Biomedical subjects

Augusto García-Valenzuela

Publications and source records attributed to Augusto García-Valenzuela.

5 recordsLinked to original sources

PubMed2006

Coherent optical reflectance from a monolayer of large particles adsorbed on a glass surface.

We develop a coherent-scattering model for the reflection of light from a monolayer of large particles and low surface coverage. The model takes into account multiple scattering between particles of the monolayer and with the substrate, and it can be used around the critical angle in an internal reflection configuration. We compare the results of the model with our own reflectivity data taken with latex particles adsorbed on a glass-water interface and with a simpler effective-medium model.

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PubMed2004

Monitoring particle adsorption by use of laser reflectometry near the critical angle.

We investigate the use of laser reflectometry near the critical angle to monitor particle adsorption onto a flat glass surface. Experimental results show that positive particles are adsorbed onto the glass surface and that their adsorption kinetics depend strongly on the volume fraction occupied by the particles in suspension but not appreciably on the particle size. The reflectance near the critical angle is dominated by the particles on the surface, with the contribution of the particles in suspension being very low. We compare the reflectance change near the critical angle with the change in reflectance near the Brewster angle when particles are adsorbed onto the glass surface. We find that reflectometry near the critical angle is 3000 times more sensitive than it is near the Brewster angle. Some optical images are presented to validate our results.

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PubMed2004

High-resolution optical angle sensors: approaching the diffraction limit to the sensitivity.

We carry out a detailed analysis of angle-sensitive devices based on the critical-angle effect. We consider their use in measuring small angular deflections of a laser beam. We establish the diffraction limit to the sensitivity for optical-angle sensors based on reflection and transmission of a laser beam. We find that this limit is identical to that of the triangulation scheme when using a position-sensitive detector or the autocollimation scheme. We analyze the main proposals to date of optical-angle sensors based on the critical-angle effect, focusing on their maximum sensitivity and their polarization dependence in practical conditions. We propose and analyze theoretically a novel and simple angle-sensitive device for sensing optical-beam deflections with very low polarization dependence and a maximum sensitivity close to the diffraction limit when used with typical laser beams. We discuss the basic principles for designing this type of device, provide numerical results, and point out a convenient fabrication procedure.

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PubMed2004

Technique for referencing of fiber-optic intensity-modulated sensors by use of counterpropagating signals.

We present a new all-optical fiber-referencing scheme for intensity-modulated sensors. It consists of a closed loop traversed by sensing and reference optical signals in opposite directions. With the proposed scheme the noise induced by power fluctuations of the optical source and mechanical perturbations can be greatly reduced. We experimentally demonstrate the efficiency of the scheme and discuss its use in a sensor array.

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PubMed2003

Coherent reflectance in a system of random Mie scatterers and its relation to the effective-medium approach.

We consider the coherent reflection and transmission of electromagnetic waves from a slab of a dilute system of randomly located, polarizable, spherical particles. We focus our attention on the case where the size of the spheres is comparable to the wavelength of the incident radiation. First, using wave-scattering and Mie theories, we derive expressions for the coherent fields that are transmitted and reflected by a very thin slab. Then we find the effective-current distribution that would act as a source of these fields. We conclude that if the effective currents were induced in an effective medium, this medium must possess, besides an effective electric permittivity, also an effective magnetic permeability. We find that both of these optical coefficients become functions of the angle of incidence and the polarization of the incident wave. Then we calculate the reflection coefficient of a half-space by considering a semi-infinite pile of thin slabs and compare the result with Fresnel relations. Numerical results are presented for the optical coefficients as well as for the half-space reflectance as a function of several parameters. The reflectance is compared with that obtained without considering the magnetic response. Finally, we discuss the relevance and the physics behind our results and indicate as well the measurements that could be performed to obtain an experimental verification of our theory.

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