79 research outputs found
Multi-electron transitions induced by neutron impact on helium
We explore excitation and ionization by neutron impact as a novel tool for
the investigation of electron-electron correlations in helium. We present
single and double ionization spectra calculated in accurate numerical ab-initio
simulations for incoming neutrons with kinetic energies of up to 150 keV. The
resulting electron spectra are found to be fundamentally different from
photoioniza- tion or charged particle impact due to the intrinsic many-body
character of the interaction. In particular, doubly excited resonances that are
strongly suppressed in electron or photon impact become prominent. The ratio of
double to single ionization is found to differ significantly from those of
photon and charged particle impact.Comment: 5 pages, 5 figure
Photoionization of helium by attosecond pulses: extraction of spectra from correlated wave functions
We investigate the photoionization spectrum of helium by attosecond XUV
pulses both in the spectral region of doubly excited resonances as well as
above the double ionization threshold. In order to probe for convergence, we
compare three techniques to extract photoelectron spectra from the wavepacket
resulting from the integration of the time-dependent Schroedinger equation in
a finite-element discrete variable representation basis. These techniques are:
projection on products of hydrogenic bound and continuum states, projection
onto multi-channel scattering states computed in a B-spline close-coupling
basis, and a technique based on exterior complex scaling (ECS) implemented in
the same basis used for the time propagation. These methods allow to monitor
the population of continuum states in wavepackets created with ultrashort
pulses in different regimes. Applications include photo cross sections and
anisotropy parameters in the spectral region of doubly excited resonances,
time-resolved photoexcitation of autoionizing resonances in an attosecond
pump-probe setting, and the energy and angular distribution of correlated
wavepackets for two-photon double ionization.Comment: 19 pages, 12 figure
Pump-induced Exceptional Points in Lasers
We demonstrate that the above-threshold behavior of a laser can be strongly
affected by exceptional points which are induced by pumping the laser
nonuniformly. At these singularities, the eigenstates of the non-Hermitian
operator which describes the lasing modes coalesce. In their vicinity, the
laser may turn off even when the overall pump power deposited in the system is
increased. Such signatures of a pump- induced exceptional point can be
experimentally probed with coupled ridge or microdisk lasers.Comment: 4.5 pages, 4 figures, final version including additional FDTD dat
Scalable numerical approach for the steady-state ab initio laser theory
We present an efficient and flexible method for solving the non-linear lasing
equations of the steady-state ab initio laser theory. Our strategy is to solve
the underlying system of partial differential equations directly, without the
need of setting up a parametrized basis of constant flux states. We validate
this approach in one-dimensional as well as in cylindrical systems, and
demonstrate its scalability to full-vector three-dimensional calculations in
photonic-crystal slabs. Our method paves the way for efficient and accurate
simulations of lasing structures which were previously inaccessible.Comment: 17 pages, 8 figure
Symmetry, stability, and computation of degenerate lasing modes
We present a general method to obtain the stable lasing solutions for the steady-state ab initio lasing theory (SALT) for the case of a degenerate symmetric laser in two dimensions (2D). We find that under most regimes (with one pathological exception), the stable solutions are clockwise and counterclockwise circulating modes, generalizing previously known results of ring lasers to all 2D rotational symmetry groups. Our method uses a combination of semianalytical solutions close to lasing threshold and numerical solvers to track the lasing modes far above threshold. Near threshold, we find closed-form expressions for both circulating modes and other types of lasing solutions as well as for their linearized Maxwell-Bloch eigenvalues, providing a simple way to determine their stability without having to do a full nonlinear numerical calculation. Above threshold, we show that a key feature of the circulating mode is its “chiral” intensity pattern, which arises from spontaneous symmetry breaking of mirror symmetry, and whose symmetry group requires that the degeneracy persists even when nonlinear effects become important. Finally, we introduce a numerical technique to solve the degenerate SALT equations far above threshold even when spatial discretization artificially breaks the degeneracy.United States. Army Research Office. Institute for Soldier Nanotechnologies (Grant W911NF-07-D-0004)Austrian Science Fund (Project SFB NextLite F49-P10)United States. Air Force Research Laboratory (Agreement FA8650-15-2-5220
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