27 research outputs found
Origin of degenerate bound states in the continuum in a grating waveguide: Parity symmetry breaking due to mode crossing
We explain the origin of bound states in the continuum (BICs) in a planar
grating waveguide, in particular, a mechanism for formation of degenerate BICs,
via the analytical theory of the infinite-grating eigenmodes. Conventional
symmetry-protected BICs are formed at normal incidence mainly by a single
infinite-grating eigenmode that has an odd spatial parity on both sides of the
BIC resonance. The odd parity is the reason for a cutoff from the
radiation-loss channel and appearance of such BICs. The mechanism of emergence
of a degenerate BIC in a vicinity of a degenerate frequency of two
infinite-grating eigenmodes is different. The degenerate BIC is formed by an
anti-phased coherent superposition of two crossing infinite-grating eigenmodes
both of which possess a mixed parity and experience parity symmetry breaking as
the frequency scans through the degeneracy point. In this case a cutoff from
the radiation-loss channel and extremely high-Q narrow resonance is achieved
due to the destructive interference of the two crossing eigenmodes.
Implementation of such a mechanism can be instructive for designing BICs in
other photonic crystals and structures.Comment: 25 pages, 20 figure
Multi-scale magnetic field structures in an expanding elongated plasma cloud with hot electrons subject to an external magnetic field
We carry out 3D and 2D PIC-simulations of the expansion of a magnetized
plasma that initially uniformly fills a half-space and contains a
semi-cylindrical region of heated electrons elongated along the surface of the
plasma boundary. This geometry is related, for instance, to the ablation of a
plane target by a femtosecond laser beam under quasi-cylindrical focusing. We
find that the decay of the inhomogeneous plasma--vacuum discontinuity is
strongly affected by an external magnetic field parallel to its boundary.
We observe various transient phenomena, including the anisotropic scattering
of electrons and the accompanying Weibel instability, and reveal various
spatial structures of the arising magnetic field and current, including
multiple flying apart filaments of a z-pinch type and slowly evolving current
sheets with different orientations. The magnitude of the self-generated
magnetic field can be of the order of or significantly exceed that of the
external one. Such phenomena are expected in the laser and cosmic plasmas,
including the explosive processes in the planetary magnetospheres and stellar
coronal arches.Comment: 13 pages, 6 figures, submitted to JP
Quantum electrodynamics of accelerated atoms in free space and in cavities
Journals published by the American Physical Society can be found at http://publish.aps.org/We consider a gedanken experiment with a beam of atoms in their ground state that are accelerated through a single-mode cavity. We show that taking into account of the "counterrotating" terms in the interaction Hamiltonian leads to the excitation of an atom with simultaneous emission of a photon into a field mode. In free space, when the atom-field interaction is turning on/off adiabatically, the only nonadiabatic effect that causes the excitation is the time-dependent Doppler shift. The resulting ratio of emission and absorption probabilities is exponentially small and is described by the Unruh factor. In the opposite case of rapid turn on of the interaction on the cavity boundaries the above ratio is much greater and radiation is produced with an intensity which can exceed the intensity of radiation in free space by many orders of magnitude. In both cases real photons are produced. The cavity field at steady state has a thermal density matrix. However, under some conditions laser gain is possible. We present a detailed discussion of how the acceleration of atoms affects the generated cavity field in different situations. We identify a common physical mechanism behind the Unruh effect and similar QED radiation processes
Enhancing Acceleration Radiation from Ground-State Atoms via Cavity Quantum Electrodynamics
When ground state atoms are accelerated through a high Q microwave cavity,
radiation is produced with an intensity which can exceed the intensity of Unruh
acceleration radiation in free space by many orders of magnitude. The cavity
field at steady state is described by a thermal density matrix under most
conditions. However, under some conditions gain is possible, and when the atoms
are injected in a regular fashion, the radiation can be produced in a squeezed
state