911 research outputs found
Surface plasmon polaritons and surface phonon polaritons on metallic and semiconducting spheres: Exact and semiclassical descriptions
We study the interaction of an electromagnetic field with a non-absorbing or
absorbing dispersive sphere in the framework of complex angular momentum
techniques. We assume that the dielectric function of the sphere presents a
Drude-like behavior or an ionic crystal behavior modelling metallic and
semiconducting materials. We more particularly emphasize and interpret the
modifications induced in the resonance spectrum by absorption. We prove that
"resonant surface polariton modes" are generated by a unique surface wave,
i.e., a surface (plasmon or phonon) polariton, propagating close to the sphere
surface. This surface polariton corresponds to a particular Regge pole of the
electric part (TM) of the S matrix of the sphere. From the associated Regge
trajectory we can construct semiclassically the spectrum of the complex
frequencies of the resonant surface polariton modes which can be considered as
Breit-Wigner-type resonances. Furthermore, by taking into account the Stokes
phenomenon, we derive an asymptotic expression for the position in the complex
angular momentum plane of the surface polariton Regge pole. We then describe
semiclassically the surface polariton and provide analytical expressions for
its dispersion relation and its damping in the non-absorbing and absorbing
cases. In these analytic expressions, we more particularly exhibit
well-isolated terms directly linked to absorption. Finally, we explain why the
photon-sphere system can be considered as an artificial atom (a ``plasmonic
atom" or "phononic atom") and we briefly discuss the implication of our results
in the context of the Casimir effect.Comment: v2: Typos corrected; v3: Paper extended to absorbing media,
references added and title change
Entrainment, motion and deposition of coarse particles transported by water over a sloping mobile bed
In gravel-bed rivers, bedload transport exhibits considerable variability in
time and space. Recently, stochastic bedload transport theories have been
developed to address the mechanisms and effects of bedload transport
fluctuations. Stochastic models involve parameters such as particle
diffusivity, entrainment and deposition rates. The lack of hard information on
how these parameters vary with flow conditions is a clear impediment to their
application to real-world scenarios. In this paper, we determined the closure
equations for the above parameters from laboratory experiments. We focused on
shallow supercritical flow on a sloping mobile bed in straight channels, a
setting that was representative of flow conditions in mountain rivers.
Experiments were run at low sediment transport rates under steady nonuniform
flow conditions (i.e., the water discharge was kept constant, but bedforms
developed and migrated upstream, making flow nonuniform). Using image
processing, we reconstructed particle paths to deduce the particle velocity and
its probability distribution, particle diffusivity, and rates of deposition and
entrainment. We found that on average, particle acceleration, velocity and
deposition rate were responsive to local flow conditions, whereas entrainment
rate depended strongly on local bed activity. Particle diffusivity varied
linearly with the depth-averaged flow velocity. The empirical probability
distribution of particle velocity was well approximated by a Gaussian
distribution when all particle positions were considered together. In contrast,
the particles located in close vicinity to the bed had exponentially
distributed velocities. Our experimental results provide closure equations for
stochastic or deterministic bedload transport models.Comment: Submitted to Journal of Geophysical Researc
Are there "dragon-kings” events (i.e. genuine outliers) among extreme avalanches?
Predicting the occurrence and spatial extent of extreme avalanches is a longstanding issue. Using field data pooled from various sites within the same mountain range, authors showed that the avalanche size distribution can be described using either an extreme value distribution or a thick-tailed distribution, which implies that although they are much larger than common avalanches, extreme avalanches belong to the same population of events as "small” avalanches. Yet, when looking at historical records of catastrophic avalanches, archives reveal that a few avalanches had features that made them "extra-ordinary.” Applying avalanche-dynamics or statistical models to simulate these past events runs into considerable difficulty since the model parameters or the statical properties are very different from the values usually set to model extreme avalanches. Were these events genuine outliers (also called "dragon-kings”)? What were their distinctive features? This paper reviews some of the concepts in use to model extreme events, gives examples of processes that were at play in extreme avalanches, and shows that the concept of dragon-king avalanches is of particular relevance to describing some extreme avalanche
Surface polaritons on metallic and semiconducting cylinders: A complex angular momentum analysis
We revisit scattering of electromagnetic waves from metallic and
semiconducting cylinders in the framework of complex angular momentum
techniques. We prove that "resonant surface polariton modes" are generated by a
unique surface wave, i.e. a surface polariton, propagating close to the
cylinder surface. This surface polariton corresponds to a particular Regge pole
of the -matrix of the cylinder. From the associated Regge trajectory we can
construct semiclassically the spectrum of the complex frequencies of the
resonant surface polariton modes which can be considered as Breit-Wigner-type
resonances. Furthermore, by taking into account Stokes' phenomenon, we derive
an asymptotic expression for the position in the complex angular momentum plane
of the surface polariton Regge pole. We then describe semiclassically the
surface polariton and provide analytical expressions for its the dispersion
relation and its damping. All these features allow us to consider the
photon-cylinder system as a kind of artificial atom where the photon plays the
role of the electron. Finally, we briefly discuss the implication of our
results for two-dimensional photonic crystals
- …