2,708 research outputs found
Light propagation control by finite-size effects in photonic crystals
We exhibit the strong influence on light propagation of the finite size in
photonic band-gap material. We show that light emission can be controlled by
the symmetry group of the boundary of the finite device. These results lead
simply to important practical applications.Comment: 5 pages, 4 figures, Revte
Croissance allométrique et dynamique spatiale
L'étude de la forme et de la croissance des systèmes urbains et régionaux est entravée par la difficulté que les chercheurs éprouvent à résoudre la dichotomie forme-processus. Celle-ci rend confuse la distinction entre systèmes physiques, biologiques et sociaux ; elle rend difficile le passage du langage des attributs à celui des localisations ; et finalement, elle engendre la pauvreté théorique des notions de région homogène et de région polarisée. La théorie de la croissance allométrique offre des possibilités réelles de solution à ces dilemmes. Ses fondements philosophiques structuralistes suggèrent que la forme et la croissance sont respectivement la projection dans l'espace et dans le temps des forces sociales ; et ses fondements méthodologiques systémiques offrent une démarche opératoire permettant d'effectuer la projection. Le paradigme allométrique mène à de nouvelles questions concernant, par exemple, la relation entre disparités sociales et régionales, et la part relative, dans l'Histoire, des effets cycliques et des effets cumulatifs.The study of the form and growth of urban and regional Systems is hindered by the difficulty that researchers have in resolving the form-process dichotomy. This dichotomy blurs the distinction between physical, biological and social Systems ; it also makes difficult the transition from attributional languages to locational languages ; and it is largely responsible for the theoretical shortcomings of such notions as homogeneous and polarized regions. The theory of allometric growth offers solutions to these problems. Its structuralist philosophical foundations suggest that form and growth are respectively the projection in space and time of social forces ; and its systemic methodological foundations offer an operational approach for performing this projection. The allometric paradigm also helps in formulating new questions concerning, for instance, social and regional disparities, and the relative share, in history, of cyclical and cumulative effects
Momentum space tomographic imaging of photoelectrons
We apply tomography, a general method for reconstructing 3-D distributions
from multiple projections, to reconstruct the momentum distribution of
electrons produced via strong field photoionization. The projections are
obtained by rotating the electron distribution via the polarization of the
ionizing laser beam and recording a momentum spectrum at each angle with a 2-D
velocity map imaging spectrometer. For linearly polarized light the tomographic
reconstruction agrees with the distribution obtained using an Abel inversion.
Electron tomography, which can be applied to any polarization, will simplify
the technology of electron imaging. The method can be directly generalized to
other charged particles.Comment: Accepted by J. Phys.
Alignment dependent enhancement of the photo-electron cutoff for multi-photon ionization of molecules
The multiphoton ionization rate of molecules depends on the alignment of the
molecular axis with respect to the ionizing laser polarization. By studying
molecular frame photo-electron angular distributions from N, O and
benzene, we illustrate how the angle-dependent ionization rate affects the
photo-electron cutoff energy. We find alignment can enhance the high energy
cutoff of the photo-electron spectrum when probing along a nodal plane or when
ionization is otherwise suppressed. This is supported by calculations using a
tunneling model with a single ion state.Comment: 4 pages, 4 figure
Compensatory embryonic response to allele-specific inactivation of the murine X-linked gene Hcfc1.
Early in female mammalian embryonic development, cells randomly inactivate one of the two X chromosomes to achieve overall equal inactivation of parental X-linked alleles. Hcfc1 is a highly conserved X-linked mouse gene that encodes HCF-1 - a transcriptional co-regulator implicated in cell proliferation in tissue culture cells. By generating a Cre-recombinase inducible Hcfc1 knock-out (Hcfc1(lox)) allele in mice, we have probed the role of HCF-1 in actively proliferating embryonic cells and in cell-cycle re-entry of resting differentiated adult cells using a liver regeneration model. HCF-1 function is required for both extraembryonic and embryonic development. In heterozygous Hcfc1(lox/+) female embryos, however, embryonic epiblast-specific Cre-induced Hcfc1 deletion (creating an Hcfc1(epiKO) allele) around E5.5 is well tolerated; it leads to a mixture of HCF-1-positive and -negative epiblast cells owing to random X-chromosome inactivation of the wild-type or Hcfc1(epiKO) mutant allele. At E6.5 and E7.5, both HCF-1-positive and -negative epiblast cells proliferate, but gradually by E8.5, HCF-1-negative cells disappear owing to cell-cycle exit and apoptosis. Although generating a temporary developmental retardation, the loss of HCF-1-negative cells is tolerated, leading to viable heterozygous offspring with 100% skewed inactivation of the X-linked Hcfc1(epiKO) allele. In resting adult liver cells, the requirement for HCF-1 in cell proliferation was more evident as hepatocytes lacking HCF-1 fail to re-enter the cell cycle and thus to proliferate during liver regeneration. The survival of the heterozygous Hcfc1(epiKO/+) female embryos, even with half the cells genetically compromised, illustrates the developmental plasticity of the post-implantation mouse embryo - in this instance, permitting survival of females heterozygous for an X-linked embryonic lethal allele
Simulation of cellular irradiation with the CENBG microbeam line using GEANT4
Light-ion microbeams provide a unique opportunity to irradiate biological
samples at the cellular level and to investigate radiobiological effects at low
doses of high LET ionising radiation. Since 1998 a single-ion irradiation
facility has been developed on the focused horizontal microbeam line of the
CENBG 3.5 MV Van de Graaff accelerator. This setup delivers in air single
protons and alpha particles of a few MeV onto cultured cells, with a spatial
resolution of a few microns, allowing subcellular targeting. In this paper, we
present results from the use of the GEANT4 toolkit to simulate cellular
irradiation with the CENBG microbeam line, from the entrance to the microprobe
up to the cellular medium.Comment: 6 pages, 8 figures, presented at the 2003 IEEE-NSS conference,
Portland, OR, USA, October 20-24, 200
Generation of broad XUV continuous high harmonic spectra and isolated attosecond pulses with intense mid-infrared lasers
We present experimental results showing the appearance of a near-continuum in
the high-order harmonic generation (HHG) spectra of atomic and molecular
species as the driving laser intensity of an infrared pulse increases. Detailed
macroscopic simulations reveal that these near-continuum spectra are capable of
producing IAPs in the far field if a proper spatial filter is applied. Further,
our simulations show that the near-continuum spectra and the IAPs are a product
of strong temporal and spatial reshaping (blue shift and defocusing) of the
driving field. This offers a possibility of producing IAPs with a broad range
of photon energy, including plateau harmonics, by mid-IR laser pulses even
without carrier-envelope phase stabilization.Comment: 7 pages, 5 figures, submitted to J.Phys. B (Oct 2011
Un colloque sur la pluridisciplinariaté dans les problèmes d'environnement : quelques enseignements et orientations pour l'avenir
International audienceAn experiment has been carried out in Rennes (Brittany, West of France) in June 2000 to estimate atrazine and alachlor volatilisation fluxes after application over a maize crop. Fluxes were calculated using the classical aerodynamic micrometeorological method from vertical gradients of pesticide concentrations, temperature and wind speed. Volatilisation fluxes showed a diurnal pattern of the order of few ng/m2/s for atrazine and the order of a few 10 ng/m2/s for alachlor, leading to cumulated losses of approximately 0.1% of the theoretical application dose for atrazine and several-fold of that in the case of alachlor
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