857 research outputs found
Spectral dependence of purely-Kerr driven filamentation in air and argon
Based on numerical simulations, we show that higher-order nonlinear indices
(up to and , respectively) of air and argon have a dominant
contribution to both focusing and defocusing in the self-guiding of ultrashort
laser pulses over most of the spectrum. Plasma generation and filamentation are
therefore decoupled. As a consequence, ultraviolet wavelength may not be the
optimal wavelengths for applications requiring to maximize ionization.Comment: 14 pages, 4 figures (14 panels
Orientation and Alignment Echoes
We present what is probably the simplest classical system featuring the echo
phenomenon - a collection of randomly oriented free rotors with dispersed
rotational velocities. Following excitation by a pair of time-delayed impulsive
kicks, the mean orientation/alignment of the ensemble exhibits multiple echoes
and fractional echoes. We elucidate the mechanism of the echo formation by
kick-induced filamentation of phase space, and provide the first experimental
demonstration of classical alignment echoes in a thermal gas of CO_2 molecules
excited by a pair of femtosecond laser pulses
Higher-order Kerr terms allow ionization-free filamentation in gases
We show that higher-order nonlinear indices (, , , )
provide the main defocusing contribution to self-channeling of ultrashort laser
pulses in air and Argon at 800 nm, in contrast with the previously accepted
mechanism of filamentation where plasma was considered as the dominant
defocusing process. Their consideration allows to reproduce experimentally
observed intensities and plasma densities in self-guided filaments.Comment: 11 pages, 6 figures (11 panels
From higher-order Kerr nonlinearities to quantitative modeling of 3rd and 5th harmonic generation in argon
The recent measurement of negative higher-order Kerr effect (HOKE) terms in
gases has given rise to a controversial debate, fed by its impact on short
laser pulse propagation. By comparing the experimentally measured yield of the
third and fifth harmonics, with both an analytical and a full comprehensive
numerical propagation model, we confirm the absolute and relative values of the
reported HOKE indices.Comment: 3 pages, 2 figure
High-Resolution Nonlinear Raman Spectroscopy in Gases
The applicability of Raman spectroscopy to the investigation
of gases has been greatly improved by the development of the
different methods of nonlinear Raman scattering. When two laser
beams, one of which has a tunable frequency, are brought to a
common focus in a sample, a stimulated Raman process occurs, as
soon as the frequency difference between the two lasers is equal
to aRaman active rovibrational or rotational transition frequency
of the sarnple, and the corresponding state is popuJated above
equilibrium. The Raman resonance can be detected in different
ways: by coherent anti-Stokes Raman scattering (CARS) or the
corresponding Stokes process (CSRS), by again in one of the
beams (stimulated Raman gain spectroscopy, SRGS) or a loss in
the other one (inverse Raman spectroscopy, IRS), or even by detection of a photoacoustic signal (photoacoustic Raman spectroscopy,PARS). The selective ionisation of the excited molecules by a third ultraviolet laser pulse (ionisation detected stimulated Raman scattering, IDSRS) has considerably increased the sensitivity in special cases. The instrumental resolution of the se techniques is determined by the convoluted linewidths of the lasers used for excitation. This is of special importance for the investigation of high resolution rotation-vibrational spectra of gases
Adapting land restoration to a changing climate: Embracing the knowns and unknowns
CIFOR Infobrief 249, Center for International Forestry Research, Bogor, doi:10.17528/cifor/007261Land restoration will happen under climate change and different knowledge systems are needed to navigate uncertainties and plan adaptation. • The emergence of novel ecosystems presents a challenge for land restoration; they harbor unknown unknowns. • This brief presents key research linking land restoration and societal adaptation and an example of a practical framework for transformative adaptation. • It also proposes questions that can guide stakeholders in exploring different change narratives for adaptation and restoration planning
Assessing bundles of ecosystem services from regional to landscape scale: Insights from the French Alps
Copyright © 2015 The Authors. Journal of Applied Ecology © 2015 British Ecological SocietyAssessments of ecosystem services (ES) and biodiversity (hereafter ecological parameters) provide a comprehensive view of the links between landscapes, ecosystem functioning and human well-being. The investigation of consistent associations between ecological parameters, called bundles, and of their links to landscape composition and structure is essential to inform management and policy, yet it is still in its infancy. We mapped over the French Alps an unprecedented array of 18 ecological parameters (16 ES and two biodiversity parameters) and explored their co-occurrence patterns underpinning the supply of multiple ecosystem services in landscapes. We followed a three-step analytical framework to i) detect the ES and biodiversity associations relevant at regional scale, ii) identify the clusters supplying consistent bundles of ES at subregional scale and iii) explore the links between landscape heterogeneity and ecological parameter associations at landscape scale. We used successive correlation coefficients, overlap values and self-organizing maps to characterize ecological bundles specific to given land cover types and geographical areas of varying biophysical characteristics and human uses at nested scales from regional to local. The joint analysis of land cover richness and ES gamma diversity demonstrated that local landscape heterogeneity alone did not imply compatibility across multiple ecosystem services, as some homogeneous landscape could supply multiple ecosystem services. Synthesis and applications. Bundles of ecosystem services and biodiversity parameters are shaped by the joint effects of biophysical characteristics and of human history. Due to spatial congruence and to underlying functional interdependencies, ecological parameters should be managed as bundles even when management targets specific objectives. Moreover, depending on the abiotic context, the supply of multiple ecosystem services can arise either from deliberate management in homogeneous landscapes or from spatial heterogeneity.ERAnet BiodivERsA project CONNECTFrench Agence Nationale pour la RechercheEU project VOLANT
On negative higher-order Kerr effect and filamentation
As a contribution to the ongoing controversy about the role of higher-order
Kerr effect (HOKE) in laser filamentation, we first provide thorough details
about the protocol that has been employed to infer the HOKE indices from the
experiment. Next, we discuss potential sources of artifact in the experimental
measurements of these terms and show that neither the value of the observed
birefringence, nor its inversion, nor the intensity at which it is observed,
appear to be flawed. Furthermore, we argue that, independently on our values,
the principle of including HOKE is straightforward. Due to the different
temporal and spectral dynamics, the respective efficiency of defocusing by the
plasma and by the HOKE is expected to depend substantially on both incident
wavelength and pulse duration. The discussion should therefore focus on
defining the conditions where each filamentation regime dominates.Comment: 22 pages, 11 figures. Submitted to Laser physics as proceedings of
the Laser Physics 2010 conferenc
- …