed to allow for efficient cooling. To
compensate for frequency decrease, machine vendors are making use of long SIMD
instruction registers that are able to process multiple data with one
arithmetic operator in one clock cycle. SIMD register length is expected to
double every four years. As a consequence, Particle-In-Cell (PIC) codes will
have to achieve good vectorization to fully take advantage of these upcoming
architectures. In this paper, we present a new algorithm that allows for
efficient and portable SIMD vectorization of current/charge deposition routines
that are, along with the field gathering routines, among the most time
consuming parts of the PIC algorithm. Our new algorithm uses a particular data
structure that takes into account memory alignement constraints and avoids
gather/scatter instructions that can significantly affect vectorization
performances on current CPUs. The new algorithm was successfully implemented in
the 3D skeleton PIC code PICSAR and tested on Haswell Xeon processors (AVX2-256
bits wide data registers). Results show a factor of to
speed-up in double precision for particle shape factor of order to . The
new algorithm can be applied as is on future KNL (Knights Landing)
architectures that will include AVX-512 instruction sets with 512 bits register
lengths (8 doubles/16 singles).Comment: 36 pages, 5 figure
Harmonic generation and energy transport in dielectric and semiconductors at visible and UV wavelengths: the case of GaP
We study inhibition of absorption, transparency, energy and momentum
transport of the inhomogeneous component of harmonic pulses in dielectrics and
semiconductors, at visible and UV wavelengths, focusing on materials like GaP.
In these spectral regions GaP is characterized by large absorption, metallic
behavior or a combination of both. We show that phase locking causes the
generated inhomogeneous signals to propagate through a bulk metallic medium
without being absorbed, that is occurs even in centrosymmetric materials via
the magnetic Lorentz force, and that the transport of energy and momentum is
quite peculiar and seemingly anomalous. These results make it clear that there
are new opportunities in ultrafast nonlinear optics and nano-plasmonics in new
wavelength ranges.Comment: 16 pages, 5 figures, 1 vide
Characterizing pre-transplant and post-transplant kidney rejection risk by B cell immune repertoire sequencing.
Studying immune repertoire in the context of organ transplant provides important information on how adaptive immunity may contribute and modulate graft rejection. Here we characterize the peripheral blood immune repertoire of individuals before and after kidney transplant using B cell receptor sequencing in a longitudinal clinical study. Individuals who develop rejection after transplantation have a more diverse immune repertoire before transplant, suggesting a predisposition for post-transplant rejection risk. Additionally, over 2 years of follow-up, patients who develop rejection demonstrate a specific set of expanded clones that persist after the rejection. While there is an overall reduction of peripheral B cell diversity, likely due to increased general immunosuppression exposure in this cohort, the detection of specific IGHV gene usage across all rejecting patients supports that a common pool of immunogenic antigens may drive post-transplant rejection. Our findings may have clinical implications for the prediction and clinical management of kidney transplant rejection
A Dynamical Model of Harmonic Generation in Centrosymmetric Semiconductors
We study second and third harmonic generation in centrosymmetric
semiconductors at visible and UV wavelengths in bulk and cavity environments.
Second harmonic generation is due to a combination of symmetry breaking, the
magnetic portion of the Lorentz force, and quadrupolar contributions that
impart peculiar features to the angular dependence of the generated signals, in
analogy to what occurs in metals. The material is assumed to have a non-zero,
third order nonlinearity that gives rise to most of the third harmonic signal.
Using the parameters of bulk Silicon we predict that cavity environments can
significantly modify second harmonic generation (390nm) with dramatic
improvements for third harmonic generation (266nm). This occurs despite the
fact that the harmonics may be tuned to a wavelength range where the dielectric
function of the material is negative: a phase locking mechanism binds the pump
to the generated signals and inhibits their absorption. These results point the
way to novel uses and flexibility of materials like Silicon as nonlinear media
in the visible and UV ranges
Resonant, broadband and highly efficient optical frequency conversion in semiconductor nanowire gratings at visible and UV wavelengths
Using a hydrodynamic approach we examine bulk- and surface-induced second and
third harmonic generation from semiconductor nanowire gratings having a
resonant nonlinearity in the absorption region. We demonstrate resonant,
broadband and highly efficient optical frequency conversion: contrary to
conventional wisdom, we show that harmonic generation can take full advantage
of resonant nonlinearities in a spectral range where nonlinear optical
coefficients are boosted well beyond what is achievable in the transparent,
long-wavelength, non-resonant regime. Using femtosecond pulses with
approximately 500 MW/cm2 peak power density, we predict third harmonic
conversion efficiencies of approximately 1% in a silicon nanowire array, at
nearly any desired UV or visible wavelength, including the range of negative
dielectric constant. We also predict surface second harmonic conversion
efficiencies of order 0.01%, depending on the electronic effective mass,
bistable behavior of the signals as a result of a reshaped resonance, and the
onset fifth order nonlinear effects. These remarkable findings, arising from
the combined effects of nonlinear resonance dispersion, field localization, and
phase-locking, could significantly extend the operational spectral bandwidth of
silicon photonics, and strongly suggest that neither linear absorption nor skin
depth should be motivating factors to exclude either semiconductors or metals
from the list of useful or practical nonlinear materials in any spectral range.Comment: 12 pages, 4 figure
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