Biomedical subjects

E R Méndez

Publications and source records attributed to E R Méndez.

6 recordsLinked to original sources

PubMed2005

Interference of a pair of symmetric Collett-Wolf beams.

We present a study of the interference of light produced by a pair of mutually correlated Gaussian Schell-model sources. The spatial distributions of the fields produced by these sources are symmetric with respect to a plane through their common center and differ by a phase factor exp(i phi). When phi = 0, the resulting radiation is a beam with an intensity distribution that displays a narrow bright line at its center. When the sources can be regarded as Collett-Wolf sources, the resulting bright line diverges much more slowly than the beam itself. When phi = pi the radiated beam has an intensity distribution with a narrow dark line at its center. The theoretical results are supported by experimental results obtained by use of a modified Michelson interferometer and suggest that the interference of a pair of correlated Collett-Wolf beams can be used to produce a pseudo-nondiffracting beam.

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PubMed2005

Intermode light diffusion in multimode optical waveguides with rough surfaces.

A theoretical analysis of incoherent intermode light power diffusion in multimode dielectric waveguides with rough (corrugated) surfaces is presented. The correlation length a of the surface-profile variations is assumed to be sufficiently large (a less less than lambda/2pi) to permit light scattering into the outer space only from the modes close to the critical angles of propagation and yet sufficiently small (a less less than d, where d is the average width of the waveguide) to permit direct interaction between a given mode and a large number of neighboring ones. The cases of a one-dimensional (1D) slab waveguide and a two-dimensional cylindrical waveguide (optical fiber) are analyzed, and we find that in both cases the partial differential equations that govern the evolution of the angular light power profile propagating along the waveguide are 1D and of the diffusion type. However, whereas in the former case the effective conductivity coefficient proves to be linearly dependent on the transverse-mode wave number, in the latter one the linear dependence is for the effective diffusion coefficient. The theoretical predictions are in reasonable agreement with experimental results for the intermode power diffusion in multimode (700 x 700) optical fibers with etched surfaces. The characteristic length of dispersion of a narrow angular power profile evaluated from the correlation length and standard deviation of heights of the surface profile proved to be in good agreement with the experimentally observed changes in the output angular power profiles.

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PubMed2003

Two-dimensional random surfaces that act as circular diffusers.

We propose a method for designing a two-dimensional random Dirichlet surface that, when it is illuminated at normal incidence by a scalar plane wave, scatters the wave with a circularly symmetric distribution of intensity. The method is applied to the design of a surface that acts as a Lambertian diffuser. The method is tested by computer simulations, and a procedure for fabricating such surfaces on photoresist is described.

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PubMed2003

Enhanced specular peaks in diffuse light scattering from weakly rough metal surfaces.

We employ Monte Carlo techniques based on the reduced Rayleigh equations to study an enhanced specular peak that appears in the light scattered from weakly rough metal surfaces. This peak is not associated with the specular reflection but instead appears, with finite angular width, at the specular angle of the mean diffusely scattered intensity. As is the case with backscattering enhancement, the specular peak arises from the interference of contributions of multiple-scattering processes related to surface plasmon polariton excitation. We demonstrate that the specular peak is seen clearly for surface roughness that has a conventional Gaussian power spectrum. Further, we show that the peak appears more distinctly for roughness whose power spectrum has a new rectangular form, which is proposed here with the intent of better isolation of the scattering processes essential to the specular peak. Finally, for a pair of rough surfaces that have appropriately correlated surface roughness, it is found that the cross correlation of scattered amplitudes presents a well-isolated specular peak, which directly demonstrates the constructive interference that produces the effect.

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PubMed2001

Nonreciprocal effects in the double passage of light through a random diffuser and a birefringent crystal.

We present a theoretical and experimental study of the scattering of light by double passage through a system that consists of a strong diffuser, a piece of birefringent crystal, and a plane mirror. We show that this arrangement can produce not only enhanced backscattering and satellite peaks but also satellite dips in the angular distribution of the mean intensity. The experiments are in agreement with theoretical results based on scalar diffraction theory in the paraxial approximation.

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PubMedDate not supplied

Polarization properties of the near-field intensity reflected by metallic and dielectric one-dimensional structures.

We investigate the state of polarization and near-field intensity distribution in the vicinity of rectangular groove objects ruled on metallic and dielectric materials. The sample is illuminated from the vacuum side by a linear combination of p- and s-polarised waves. Two rigorous methods of solution are used and compared in calculations of the total intensity at constant height when the light is incident normally onto the surface. Some calculations of the total intensity in the 'follow-the-profile mode' are also presented. It is shown that in the constant height mode, the contrast in the image can be reversed as the plane of observation moves away from the mean plane of the sample. We also found that the state of polarization depends strongly on the material and the distance to the plane of detection.

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