RESUMO
It is well known that optics and classical mechanics are intimately related. One of the most important concepts in classical mechanics is that of a particle in a central potential that leads to the Newtonian description of the planetary dynamics. Within this, a relevant result is Kepler's second law that is related to the conservation of orbital angular momentum, one of the fundamental laws in physics. In this paper, we demonstrate that it is possible to find the conditions that allow us to state Kepler's second law for optical beams with orbital angular momentum by analyzing the streamlines of their energy flow. We find that the optical Kepler's law is satisfied only for cylindrical symmetric beams in contrast to the classical mechanics situation that is satisfied for the other conic geometries, namely, parabolic, elliptical and hyperbolic. We propose a novel approach to confirm our analytic results: we observe the propagation of the Arago's spot created by a beam with orbital angular momentum as a local "light-tracer" instead of looking at the propagation of the whole beam. The observed patterns fully agree with the prediction of our formalism.
RESUMO
In this work we present numerical results of the far field intensity distributions obtained for a Gaussian beam after crossing a thin nonlinear nonlocal material that exhibit nonlinear refraction and absorption. The distributions are obtained for different positions along the Z axis and different signs of the nonlinear absorption. The results demonstrate that the far field intensity patterns obtained for strong nonlocal media are more affected by the presence of the nonlinear absorption than weak nonlocal media.
RESUMO
In this paper, we demonstrate, numerically and experimentally that using the mask-lens setup used by Durnin to generate Bessel beams Durnin [Phys. Rev. Lett. 58, 1499 (1987)], it is possible to generate different kinds of propagation invariant beams. A modification in the amplitude or phase of the field that illuminates the annular slit is proposed that corresponds to modulation in frequency space. In particular, we characterize the new invariant beams that were obtained by modulating the amplitude of the annular mask and when the incident field was modulated with a one-dimensional quadratic or cubic phase. Experimental results using an amplitude mask are shown in order to corroborate the numerical predictions.
RESUMO
In this paper, the nonlinear refractive index of colloidal gold nanoparticles under continuous wave illumination is investigated with the z-scan technique. Gold nanoparticles were synthesized using ascorbic acid as reductant, phosphates as stabilizer and cetyltrimethylammonium chloride (CTAC) as surfactant agent. The nanoparticle size was controlled with the CTAC concentration. Experiments changing incident power and sample concentration were done. The experimental z-scan results were fitted with three models: thermal lens, aberrant thermal lens and the nonlocal model. It is shown that the nonlocal model reproduces with exceptionally good agreement; the obtained experimental behaviour.
RESUMO
We explore the polarization dependence of the nonlinear response of a planar nematic liquid crystal cell doped with 1% wt of methyl red dye. The results obtained show that the refractive index change can be switched from a positive value to a negative one as the polarization of the beam changes from parallel to perpendicular with respect to the rubbing direction. This property is exploited in a phase contrast system, where a dynamic phase filter is photoinduced in a liquid crystal cell placed in the system's Fourier plane. Real-time contrast inversion in the resulting images is demonstrated.
RESUMO
Based on the separability of the Helmholtz equation into elliptical cylindrical coordinates, we present another class of invariant optical fields that may have a highly localized distribution along one of the transverse directions and a sharply peaked quasi-periodic structure along the other. These fields are described by the radial and angular Mathieu functions. We identify the corresponding function in the McCutchen sphere that produces this kind of beam and propose an experimental setup for the realization of an invariant optical field.
RESUMO
We propagate two coherent and parallel beams of a He-Ne laser through a Bi(12)TiO(20) photorefractive crystal in the presence of drift nonlinearity. Our experimental results demonstrate that the beams attract or repel each other according to their initial phase difference. They attract each other when they are initially in phase and they repel each other when they are initially out of phase. These experimental results agree with numerical predictions recently published.
RESUMO
We present experimental results on the propagation of an interference pattern of two He-Ne laser beams of unequal amplitudes through a photorefractive Bi(12)TiO(20) crystal in the presence of drift nonlinearity. The phenomenon that we have observed is the focusing of the fringes as the nonlinearity of the crystal is increased. We show that such a phenomenon can be quantitatively interpreted in the framework of modulation instability theory.
RESUMO
We consider the operation of an active Talbot spatial filter in semiconductor laser arrays. High linearity of filtering can be achieved in lasers with separate driving systems for the array and filter regions by the use of a constant driving current in the filter region. If the regions are pumped equally, then lasers with high threshold current will show nonlinear light-current characteristics.