49 research outputs found
Generalized composition law from 2x2 matrices
Many results that are difficult can be found more easily by using a
generalization in the complex plane of Einstein's addition law of parallel
velocities. Such a generalization is a natural way to add quantities that are
limited to bounded values. We show how this generalization directly provides
phase factors such as the Wigner angle in special relativity and how this
generalization is connected in the simplest case with the composition of 2x2 S
matrices.Comment: Accepted for publication in Am. J. Phy
Composition law for polarizers
The polarization process when polarizers act on an optical field is studied.
We give examples for two kinds of polarizers. The first kind presents an
anisotropic absorption - as in a polaroid film - and the second one is based on
total reflection at the interface with a birefringent medium. Using the Stokes
vector representation, we determine explicitly the trajectories of the wave
light polarization during the polarization process. We find that such
trajectories are not always geodesics of the Poincar\'e sphere as it is usually
thought. Using the analogy between light polarization and special relativity,
we find that the action of successive polarizers on the light wave polarization
is equivalent to the action of a single resulting polarizer followed by a
rotation achieved for example by a device with optical activity. We find a
composition law for polarizers similar to the composition law for noncollinear
velocities in special relativity. We define an angle equivalent to the
relativistic Wigner angle which can be used to quantify the quality of two
composed polarizers.Comment: 23 pages, 9 figures, accepted for publication in Physical Review
Sending femtosecond pulses in circles: highly non-paraxial accelerating beams
We use caustic beam shaping on 100 fs pulses to experimentally generate
non-paraxial accelerating beams along a 60 degree circular arc, moving
laterally by 14 \mum over a 28 \mum propagation length. This is the highest
degree of transverse acceleration reported to our knowledge. Using diffraction
integral theory and numerical beam propagation simulations, we show that
circular acceleration trajectories represent a unique class of non-paraxial
diffraction-free beam profile which also preserves the femtosecond temporal
structure in the vicinity of the caustic
Arbitrary non-paraxial accelerating periodic beams and spherical shaping of light
We report the observation of arbitrary accelerating beams designed using a
non-paraxial description of optical caustics. We use a spatial light
modulator-based setup and techniques of Fourier optics to generate circular and
Weber beams subtending over 95 degrees of arc. Applying a complementary binary
mask also allows the generation of periodic accelerating beams taking the forms
of snake-like trajectories, and the application of a rotation to the caustic
allows the first experimental synthesis of optical accelerating beams upon the
surface of a sphere in three dimensions.Comment: 4 pages, 4 figures articl
EFFETTO DELLA DENSITĂ€ SULLA CONDIZIONE DI TRE SPECIE DI DECAPODI NATANTI NELLA LAGUNA DI VENEZIA
Quantitative samples of three species of decapod crustaceans were collected across different
seasons in three habitat types of the Venice lagoon, in order to evaluate how species density may affect the
individual condition. The three species showed a different spatial distribution, with significant differences
in density across lagoon habitats. A possible negative effect of density on individual weight was recorded
for all the species considere
High speed cleaving of crystals with ultrafast Bessel beams
International audienceWe develop a novel concept for ultra-high speed cleaving of crystalline materials with femtosecond lasers. Using Bessel beams in single shot, fracture planes can be induced nearly all along the Bessel zone in sapphire. For the first time, we show that only for a pulse duration below 650 fs, a single fracture can be induced in sapphire, while above this duration, cracks appear in all crystallographic orientations. We determine the influential parameters which are polarization direction, crystallographic axes and scanning direction. This is applied to cleave sapphire with a spacing as high as 25 ÎĽm between laser impacts
Void reactivity coefficient benchmark results for a 10x10 BWR assembly in the full 0-100% void fraction range
A boiling water reactor SVEA-96+ fresh fuel lattice has been used as the basis for a benchmark study of the void reactivity coefficient at assembly level in the full voidage range. Results have been obtained using the deterministic codes CASMO-4, HELIOS, PHOENIX, BOXER and the probabilistic code MCNP4C, combined for almost all cases with different cross section libraries. A statistical analysis of the results obtained showed that the void reactivity coefficient tends to become less negative beyond 80% void and that the discrepancies between codes tend to increase from less than 15% at voidages lower than 40% to more than 25% at voidages higher than 70%. The void reactivity coefficient results and the corresponding differences between codes were isotopically decomposed to interpret discrepancies. The isotopic decomposition shows that the minimum observed in the void reactivity coefficient between 80% and 90% void is largely due to the decrease in the relative importance of the 157Gd(n, ) rate with increasing voidage, and that the fundamental discrepancies between codes or libraries are mainly governed by the different predictions of the 238U(n, ) variation with voidage. [All rights reserved Elsevier]