25 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
Coherent radiation from neutral molecules moving above a grating
We predict and study the quantum-electrodynamical effect of parametric
self-induced excitation of a molecule moving above the dielectric or conducting
medium with periodic grating. In this case the radiation reaction force
modulates the molecular transition frequency which results in a parametric
instability of dipole oscillations even from the level of quantum or thermal
fluctuations. The present mechanism of instability of electrically neutral
molecules is different from that of the well-known Smith-Purcell and transition
radiation in which a moving charge and its oscillating image create an
oscillating dipole.
We show that parametrically excited molecular bunches can produce an easily
detectable coherent radiation flux of up to a microwatt.Comment: 4 page
Anomalous Statistics of Bose-Einstein Condensate in an Interacting Gas: An Effect of the Trap’s Form and Boundary Conditions in the Thermodynamic Limit
We analytically calculate the statistics of Bose-Einstein condensate (BEC) fluctuations in an interacting gas trapped in a three-dimensional cubic or rectangular box with the Dirichlet, fused or periodic boundary conditions within the mean-field Bogoliubov and Thomas-Fermi approximations. We study a mesoscopic system of a finite number of trapped particles and its thermodynamic limit. We find that the BEC fluctuations, first, are anomalously large and non-Gaussian and, second, depend on the trap’s form and boundary conditions. Remarkably, these effects persist with increasing interparticle interaction and even in the thermodynamic limit—only the mean BEC occupation, not BEC fluctuations, becomes independent on the trap’s form and boundary conditions