22 research outputs found
Smith-Purcell Radiation from Rough Surfaces
Radiation of a charged particle moving parallel to a inhomogeneous surface is
considered. Within a single formalism periodic and random gratings are
examined. For the periodically inhomogeneous surface we derive new expressions
for the dispersion relation and the spectral-angular intensity. In particular,
for a given observation direction two wavelengths are emitted instead of one
wavelength of the standard Smith-Purcell effect. For a rough surface we show
that the main contribution to the radiation intensity is given by surface
polaritons induced on the interface between two media. These polaritons are
multiply scattered on the roughness of surface and convert into real photons.
The spectral-angular intensity is calculated and its dependence on different
parameters is revealed.Comment: 6 pages, 2 figures, submitted to Phys. Rev. ST Accel. Beam
Bloch states in light transport through a perforated metal
Light transport in a metal with two-dimensional hole arrays is considered.
Analytical expression for a transmission coefficient in periodic, isolated and
disordered cases are derived, assuming the existence of waveguide modes
transverse tunneling in two-dimensional plane perpendicular to traveling
direction of light. The one dimensional case of periodic holes, due to its
simplicity, is investigated in detail. In the dilute metal regime, when metal
fraction is small, our numerical study of the transmission coefficient of
central diffracted wave indicates the existence of a minimum which is
completely independent of an incident wavelength. Further increasing of metal
fraction leads to the unusual monotonic increasing of central diffracted wave
transmission. The role of the surface plasmons is discussed.Comment: 14pages, 2figure
Spectrum of Radiation from Rough Surfaces
Radiation from a charged particle travelling parallel to a rough surface has
been considered. Spectral-angular intensity is calculated in the weak
scattering regime. It is shown that the main contribution to the radiation
intensity is determined by the multiple scattering of polaritons induced by a
charge on the surface. Multiple scattering effects lead to strong frequency
dependence of radiation intensity. Possible applications in beam and surface
diagnostics are discussed.Comment: submitted to Europhys.Let
Observation of Resonant Diffusive Radiation in Random Multilayered Systems
Diffusive Radiation is a new type of radiation predicted to occur in randomly
inhomogeneous media due to the multiple scattering of pseudophotons. This
theoretical effect is now observed experimentally. The radiation is generated
by the passage of electrons of energy 200KeV-2.2MeV through a random stack of
films in the visible light region. The radiation intensity increases resonantly
provided the Cherenkov condition is satisfied for the average dielectric
constant of the medium. The observed angular dependence and electron resonance
energy are in agreement with the theoretical predictions. These observations
open a road to application of diffusive radiation in particle detection,
astrophysics, soft X-ray generation and etc.. `Comment: 4pages, 4figure
Adhesion Induced DNA Naturation
DNA adsorption and naturation is modeled via two interacting flexible
homopolymers coupled to a solid surface. DNA denatures if the entropy gain for
unbinding the two strands overcomes the loss of binding energy. When adsorbed
to a surface, the entropy gain is smaller than in the bulk, leading to a
stronger binding and, upon neglecting self-avoidance, absence of a denatured
phase. Now consider conditions where the binding potentials are too weak for
naturation, and the surface potential too weak to adsorb single strands. In a
variational approach it is shown that their combined action may lead to a
naturated adsorbed phase. Conditions for the absence of naturation and
adsorption are derived too. The phase diagram is constructed qualitatively.Comment: 4 pages, 1 figur
Unzipping of DNA with correlated base-sequence
We consider force-induced unzipping transition for a heterogeneous DNA model
with a correlated base-sequence. Both finite-range and long-range correlated
situations are considered. It is shown that finite-range correlations increase
stability of DNA with respect to the external unzipping force. Due to
long-range correlations the number of unzipped base-pairs displays two widely
different scenarios depending on the details of the base-sequence: either there
is no unzipping phase-transition at all, or the transition is realized via a
sequence of jumps with magnitude comparable to the size of the system. Both
scenarios are different from the behavior of the average number of unzipped
base-pairs (non-self-averaging). The results can be relevant for explaining the
biological purpose of correlated structures in DNA.Comment: 22 pages, revtex4, 14 eps figures; reprinted in the June 15, 2004
issue of Virtual Journal of Biological Physics Researc