1,615 research outputs found
Scattering of accelerated wave packets
Wave-packet scattering from a stationary potential is significantly modified
when the wave-packet is subject to an external time-dependent force during the
interaction. In the semiclassical limit, wave--packet motion is simply
described by Newtonian equations and the external force can, for example,
cancel the potential force making a potential barrier transparent. Here we
consider wave-packet scattering from reflectionless potentials, where in
general the potential becomes reflective when probed by an accelerated
wave-packet. In the particular case of the recently-introduced class of complex
Kramers-Kronig potentials we show that a broad class of time dependent forces
can be applied without inducing any scattering, while there is a breakdown of
the reflectionless property when there is a broadband distribution of initial
particle momentum, involving both positive and negative components.Comment: 13 pages, 4 figures, to appear in Phys. Rev.
A folding inhibitor of the HIV-1 Protease
Being the HIV-1 Protease (HIV-1-PR) an essential enzyme in the viral life
cycle, its inhibition can control AIDS. The folding of single domain proteins,
like each of the monomers forming the HIV-1-PR homodimer, is controlled by
local elementary structures (LES, folding units stabilized by strongly
interacting, highly conserved, as a rule hydrophobic, amino acids). These LES
have evolved over myriad of generations to recognize and strongly attract each
other, so as to make the protein fold fast and be stable in its native
conformation. Consequently, peptides displaying a sequence identical to those
segments of the monomers associated with LES are expected to act as competitive
inhibitors and thus destabilize the native structure of the enzyme. These
inhibitors are unlikely to lead to escape mutants as they bind to the protease
monomers through highly conserved amino acids which play an essential role in
the folding process. The properties of one of the most promising inhibitors of
the folding of the HIV-1-PR monomers found among these peptides is demonstrated
with the help of spectrophotometric assays and CD spectroscopy
Observation of surface states with algebraic localization
We introduce and experimentally demonstrate a class of surface bound states
with algebraic decay in a one-dimensional tight-binding lattice. Such states
have an energy embedded in the spectrum of scattered states and are
structurally stable against perturbations of lattice parameters. Experimental
demonstration of surface states with algebraic localization is presented in an
array of evanescently-coupled optical waveguides with tailored coupling rates.Comment: revised version with Supplemental Material, to appear in Phys. Rev.
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Nonlocal reflection by photonic barriers
The time behaviour of microwaves undergoing partial reflection by photonic
barriers was measured in the time and in the frequency domain. It was observed
that unlike the duration of partial reflection by dielectric layers, the
measured reflection duration of barriers is independent of their length. The
experimental results point to a nonlocal behaviour of evanescent modes at least
over a distance of some ten wavelengths. Evanescent modes correspond to
photonic tunnelling in quantum mechanics.Comment: 8 pages, 5 figure
Optical realization of the two-site Bose-Hubbard model in waveguide lattices
A classical realization of the two-site Bose-Hubbard Hamiltonian, based on
light transport in engineered optical waveguide lattices, is theoretically
proposed. The optical lattice enables a direct visualization of the
Bose-Hubbard dynamics in Fock space.Comment: to be published, J Phys. B (Fast Track Communication
Nonlinearity-induced broadening of resonances in dynamically modulated couplers
We report the observation of nonlinearity-induced broadening of resonances in
dynamically modulated directional couplers. When the refractive index of the
guiding channels in the coupler is harmonically modulated along the propagation
direction and out-of-phase in two channels, coupling can be completely
inhibited at resonant modulation frequencies. We observe that nonlinearity
broadens such resonances and that localization can be achieved even in detuned
systems at power levels well below those required in unmodulated couplers.Comment: 14 pages, 4 figures, to appear in Optics Letter
Stability of the self-phase-locked pump-enhanced singly resonant parametric oscillator
Steady-state and dynamics of the self-phase-locked (3\omega ==> 2\omega,
\omega) subharmonic optical parametric oscillator are analyzed in the
pump-and-signal resonant configuration, using an approximate analytical model
and a full propagation model. The upper branch solutions are found always
stable, regardless of the degree of pump enhancement. The domain of existence
of stationary states is found to critically depend on the phase-mismatch of the
competing second-harmonic process.Comment: LateX2e/RevteX4, 4 pages, 5 figures. Submitted to Phys. Rev. A
(accepted on Jan. 17, 2003
Multistable Pulse-like Solutions in a Parametrically Driven Ginzburg-Landau Equation
It is well known that pulse-like solutions of the cubic complex
Ginzburg-Landau equation are unstable but can be stabilised by the addition of
quintic terms. In this paper we explore an alternative mechanism where the role
of the stabilising agent is played by the parametric driver. Our analysis is
based on the numerical continuation of solutions in one of the parameters of
the Ginzburg-Landau equation (the diffusion coefficient ), starting from the
nonlinear Schr\"odinger limit (for which ). The continuation generates,
recursively, a sequence of coexisting stable solutions with increasing number
of humps. The sequence "converges" to a long pulse which can be interpreted as
a bound state of two fronts with opposite polarities.Comment: 13 pages, 6 figures; to appear in PR
Polarization coupling and pattern selection in a type-II optical parametric oscillator
We study the role of a direct intracavity polarization coupling in the
dynamics of transverse pattern formation in type-II optical parametric
oscillators. Transverse intensity patterns are predicted from a stability
analysis, numerically observed, and described in terms of amplitude equations.
Standing wave intensity patterns for the two polarization components of the
field arise from the nonlinear competition between two concentric rings of
unstable modes in the far field. Close to threshold a wavelength is selected
leading to standing waves with the same wavelength for the two polarization
components. Far from threshold the competition stabilizes patterns in which two
different wavelengths coexist.Comment: 14 figure
Spinning Particles on Spacelike Hypersurfaces and their Rest-Frame Description
A new spinning particle with a definite sign of the energy is defined on
spacelike hypersurfaces after a critical discussion of the standard spinning
particles. They are the pseudoclassical basis of the positive energy [or negative energy ] parts of the solutions of the Dirac equation. The study of the isolated system of N
such spinning charged particles plus the electromagnetic field leads to their
description in the rest-frame Wigner-covariant instant form of dynamics on the
Wigner hyperplanes orthogonal to the total 4-momentum of the isolated system
(when it is timelike). We find that on such hyperplanes these spinning
particles have a nonminimal coupling only of the type "spin-magnetic field"
like the nonrelativistic Pauli particles to which they tend in the
nonrelativistic limit. The Lienard-Wiechert potentials associated with these
charged spinning particles are found. Then, a comment on how to quantize the
spinning particles respecting their fibered structure describing the spin
structure is done.Comment: 70 pages, revte
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