845 research outputs found
Making Distinct Dynamical Systems Appear Spectrally Identical
We show that a laser pulse can always be found that induces a desired optical
response from an arbitrary dynamical system. As illustrations, driving fields
are computed to induce the same optical response from a variety of distinct
systems (open and closed, quantum and classical). As a result, the observed
induced dipolar spectra without detailed information on the driving field is
not sufficient to characterize atomic and molecular systems. The formulation
may also be applied to design materials with specified optical characteristics.
These findings reveal unexplored flexibilities of nonlinear optics.Comment: 9 pages, 5 figure
Multidimensional quantum tunneling in the Schwinger effect
We study the Schwinger effect, in which the external field having a
spatiotemporal profile creates electron-positron pairs via multidimensional
quantum tunneling. Our treatment is based on the trace formula for the QED
effective action, whose imaginary part is represented by a sum over complex
worldline solutions. The worldlines are multiperiodic, and the periods of
motion collectively depend on the strength of spatial and temporal
inhomogeneity. We argue that the classical action that leads to the correct
tunneling amplitude must take into account both the full period,
and the first fundamental period, . In view of this argument we
investigate pair production in an exponentially damped sinusoidal field and
find that the initial momenta for multiperiodic trajectories lie on parabolic
curves, such that on each curve the ratio stays uniform.
Evaluation of the tunneling amplitude using these trajectories shows that
vacuum decay rate is reduced by an order of magnitude, with respect to the
purely time-dependent case, due to the presence of magnetic field.Comment: 6 pages, 4 figures. Revised and extende
Resonant tunneling controlled by laser and constant electric fields
We develop the concept of scattering matrix and we use it to perform stable
numerical calculations of resonant tunneling of electrons through a multiple
potential barrier in a semiconductor heterostructure. Electrons move in two
external nonperturbative electric fields: constant and oscillating in time. We
apply our algorithm for different strengths and spatial configurations of the
fields.Comment: 10 pages, 11 figure
Photoelectron Angular Distributions for Two-photon Ionization of Helium by Ultrashort Extreme Ultraviolet Free Electron Laser Pulses
Phase-shift differences and amplitude ratios of the outgoing and
continuum wave packets generated by two-photon ionization of helium atoms are
determined from the photoelectron angular distributions obtained using velocity
map imaging. Helium atoms are ionized with ultrashort extreme-ultraviolet
free-electron laser pulses with a photon energy of 20.3, 21.3, 23.0, and 24.3
eV, produced by the SPring-8 Compact SASE Source test accelerator. The measured
values of the phase-shift differences are distinct from scattering phase-shift
differences when the photon energy is tuned to an excited level or Rydberg
manifold. The difference stems from the competition between resonant and
non-resonant paths in two-photon ionization by ultrashort pulses. Since the
competition can be controlled in principle by the pulse shape, the present
results illustrate a new way to tailor the continuum wave packet.Comment: 5 pages, 1 table, 3 figure
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