128 research outputs found
Probing time-ordering in two-photon double ionization of helium on the attosecond time scale
We show that time ordering underlying time-dependent quantum dynamics is a
physical observable accessible by attosecond streaking. We demonstrate the
extraction of time ordering for the prototypical case of time-resolved
two-photon double ionization (TPDI) of helium by an attosecond XUV pulse. The
Eisenbud-Wigner-Smith time delay for the emission of a two-electron wavepacket
and the time interval between subsequent emission events can be unambiguously
determined by attosecond streaking. The delay between the two emission events
sensitively depends on the energy, pulse duration, and angular distribution of
the emitted electron pair. Our fully-dimensional ab-initio quantum mechanical
simulations provide benchmark data for experimentally accessible observables.Comment: 8 pages, 5 figures; revised version, added appendi
Angular distribution in two-photon double ionization of helium by intense attosecond soft X-ray pulses
We investigate two-photon double ionization of helium by intense () ultrashort ( as) soft X-ray pulses (E = 91.6 eV). The
time-dependent two-electron Schr\"odinger equation is solved using a coupled
channel method. We show that for ultrashort pulses the angular distribution of
ejected electrons depends on the pulse duration and provides novel insights
into the role of electron correlations in the two-electron photoemission
process. The angular distribution at energies near the ``independent electron''
peaks is close to dipolar while it acquires in the ``valley'' of correlated
emission a significant quadrupolar component within a few hundred attoseconds.Comment: 17 pages, 6 fig
Large optical field enhancement for nanotips with large opening angles
We theoretically investigate the dependence of the enhancement of optical
near-fields at nanometric tips on the shape, size, and material of the tip. We
confirm a strong dependence of the field enhancement factor on the radius of
curvature. In addition, we find a surprisingly strong increase of field
enhancement with increasing opening angle of the nanotips. For gold and
tungsten nanotips in the experimentally relevant parameter range (radius of
curvature nm at nm laser wavelength), we obtain field
enhancement factors of up to for Au and for W for large
opening angles. We confirm this strong dependence on the opening angle for many
other materials featuring a wide variety in their dielectric response. For
dielectrics, the opening angle dependence is traced back to the electrostatic
force of the induced surface charge at the tip shank. For metals, the plasmonic
response strongly increases the field enhancement and shifts the maximum field
enhancement to smaller opening angles.Comment: 16 pages, 12 figure
Photovoltaic effect in an electrically tunable van der Waals heterojunction
Semiconductor heterostructures form the cornerstone of many electronic and
optoelectronic devices and are traditionally fabricated using epitaxial growth
techniques. More recently, heterostructures have also been obtained by vertical
stacking of two-dimensional crystals, such as graphene and related two-
dimensional materials. These layered designer materials are held together by
van der Waals forces and contain atomically sharp interfaces. Here, we report
on a type- II van der Waals heterojunction made of molybdenum disulfide and
tungsten diselenide monolayers. The junction is electrically tunable and under
appropriate gate bias, an atomically thin diode is realized. Upon optical
illumination, charge transfer occurs across the planar interface and the device
exhibits a photovoltaic effect. Advances in large-scale production of
two-dimensional crystals could thus lead to a new photovoltaic solar
technology.Comment: 26 pages, 14 figures, Nano Letters 201
Decreasing excitation gap in Andreev billiards by disorder scattering
We investigate the distribution of the lowest-lying energy states in a
disordered Andreev billiard by solving the Bogoliubov-de Gennes equation
numerically. Contrary to conventional predictions we find a decrease rather
than an increase of the excitation gap relative to its clean ballistic limit.
We relate this finding to the eigenvalue spectrum of the Wigner-Smith time
delay matrix between successive Andreev reflections. We show that the longest
rather than the mean time delay determines the size of the excitation gap. With
increasing disorder strength the values of the longest delay times increase,
thereby, in turn, reducing the excitation gap.Comment: 6 pages, 5 figures, submitted to EP
Time Double-Slit Interference in Tunneling Ionization
We show that interference phenomena plays a big role for the electron yield
in ionization of atoms by an ultra-short laser pulse. Our theoretical study of
single ionization of atoms driven by few-cycles pulses extends the
photoelectron spectrum observed in the double-slit experiment by Lindner et al,
Phys. Rev. Lett. \textbf{95}, 040401 (2005) to a complete three-dimensional
momentum picture. We show that different wave packets corresponding to the same
single electron released at different times interfere, forming interference
fringes in the two-dimensional momentum distributions. These structures
reproduced by means of \textit{ab initio} calculations are understood within a
semiclassical model.Comment: 7 pages, 5 figure
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