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.

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

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$_2$, O$_2$ 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

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

Excitation Spectrum and Superexchange Pathways in the Spin Dimer VODPO_4 . 1/2 D_2O

Magnetic excitations have been investigated in the spin dimer material VODPO_4 \cdot 1/2 D_2O using inelastic neutron scattering. A dispersionless magnetic mode was observed at an energy of 7.81(4) meV. The wavevector dependence of the scattering intensityfrom this mode is consistent with the excitation of isolated V^{4+} spin dimers with a V-V separation of 4.43(7) \AA. This result is unexpected since the V-V pair previously thought to constitute themagnetic dimer has a separation of 3.09 \AA. We identify an alternative V-V pair as the likely magnetic dimer, which involves superexchange pathways through a covalently bonded PO_4 group. This surprising result casts doubt on the interpretation of (VO)_2P_2O_7 as a spin ladder.Comment: 4 pages, 4 postscript figures - identical to previous paper but figure 2 and 3 hopefully more compatible .p

Advanced deep space communication systems study Final report

Deep space communication system requirements for period 1970 to 198

Partitioning of the linear photon momentum in multiphoton ionization

The balance of the linear photon momentum in multiphoton ionization is studied experimentally. In the experiment argon and neon atoms are singly ionized by circularly polarized laser pulses with a wavelength of 800 nm and 1400 nm in the intensity range of 10^{14} - 10^{15} W/cm^2. The photoelectrons are measured using velocity map imaging. We find that the photoelectrons carry linear momentum corresponding to the photons absorbed above the field free ionization threshold. Our finding has implications for concurrent models of the generation of terahertz radiation in filaments.Comment: 4 pages, 3 figure

Degeneracy analysis for a super cell of a photonic crystal and its application to the creation of band gaps

A method is introduced to analyze the degeneracy properties of the band structure of a photonic crystal making use of the super cells. The band structure associated with a super cell of a photonic crystal has degeneracies at the edge of the Brillouin zone if the photonic crystal has some kind of point group symmetry. Both E-polarization and H-polarization cases have the same degeneracies for a 2-dimensional (2D) photonic crystal. Two theorems are given and proved. These degeneracies can be lifted to create photonic band gaps by changing the transform matrix between the super cell and the smallest unit cell. The existence of the photonic band gaps for many known 2D photonic crystals is explained through the degeneracy analysis.Comment: 19 pages, revtex4, 14 figures, p

Atypical sulcal anatomy in young children with autism spectrum disorder

AbstractAutism spectrum disorder is associated with an altered early brain development. However, the specific cortical structure abnormalities underlying this disorder remain largely unknown. Nonetheless, atypical cortical folding provides lingering evidence of early disruptions in neurodevelopmental processes and identifying changes in the geometry of cortical sulci is of primary interest for characterizing these structural abnormalities in autism and their evolution over the first stages of brain development. Here, we applied state-of-the-art sulcus-based morphometry methods to a large highly-selective cohort of 73 young male children of age spanning from 18 to 108 months. Moreover, such large cohort was selected through extensive behavioral assessments and stringent inclusion criteria for the group of 59 children with autism. After manual labeling of 59 different sulci in each hemisphere, we computed multiple shape descriptors for each single sulcus element, hereby separating the folding measurement into distinct factors such as the length and depth of the sulcus. We demonstrated that the central, intraparietal and frontal medial sulci showed a significant and consistent pattern of abnormalities across our different geometrical indices. We also found that autistic and control children exhibited strikingly different relationships between age and structural changes in brain morphology. Lastly, the different measures of sulcus shapes were correlated with the CARS and ADOS scores that are specific to the autistic pathology and indices of symptom severity. Inherently, these structural abnormalities are confined to regions that are functionally relevant with respect to cognitive disorders in ASD. In contrast to those previously reported in adults, it is very unlikely that these abnormalities originate from general compensatory mechanisms unrelated to the primary pathology. Rather, they most probably reflect an early disruption on developmental trajectory that could be part of the primary pathology

Intensity Dependence of Multiple Orbital Contributions and Shape Resonance in High-Order Harmonic Generation of Aligned N₂ Molecules

We report measurements and theoretical simulations of high-order harmonic generation (HHG) in aligned N₂ molecules using a 1200-nm intense laser field when the generating pulse is perpendicular to the aligning one. With increasing laser intensity, the minimum in the HHG spectra first shifts its position and then disappears. Theoretical simulations including the macroscopic propagation effects in the medium reproduce these observations and the disappearance of the minimum is attributed to the additional contribution of HHG from inner orbitals. We also predict that the well-known shape resonance in the photoionization spectra of N₂ should exist in the HHG spectra. It is most clearly seen when the generating laser is parallel to the aligning one and disappears gradually as the angle between the two lasers increases. No clear evidence of this shape resonance has been reported so far when using lasers with different wavelengths. Further experimentation is needed to draw conclusions