542 research outputs found
Experimental setup for low-energy laser-based angle resolved photoemission spectroscopy
A laser-based angle resolved photoemission (APRES) system utilizing 6 eV
photons from the fourth harmonic of a mode-locked Ti:sapphire oscillator is
described. This light source greatly increases the momentum resolution and
photoelectron count rate, while reducing extrinsic background and surface
sensitivity relative to higher energy light sources. In this review, the
optical system is described, and special experimental considerations for
low-energy ARPES are discussed. The calibration of the hemispherical electron
analyzer for good low-energy angle-mode performance is also described. Finally,
data from the heavily studied high T_c superconductor Bi2Sr2CaCu2O8+\delta
(Bi2212) is compared to the results from higher photon energies.Comment: Please download final version from Journal-Re
Phase-matching conditions for nonlinear frequency conversion by use of aligned molecular gases
Includes bibliographical references (page 348).Transient birefringence can be induced in a gas of anisotropic molecules by an intense polarized laser pulse. We propose to use this birefringence to phase match nonlinear optical frequency-conversion processes. The conditions for anisotropic phase matching are derived, and experimental conditions required for phase-matched third-harmonic generation in a gas-filled hollow-core fiber are presented. We show that these conditions are experimentally feasible over a significant parameter range, making possible a new type of nonlinear optics
Light with a self-torque: extreme-ultraviolet beams with time-varying orbital angular momentum
Twisted light fields carrying orbital angular momentum (OAM) provide powerful
capabilities for applications in optical communications, microscopy, quantum
optics and microparticle rotation. Here we introduce and experimentally
validate a new class of light beams, whose unique property is associated with a
temporal OAM variation along a pulse: the self-torque of light. Self-torque is
a phenomenon that can arise from matter-field interactions in electrodynamics
and general relativity, but to date, there has been no optical analog. In
particular, the self-torque of light is an inherent property, which is
distinguished from the mechanical torque exerted by OAM beams when interacting
with physical systems. We demonstrate that self-torqued beams in the
extreme-ultraviolet (EUV) naturally arise as a necessary consequence of angular
momentum conservation in non-perturbative high-order harmonic generation when
driven by time-delayed pulses with different OAM. In addition, the
time-dependent OAM naturally induces an azimuthal frequency chirp, which
provides a signature for monitoring the self-torque of high-harmonic EUV beams.
Such self-torqued EUV beams can serve as unique tools for imaging magnetic and
topological excitations, for launching selective excitation of quantum matter,
and for manipulating molecules and nanostructures on unprecedented time and
length scales.Comment: 24 pages, 4 figure
Ultrafast modulation of the chemical potential in BaFeAs by coherent phonons
Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is
used to study the electronic structure dynamics in BaFeAs around the
high-symmetry points and . A global oscillation of the Fermi level
at the frequency of the (As) phonon mode is observed. It is argued that
this behavior reflects a modulation of the effective chemical potential in the
photoexcited surface region that arises from the high sensitivity of the band
structure near the Fermi level to the phonon mode combined with a low
electron diffusivity perpendicular to the layers. The results establish a novel
way to tune the electronic properties of iron pnictides: coherent control of
the effective chemical potential. The results further suggest that the
equilibration time for the effective chemical potential needs to be considered
in the ultrafast electronic structure dynamics of materials with weak
interlayer coupling.Comment: 6 pages, 3 figure
Time- and angle-resolved photoemission spectroscopy with optimized high-harmonic pulses using frequency-doubled Ti:Sapphire lasers
Time- and angle-resolved photoemission spectroscopy (trARPES) using femtosecond extreme ultraviolet high harmonics has recently emerged as a powerful tool for investigating ultrafast quasiparticle dynamics in correlated-electron materials. However, the full potential of this approach has not yet been achieved because, to date, high harmonics generated by 800 nm wavelength Ti:Sapphire lasers required a trade-off between photon flux, energy and time resolution. Photoemission spectroscopy requires a quasi-monochromatic output, but dispersive optical elements that select a single harmonic can significantly reduce the photon flux and time resolution. Here we show that 400 nm driven high harmonic extreme-ultraviolet trARPES is superior to using 800 nm laser drivers since it eliminates the need for any spectral selection, thereby increasing photon flux and energy resolution to < 150 meV while preserving excellent time resolution of about 30 fs. © 2014 The Authors
Demonstration of a sub‐picosecond x‐ray streak camera
A novel design, magnetically focused, x‐ray streak camera was designed and tested using sub‐20 fs soft‐x‐ray pulses generated by high harmonic emission in a gas. The temporal resolution of the camera was demonstrated to be under 0.9 ps throughout the ultraviolet to soft‐x‐ray wavelength region. Our streak camera represents the fastest x‐ray detector developed to date. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69727/2/APPLAB-69-1-133-1.pd
The dark matter halo shape of edge-on disk galaxies - I. HI observations
This is the first paper of a series in which we will attempt to put
constraints on the flattening of dark halos in disk galaxies. We observe for
this purpose the HI in edge-on galaxies, where it is in principle possible to
measure the force field in the halo vertically and radially from gas layer
flaring and rotation curve decomposition respectively. In this paper, we define
a sample of 8 HI-rich late-type galaxies suitable for this purpose and present
the HI observations.Comment: Accepted for publication by Astronomy & Astrophysics. For a higher
resolution version see
http://www.astro.rug.nl/~vdkruit/jea3/homepage/12565.pd
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Electronic initiation and optimization of nonlinear polarization evolution mode-locking in a fiber laser
We describe a system for automated modelocking and optimization of a fiber laser oscillator employing nonlinear polarization evolution. Using four liquid crystal variable retarders, we fully control the fiber launch and output polarization states, enabling compensation for mechanical and environmental perturbations to the fiber cavity. We demonstrate mapping of the modelocking regions for an ANDi fiber oscillator and demonstrate that local and global optimization algorithms can be used to maintain the laser in the same operating state. This technique enables robust operation of nonlinear polarization evolution modelocked fiber lasers, rivaling the stability of PM fiber lasers while maintaining the advantages of the NPE modelocking mechanism
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