136 research outputs found
All-optical three-dimensional electron pulse compression
We propose an all-optical, three-dimensional electron pulse compression
scheme in which Hermite-Gaussian optical modes are used to fashion a
three-dimensional optical trap in the electron pulse's rest frame. We show that
the correct choices of optical incidence angles are necessary for optimal
compression. We obtain analytical expressions for the net impulse imparted by
Hermite-Gaussian free-space modes of arbitrary order. Although we focus on
electrons, our theory applies to any charged particle and any particle with
non-zero polarizability in the Rayleigh regime. We verify our theory
numerically using exact solutions to Maxwell's equations for first-order
Hermite-Gaussian beams, demonstrating single-electron pulse compression factors
of in both longitudinal and transverse dimensions with experimentally
realizable optical pulses. The proposed scheme is useful in ultrafast electron
imaging for both single- and multi-electron pulse compression, and as a means
of circumventing temporal distortions in magnetic lenses when focusing
ultrashort electron pulses.Comment: 21 pages, 7 figure
Disentangling the Electronic and Lattice Contributions to the Dielectric Response of Photoexcited Bismuth
Elucidating the interplay between nuclear and electronic degrees of freedom
that govern the complex dielectric behavior of materials under intense
photoexcitation is essential for tailoring optical properties on demand.
However, conventional transient reflectivity experiments have been unable to
differentiate between real and imaginary components of the dielectric response,
omitting crucial electron-lattice interactions. Utilizing thin film
interference we unambiguously determined the photoinduced change in complex
dielectric function in the Peierls semimetal bismuth and examined its
dependence on the excitation density and nuclear motion of the A phonon.
Our modeled transient reflectivity data reveals a progressive broadening and
redshift of Lorentz oscillators with increasing excitation density and
underscores the importance of both, electronic and nuclear coordinates in the
renormalization of interband transitions.Comment: Manuscript (6 pages) plus supplemental material (6 pages
Ultrafast electronic and lattice dynamics in laser-excited crystalline bismuth
Femtosecond spectroscopy is applied to study transient electronic and lattice
processes in bismuth. Components with relaxation times of 1 ps, 7 ps and ~ 1 ns
are detected in the photoinduced reflectivity response of the crystal. To
facilitate the assignment of the observed relaxation to the decay of particular
excited electronic states we use pump pulses with central wavelengths ranging
from 400 nm to 2.3 mum. Additionally, we examine the variation of parameters of
coherent A1g phonons upon the change of excitation and probing conditions. Data
analysis reveals a significant wavevector dependence of electron-hole and
electron- phonon coupling strength along \Gamma--T direction of the Brillouin
zone.Comment: 19 pages, 9 figure
Ring closing reaction in diarylethene captured by femtosecond electron crystallography
The photoinduced ring-closing reaction in diarylethene, which serves as a model system for understanding reactive crossings through conical intersections, was directly observed with atomic resolution using femtosecond electron diffraction. Complementary ab initio calculations were also performed. Immediately following photoexcitation, subpicosecond structural changes associated with the formation of an open-ring excited-state intermediate were resolved. The key motion is the rotation of the thiophene rings, which significantly decreases the distance between the reactive carbon atoms prior to ring closing. Subsequently, on the few picosecond time scale, localized torsional motions of the carbon atoms lead to the formation of the closed-ring photoproduct. These direct observations of the molecular motions driving an organic chemical reaction were only made possible through the development of an ultrabright electron source to capture the atomic motions within the limited number of sampling frames and the low data acquisition rate dictated by the intrinsically poor thermal conductivity and limited photoreversibility of organic materials
Incorporating fine-scale environmental heterogeneity into broad-extent models
A key aim of ecology is to understand the drivers of ecological patterns, so that we can accurately predict the effects of global environmental change. However, in many cases, predictors are measured at a finer resolution than the ecological response. We therefore require data aggregation methods that avoid loss of information on fine-grain heterogeneity. We present a data aggregation method that, unlike current approaches, reduces the loss of information on fine-grain spatial structure in environmental heterogeneity for use with coarse-grain ecological datasets. Our method contains three steps: (a) define analysis scales (predictor grain, response grain, scale-of-effect); (b) use a moving window to calculate a measure of variability in environment (predictor grain) at the process-relevant scale (scale-of-effect); and (c) aggregate the moving window calculations to the coarsest resolution (response grain). We show the theoretical basis for our method using simulated landscapes and the practical utility with a case study. Our method is available as the grainchanger r package. The simulations show that information about spatial structure is captured that would have been lost using a direct aggregation approach, and that our method is particularly useful in landscapes with spatial autocorrelation in the environmental predictor variable (e.g. fragmented landscapes) and when the scale-of-effect is small relative to the response grain. We use our data aggregation method to find the appropriate scale-of-effect of land cover diversity on Eurasian jay Garrulus glandarius abundance in the UK. We then model the interactive effect of land cover heterogeneity and temperature on G. glandarius abundance. Our method enables us quantify this interaction despite the different scales at which these factors influence G. glandarius abundance. Our data aggregation method allows us to integrate variables that act at varying scales into one model with limited loss of information, which has wide applicability for spatial analyses beyond the specific ecological context considered here. Key ecological applications include being able to estimate the interactive effect of drivers that vary at different scales (such as climate and land cover), and to systematically examine the scale dependence of the effects of environmental heterogeneity in combination with the effects of climate change on biodiversity
Ultrafast changes in lattice symmetry probed by coherent phonons
The electronic and structural properties of a material are strongly
determined by its symmetry. Changing the symmetry via a photoinduced phase
transition offers new ways to manipulate material properties on ultrafast
timescales. However, in order to identify when and how fast these phase
transitions occur, methods that can probe the symmetry change in the time
domain are required. We show that a time-dependent change in the coherent
phonon spectrum can probe a change in symmetry of the lattice potential, thus
providing an all-optical probe of structural transitions. We examine the
photoinduced structural phase transition in VO2 and show that, above the phase
transition threshold, photoexcitation completely changes the lattice potential
on an ultrafast timescale. The loss of the equilibrium-phase phonon modes
occurs promptly, indicating a non-thermal pathway for the photoinduced phase
transition, where a strong perturbation to the lattice potential changes its
symmetry before ionic rearrangement has occurred.Comment: 14 pages 4 figure
How Plasmonic excitation influences the LIPSS formation on diamond during multipulse femtosecond laser irradiation ?
A generalized plasmonic model is proposed to calculate the nanostructure period induced by multipulse laser femtosecond on diamond at 800 nm wavelengths. We follow the evolution of LIPSS formation by changing diamond optical parameters in function of electron plasma excitation during laser irradiation. Our calculations shows that the ordered nanostructures can be observed only in the range of surface plasmon polariton excitation
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