120 research outputs found
Enhancement of the nonlinear optical absorption of the E7 liquid crystal at the nematic-isotropic transition
We present an experimental study of the nonlinear optical absorption of the
eutectic mixture E7 at the nematic-isotropic phase transition by the Z-scan
technique, under a continuos wave excitation at 532 nm. In the nematic region,
the effective nonlinear optical coefficient \beta for an extraordinary beam is
negative and for an ordinary beam is positive, being null in the isotropic
phase. The parameter S_{NL} defined in terms of the nonlinear absorption
coefficients in a similar way than the optical order parameter from the linear
dichoic ratio behavies like an order parameter with a critical exponent
0.22+-0.05, in good agreement with the tri-critical hipothesis of the N-I
transition.Comment: 11 pages, 6 figures, to be published in the vol 42 of the Brazilian
Jurnal of physic
Linear and Nonlinear Optical Properties of Aligned Elongated Silver Nanoparticles Embedded in Silica
Observaciones petrográficas y tectónicas sobre las ofitas del área de Aulet (Prepirineo meridional, prov. de Huesca)
Se describen las diversas facies petrográficas y se indica su distribución en dos de las masas ofÃticas del afloramiento de materiales triásicos de los alrededores de Aulet (Huesca). Se expresa su composición quimica que refleja elevada uniformidad y escasa diferenciación, mostrando analogÃa con los basaltos toleÃticos continentales. Se ratifica su edad (Keuper-LÃas) por el método óptico de la dispersión de la birrefringencia (Kd) y se adjunta una breve exposición de los fundamentos del método. Finalmente se analiza la fracturación y su comportamiento en relación con las deformaciones que afectan a la cobertera supratriásica
On the precision and accuracy of the acoustic birefringence technique for stress evaluation
This paper presents a numerical procedure for estimation of the precision and accuracy of the acoustic birefringence technique as used in the Instituto de Engenharia Nuclear (IEN) for evaluation of residual and applied stresses in structures and components, mainly of the nuclear power industry. This procedure shall be incorporated to the signal processing module of the ultrasonic system used at IEN’s Ultrasonic Laboratory to account in an automatic and systematic way for the uncertainties in the input data and their propagation throughout the calculations. The acoustic birefringence is generally defined from the speeds of two mutually
orthogonal volumetric waves of normal incidence, but when the use of a pulse-echo measurement system is feasible, the birefringence can be defined directly from the time-of-flight of the waves, since they travel the same physical space. The times-of-flight of the waves can thus be regarded as the primary variables of interest. They are estimated by coupling the mathematical techniques of cross correlation and data interpolation, whereas the material’s acoustoelastic constant is determined via a weighted linear regression. An Excel spreadsheet performs all calculations taking into account the uncertainties and the number of significant digits in the results. As an example of the procedure developed, the estimation of the precision and accuracy in the evaluation of the stresses acting in a beam under bending is presented. The analytical solution derived from the strength of materials theory was used as the reference value for accuracy estimation purpos
Observers and their notion of spacetime beyond special relativity
It is plausible that quantum gravity effects may lead us to a description of
Nature beyond the framework of special relativity. In this case, either the
relativity principle is broken or it is maintained. These two scenarios (a
violation or a deformation of special relativity) are very different, both
conceptually and phenomenologically. We discuss some of their implications on
the description of events for different observers and the notion of spacetime.Comment: 10 page
Causal Structure and Birefringence in Nonlinear Electrodynamics
We investigate the causal structure of general nonlinear electrodynamics and
determine which Lagrangians generate an effective metric conformal to
Minkowski. We also proof that there is only one analytic nonlinear
electrodynamics presenting no birefringence.Comment: 11 pages, no figure
- …