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## Geração de harmônicos de pulsos laser de femtossegundo pela técnica de conversão de frequência em capilares preenchidos com gases nobres; Harmonics generation of femtosecond laser pulses by the technique of frequency conversion in noble gas filled capillaries

Fonte: Biblioteca Digitais de Teses e Dissertações da USP
Publicador: Biblioteca Digitais de Teses e Dissertações da USP

Tipo: Tese de Doutorado
Formato: application/pdf

Publicado em 26/06/2012
PT

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#Capilares preenchidos com gases#Conversão paramétrica de frequência#Femtosecond pulses. Gas filled capillaries#Geração de altos harmônicos#High-harmonic generation#Modulação da fase espectral#Parametric frequency conversion#Pulsos de femtossegundo#Spectral phase modulation

O propósito principal desta tese foi a implementação e estudo da geração de pulsos laser de femtossegundos em comprimentos de onda curtos (ultravioleta profundo, ultravioleta de vácuo e ultravioleta extremo) pela técnica de conversão de frequência em capilar preenchido com gás nobre. Esta técnica de conversão de frequência tem feito diversas contribuições nas últimas décadas para o avanço da geração de pulsos laser ultracurtos nesta região espectral. O desenvolvimento de tais fontes de luz coerente possui importantes implicações nos estudos de espectroscopia resolvida no tempo em átomos, moléculas e materiais. Através da implementação da técnica de conversão de frequência com casamento de fase em capilar preenchido com gás argônio, foi possível a obtenção de pulsos de femtossegundos centrados em 260 nm e 195 nm utilizando um sistema laser amplificado Ti: safira (780 nm, 1.5 mJ, 43 fs, 1 KHz). Estes comprimentos de onda correspondem, respectivamente, aos terceiro e quarto harmônicos da frequência fundamental do laser utilizado. Pulsos centrados em 260 nm com excelente perfil espacial, energias da ordem de microjoules e durações temporais tão curtas quanto 18 fs, possibilitadas pela recompressão por um par de prismas...

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## Monitoring molecular dynamics using coherent electrons from high harmonic generation

Fonte: National Academy of Sciences
Publicador: National Academy of Sciences

Tipo: Artigo de Revista Científica

EN

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We report a previously undescribed spectroscopic probe that makes use of electrons rescattered during the process of high-order harmonic generation. We excite coherent vibrations in SF6 using impulsive stimulated Raman scattering with a short laser pulse. A second, more intense laser pulse generates high-order harmonics of the fundamental laser, at wavelengths of ≈20–50 nm. The high-order harmonic yield is observed to oscillate, at frequencies corresponding to all of the Raman-active modes of SF6, with an asymmetric mode most visible. The data also show evidence of relaxation dynamics after impulsive excitation of the molecule. Theoretical modeling indicates that the high harmonic yield should be modulated by both Raman and infrared-active vibrational modes. Our results indicate that high harmonic generation is a very sensitive probe of vibrational dynamics and may yield more information simultaneously than conventional ultrafast spectroscopic techniques. Because the de Broglie wavelength of the recolliding electron is on the order of interatomic distances, i.e., ≈1.5 Å, small changes in the shape of the molecule lead to large changes in the high harmonic yield. This work therefore demonstrates a previously undescribed spectroscopic technique for probing ultrafast internal dynamics in molecules and...

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## Oriented rotational wave-packet dynamics studies via high harmonic generation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 18/05/2012

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We produce oriented rotational wave packets in CO and measure their
characteristics via high harmonic generation. The wavepacket is created using
an intense, femtosecond laser pulse and its second harmonic. A delayed 800 nm
pulse probes the wave packet, generating even-order high harmonics that arise
from the broken symmetry induced by the orientation dynamics. The even-order
harmonic radiation that we measure appears on a zero background, enabling us to
accurately follow the temporal evolution of the wave packet. Our measurements
reveal that, for the conditions optimum for harmonic generation, the
orientation is produced by preferential ionization which depletes the sample of
molecules of one orientation.

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## A theory of intense-field dynamic alignment and high harmonic generation from coherently rotating molecules and interpretation of intense-field ultrafast pump-probe experiments

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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A theory of ultra-fast pump-probe experiments proposed by us earlier [F.H.M.
Faisal et al., Phys. Rev. Lett. 98, 143001 (2007) and F.H.M. Faisal and A.
Abdurrouf, Phys. Rev. Lett. 100, 123005 (2008)] is developed here fully and
applied to investigate the phenomena of dynamic alignment and high harmonic
generation (HHG) from coherently rotating linear molecules. The theory provides
essentially analytical results for the signals that allow us to investigate the
simultaneous dependence of the HHG signals on the two externally available
control parameters, namely, the relative angle between the polarizations, and
the delay-time between the two pulses. It is applied to investigate the
characteristics of high harmonic emission from nitrogen and oxygen molecules
that have been observed experimentally in a number of laboratories. The results
obtained both in the time-domain and in the frequency-domain are compared with
the observed characteristics as well as directly with the data and are found to
agree remarkably well. In addition we have predicted the existence of a "magic"
polarization angle at which all modulations of the harmonic emission from
nitrogen molecule changes to a steady emission at the harmonic frequency. Among
other things we have also shown a correlation between the existence of the
"magic" or critical polarization angles and the symmetry of the active
molecular orbitals...

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## Efficient output coupling of intracavity high harmonic generation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 18/03/2008

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We demonstrate a novel technique for coupling XUV harmonic radiation out of a
femtosecond enhancement cavity. We use a small-period diffraction grating
etched directly into the surface of a dielectric mirror. For the fundamental
light, this element acts as a high reflector. For harmonic wavelengths, it acts
as a diffraction grating, coupling XUV radiation out of the cavity. Using this
method, we observed the 3rd through 21st odd harmonics with a dramatic increase
in usable power over previous results of high harmonic generation at high
repetition rates.; Comment: 3 pages, 2 figures

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## High-harmonic generation and periodic level crossings: time profile and control

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 21/12/2001

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We investigate high-harmonic generation in closed systems, using the
two-level atom as a simplified model. By means of a windowed Fourier transform
of the time-dependent dipole acceleration, we extract the main contributions to
this process within a cycle of the driving field. We show that the patterns
obtained can be understood by establishing a parallel between the two-level
atom and the three-step model. In both models, high-harmonic generation is a
consequence of a three-step process, which involves either the continuum and
the ground state, or the adiabatic states of the two-level Hamiltonian. The
knowledge of this physical mechanism allows us to manipulate the adiabatic
states, and consequently the harmonic spectra, by means of a bichromatic
driving field. Furthermore, using scaling laws, we establish sharp criteria for
the invariance of the physical quantities involved. Consequently, our results
can be extended to a broader parameter range, as for instance those
characteristic of solid-state systems in strong fields.; Comment: 17 pages revtex, 15 figures (ps and eps files); figs. 1, 4, 6, 7, 10,
12 and 14 have been compressed due to memory problems

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## Theory of plasmon-enhanced high-harmonic generation in the vicinity of metal nanostructures in noble gases

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 21/09/2010

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We present a semiclassical model for plasmon-enhanced high-harmonic
generation (HHG) in the vicinity of metal nanostructures. We show that both the
inhomogeneity of the enhanced local fields and electron absorption by the metal
surface play an important role in the HHG process and lead to the generation of
even harmonics and to a significantly increased cutoff. For the examples of
silver-coated nanocones and bowtie antennas we predict that the required
intensity reduces by up to three orders of magnitudes and the HHG cutoff
increases by more than a factor of two. The study of the enhanced high-harmonic
generation is connected with a finite-element simulation of the electric field
enhancement due to the excitation of the plasmonic modes.; Comment: 4 figures

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## Effective ATI Channels in High Harmonic Generation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/07/2000

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Harmonic generation by an atom in a laser field is described by the
three-step mechanism as proceeding via above-threshold ionization (ATI)
followed by the electron propagation in the laser-dressed continuum and the
subsequent laser assisted recombination (LAR). An amplitude of harmonic
production is given by the coherent sum of contributions from different
intermediate ATI channels labeled by the number m of absorbed laser photons.
The range of m-values that gives substantial contribution is explored and found
to be rather broad for high harmonic generation. The coherence effects are of
crucial importance being responsible for the characteristic pattern of harmonic
intensities with a plateau domain followed by a cutoff region. Due to
multiphoton nature of the process, an efficient summation of m-contributions
can be carried out in the framework of the saddle point method. The saddle
points correspond to some complex-valued labels m=m_c associated with the
intermediate effective ATI channels in the three-step harmonic generation
process. The advantage of this approach stems from the fact that summation over
large number of conventional ATI m-channels is replaced by summation over small
number of effective m_c-channels. The equation governing m_c has a transparent
physical meaning: the electron ejected from the atom on the first (ATI) stage
should return to the core to make LAR possible. The calculated rates are in
good agreement with the results obtained by other approaches.; Comment: 26 pages...

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## Theoretical studies of high-harmonic generation: Effects of symmetry, degeneracy and orientation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 22/06/2007

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Using a quantum mechanical three-step model we present numerical calculations
on the high-harmonic generation from four polyatomic molecules. Ethylene
(C$_2$H$_4$) serves as an example where orbital symmetry directly affects the
harmonic yield. We treat the case of methane (CH$_4$) to address the
high-harmonic generation resulting from a molecule with degenerate orbitals. To
this end we illustrate how the single orbital contributions show up in the
total high-harmonic signal. This example illustrates the importance of adding
coherently amplitude contributions from the individual degenerate orbitals.
Finally, we study the high-harmonic generation from propane (C$_3$H$_8$) and
butane (C$_4$H$_{10}$). These two molecules, being extended and far from
spherical in structure, produce harmonics with non-trivial orientational
dependencies. In particular, propane can be oriented so that very
high-frequency harmonics are favorized, and thus the molecule contains
prospects for the generation of UV attosecond pulses.; Comment: 9 pages, 6 figures, 1 table

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## Optimal control of high-harmonic generation by intense few-cycle pulses

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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At the core of attosecond science lies the ability to generate laser pulses
of sub-femtosecond duration. In tabletop devices the process relies on
high-harmonic generation, where a major challenge is to obtain high yields and
high cutoff energies required for the generation of attosecond pulses. We
develop a computational method that can simultaneously resolve these issues by
optimizing the driving pulses using quantum optimal control theory. Our target
functional, an integral over the harmonic yield over a desired energy range,
leads to a remarkable cutoff extension and yield enhancement for a
one-dimensional model H-atom. The physical enhancement process is shown to be
twofold: the cutoff extension is of classical origin, whereas the yield
enhancement arises from increased tunneling probability. The scheme is directly
applicable to more realistic models and, within straightforward refinements,
also to experimental verification.

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## Single-shot fluctuations in waveguided high-harmonic generation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/07/2015

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For exploring the application potential of coherent soft x-ray (SXR) and
extreme ultraviolet radiation (XUV) provided by high-harmonic generation, it is
important to characterize the central output parameters. Of specific importance
are pulse-to-pulse (shot-to-shot) fluctuations of the high-harmonic output
energy, fluctuations of the direction of the emission (pointing instabilities),
and fluctuations of the beam divergence and shape that reduce the spatial
coherence. We present the first single-shot measurements of waveguided
high-harmonic generation in a waveguided (capillary-based) geometry. Using a
capillary waveguide filled with Argon gas as the nonlinear medium, we provide
the first characterization of shot-to-shot fluctuations of the pulse energy, of
the divergence and of the beam pointing. We record the strength of these
fluctuations vs. two basic input parameters, which are the drive laser pulse
energy and the gas pressure in the capillary waveguide. In correlation
measurements between single-shot drive laser beam profiles and single-shot
high-harmonic beam profiles we prove the absence of drive laser
beam-pointing-induced fluctuations in the high-harmonic output. We attribute
the main source of high-harmonic fluctuations to ionization-induced nonlinear
mode mixing during propagation of the drive laser pulse inside the capillary
waveguide.

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## Near infrared few-cycle pulses for high harmonic generation

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/04/2014

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We report on the development of tunable few-cycle pulses with central
wavelengths from 1.6 um to 2 um. Theses pulses were used as a proof of
principle for high harmonic generation in atomic and molecular targets. In
order to generate such pulses we produced a filament in a 4 bar krypton cell.
Spectral broadening by a factor of 2 to 3 of a 40 fs near infrared input pulse
was achieved. The spectrally broadened output pulses were then compressed by
fused silica plates down to the few-cycle regime close to the Fourier limit.
The auto-correlation of these pulses revealed durations of about 3 cycles for
all investigated central wavelengths. Pulses with a central wavelength of 1.7
um and up to 430 uJ energy per pulse were employed to generate high order
harmonics in Xe, Ar and N2. Moving to near infrared few-cycle pulses opens the
possibility to operate deeply in the non-perturbative regime with a Keldysh
parameter smaller than 1. Hence, this source is suitable for the study of the
non-adiabatic tunneling regime in most generating systems used for high order
harmonic generation and attoscience.; Comment: 12 pages, 4 figures

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## Quantum theory of a high harmonic generation as a three-step process

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 26/06/1999

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Fully quantum treatment explicitly presents the high harmonic generation as a
three-step process: (i) above threshold ionization (ATI) is followed by (ii)
electron propagation in a laser-dressed continuum. Subsequently (iii)
stimulated (or laser assisted) recombination brings the electron back into the
initial state with emission of a high-energy photon. Contributions of all ATI
channels add up coherently. All three stages of the process are described by
simple, mostly analytical expressions that allow a detailed physical
interpretation. A very good quantitative agreement with the previous
calculations on the harmonic generation by H-minus ion is demonstrated, thus
supplementing the conceptual significance of the theory with its practical
efficiency. The virtue of the present scheme is further supported by a good
accord between the calculations in length and velocity gauges for the
high-energy photon.; Comment: Revtex 23 pages, 4 figures. Accepted for publication in Phys.Rev.A
(1999)

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## Multielectron High Harmonic Generation: simple man on a complex plane

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 08/04/2013

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From the famous classical "simple man" model to the recent multichannel model
for polyatomic molecules, this tutorial will guide you through the several
landmarks in our understanding of high harmonic generation and high harmonic
spectroscopy. Our goal was to provide recipes and insights for modelling the
harmonic response in various regimes, from the single active electron regime
typical for noble gas atoms to the laser-driven attosecond hole dynamics in
polyatomics. We have tried to pay particular attention to both simple recipes
and their limitations, including the connection of real-valued classical and
complex-valued quantum times and velocities. In addition to physical pictures,
general approaches and their practical realizations, we have also tried to
discuss some of the sticky technical issues, such as the possibility to
factorize high harmonic response into the three steps of ionization,
propagation and recombination and practical ways to treat strong field
ionization, search for the saddle points of multi-dimensional integrals, deal
with poles arising in the quantum-mechanical integrands, remove divergences
appearing in approximate saddle point expressions, etc.; Comment: 55 pages, 16 figures, Book Chapter

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## Does high harmonic generation conserve angular momentum?

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 04/10/2013

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High harmonic generation (HHG) is a unique and useful process in which
infrared or visible radiation is frequency up converted into the extreme
ultraviolet and x ray spectral regions. As a parametric process, high harmonic
generation should conserve the radiation energy, momentum and angular momentum.
Indeed, conservation of energy and momentum have been demonstrated. Angular
momentum of optical beams can be divided into two components: orbital and spin
(polarization). Orbital angular momentum is assumed to be conserved and
recently observed deviations were attributed to propagation effects. On the
other hand, conservation of spin angular momentum has thus far never been
studied, neither experimentally nor theoretically. Here, we present the first
study on the role of spin angular momentum in extreme nonlinear optics by
experimentally generating high harmonics of bi chromatic elliptically polarized
pump beams that interact with isotropic media. While observing that the
selection rules qualitatively correspond to spin conservation, we unequivocally
find that the process of converting pump photons into a single high-energy
photon does not conserve angular momentum, i.e. this process is not self
contained. In one regime, we numerically find that this major fundamental
discrepancy can be explained if the harmonic photons are emitted in pairs. Yet
in another regime...

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## Theory of high harmonic generation in relativistic laser interaction with overdense plasma

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 28/04/2006

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High harmonic generation due to the interaction of a short ultra relativistic
laser pulse with overdense plasma is studied analytically and numerically. On
the basis of the ultra relativistic similarity theory we show that the high
harmonic spectrum is universal, i.e. it does not depend on the interaction
details. The spectrum includes the power law part $I_n\propto n^{-8/3}$ for
$n<\sqrt{8\alpha}\gamma_{\max}^3$, followed by exponential decay. Here
$\gamma_{\max}$ is the largest relativistic $\gamma$-factor of the plasma
surface and $\alpha$ is the second derivative of the surface velocity at this
moment. The high harmonic cutoff at $\propto \gamma_{\max}^3$ is parametrically
larger than the $4 \gamma_{\max}^2$ predicted by the ``oscillating mirror''
model based on the Doppler effect. The cornerstone of our theory is the new
physical phenomenon: spikes in the relativistic $\gamma$-factor of the plasma
surface. These spikes define the high harmonic spectrum and lead to attosecond
pulses in the reflected radiation.; Comment: 12 pages, 9 figures

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## Exact solution for high harmonic generation and the response to an ac driving field for a charge density wave insulator

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 13/09/2013

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We develop and exactly solve a model for electrons driven by pulsed or
continuous ac fields. The theory includes both the photoexcitation process as
well as the subsequent acceleration of the electrons. In the case of an ac
response, we examine both the nonequilibrium density of states and the current.
In the case of pulsed light for high harmonic generation, we find the radiated
light assumes a nearly universal behavior, with only limited dependence on the
parameters of the system, except for the amplitude of the driving field, which
determines the range of high harmonics generated and a tendency toward a
narrowing of the peaks in a charge density wave versus a metal. This type of
high harmonic generation can potentially be used for the creation of
solid-state-based ultrafast light sources.; Comment: (11 pages, 11 figures, ReVTeX)

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## Vacuum high harmonic generation in the shock regime

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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Electrodynamics becomes nonlinear and permits the self-interaction of fields
when the quantised nature of vacuum states is taken into account. The effect on
a plane probe pulse propagating through a stronger constant crossed background
is calculated using numerical simulation and by analytically solving the
corresponding wave equation. The electromagnetic shock resulting from vacuum
high harmonic generation is investigated and a nonlinear shock parameter
identified.; Comment: 19 pages, 16 figures

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## A Novel-type Tunable and Narrowband Extreme Ultraviolet Radiation Source Based on High-harmonic Conversion of Picosecond Laser Pulses

Fonte: Elsevier
Publicador: Elsevier

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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#Keywords: Fourier transforms#Frequency domain analysis#Harmonic generation#Resonance#Spectroscopic analysis#Ultraviolet radiation#Extreme ultraviolet radiation#High-harmonic generation (HHG)#Laser spectroscopy#Picosecond laser pulses#Laser pulses Extreme ultraviolet radiation

At the Laser Centre Vrije Universiteit a table-top size, tunable and narrowband laser-based source of extreme ultraviolet radiation was developed using high-harmonic generation of powerful laser pulses of 300 ps duration and Fourier-transform limited bandwidth. The generated radiation has unprecedented spectral purity of λ/Δλ>2.5×105 at wavelengths covering the entire tunability range of 40-100 nm. The process of high-order harmonic generation was investigated in different gases (Ar, Kr, Xe, N 2) uncovering resonance phenomena for the conversion efficiency of specific harmonic orders.

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## Density structure in low-order harmonic emission from laser-plasma interactions

Fonte: Sociedad Mexicana de Física
Publicador: Sociedad Mexicana de Física

Tipo: Artigo de Revista Científica
Formato: text/html

Publicado em 01/04/2006
EN

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Multiple harmonic generation from intense laser light incident on dense plasma targets is well-established. Both the number of harmonics generated and the corresponding power in each is strongly dependent on the intensity of the incident light. In this paper, attention is focused on an aspect of harmonic generation that has hitherto been largely overlooked, namely its dependence on plasma density. In particular we show that the conversion efficiencies of odd harmonics generated for normally incident light are strongly resonant in character, the number of resonances increasing with harmonic order. In addition to this resonant behaviour, the reflected power in low-order harmonics is further complicated by plasma emission at plasma resonant densities. Only at densities well above the resonances does the radiated harmonic power decay monotonically with density. The density dependence predicted by theory for low-order harmonics is confirmed by particle-in-cell simulations.

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