994 research outputs found
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
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
Graphene-based absorber exploiting guided mode resonances in one-dimensional gratings
A one-dimensional dielectric grating, based on a simple geometry, is proposed
and investigated to enhance light absorption in a monolayer graphene exploiting
guided mode resonances. Numerical findings reveal that the optimized
configuration is able to absorb up to 60% of the impinging light at normal
incidence for both TE and TM polarizations resulting in a theoretical
enhancement factor of about 26 with respect to the monolayer graphene
absorption (about 2.3%). Experimental results confirm this behaviour showing
CVD graphene absorbance peaks up to about 40% over narrow bands of few
nanometers. The simple and flexible design paves the way for the realization of
innovative, scalable and easy-to-fabricate graphene-based optical absorbers
Graphene-based perfect optical absorbers harnessing guided mode resonances
We numerically and experimentally investigate graphene-based optical
absorbers that exploit guided mode resonances (GMRs) achieving perfect
absorption over a bandwidth of few nanometers (over the visible and
near-infrared ranges) with a 40-fold increase of the monolayer graphene
absorption. We analyze the influence of the geometrical parameters on the
absorption rate and the angular response for oblique incidence. Finally, we
experimentally verify the theoretical predictions in a one-dimensional,
dielectric grating and placing it near either a metallic or a dielectric
mirror
Modulation of microRNA editing, expression and processing by ADAR2 deaminase in glioblastoma.
Background: ADAR enzymes convert adenosines to inosines within double-stranded RNAs, including microRNA
(miRNA) precursors, with important consequences on miRNA retargeting and expression. ADAR2 activity is impaired
in glioblastoma and its rescue has anti-tumoral effects. However, how ADAR2 activity may impact the miRNome
and the progression of glioblastoma is not known.
Results: By integrating deep-sequencing and array approaches with bioinformatics analyses and molecular studies,
we show that ADAR2 is essential to edit a small number of mature miRNAs and to significantly modulate the
expression of about 90 miRNAs in glioblastoma cells. Specifically, the rescue of ADAR2 activity in cancer cells recovers
the edited miRNA population lost in glioblastoma cell lines and tissues, and rebalances expression of onco-miRNAs and
tumor suppressor miRNAs to the levels observed in normal human brain. We report that the major effect of ADAR2 is
to reduce the expression of a large number of miRNAs, most of which act as onco-miRNAs. ADAR2 can edit miR-222/221
and miR-21 precursors and decrease the expression of the corresponding mature onco-miRNAs in vivo and in vitro, with
important effects on cell proliferation and migration.
Conclusions: Our findings disclose an additional layer of complexity in miRNome regulation and provide information
to better understand the impact of ADAR2 editing enzyme in glioblastoma. We propose that ADAR2 is a key factor for
maintaining edited-miRNA population and balancing the expression of several essential miRNAs involved in cancer
Harmonic Generation in Metallic, GaAs-Filled Nanocavities in the Enhanced Transmission Regime at Visible and UV Wavelengths
We have conducted a theoretical study of harmonic generation from a silver
grating having slits filled with GaAs. By working in the enhanced transmission
regime, and by exploiting phase-locking between the pump and its harmonics, we
guarantee strong field localization and enhanced harmonic generation under
conditions of high absorption at visible and UV wavelengths. Silver is treated
using the hydrodynamic model, which includes Coulomb and Lorentz forces,
convection, electron gas pressure, plus bulk X(3) contributions. For GaAs we
use nonlinear Lorentz oscillators, with characteristic X(2) and X(3) and
nonlinear sources that arise from symmetry breaking and Lorentz forces. We find
that: (i) electron pressure in the metal contributes to linear and nonlinear
processes by shifting/reshaping the band structure; (ii) TEand TM-polarized
harmonics can be generated efficiently; (iii) the X(2) tensor of GaAs couples
TE- and TM-polarized harmonics that create phase-locked pump photons having
polarization orthogonal compared to incident pump photons; (iv) Fabry-Perot
resonances yield more efficient harmonic generation compared to plasmonic
transmission peaks, where most of the light propagates along external metal
surfaces with little penetration inside its volume. We predict conversion
efficiencies that range from 10-6 for second harmonic generation to 10-3 for
the third harmonic signal, when pump power is 2GW/cm2
Third harmonic generation at 223 nm in the metallic regime of GaP
We demonstrate second and third harmonic generation from a GaP substrate 500 ÎĽm thick. The second harmonic field is tuned at the absorption resonance at 335 nm, and the third harmonic signal is tuned at 223 nm, in a range where the dielectric function is negative. These results show that a phase locking mechanism that triggers transparency at the harmonic wavelengths persists regardless of the dispersive properties of the medium, and that the fields propagate hundreds of microns without being absorbed even when the harmonics are tuned to portions of the spectrum that display
metallic behavior.Peer ReviewedPostprint (published version
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