<p>A setup based on the differential interference contrast imaging technique is implemented by replacing the linearly polarized beams from the conventional technique with spatially inhomogeneous polarized beams. This setup works in a reflective mode for material discrimination and for surface measurement, exploiting the information delivered by an analysis of the polarization state. A genetic algorithm that considers Malus’s law adjusts the collected data from a flat reference, and the resulting model is applied to the testing objects. Our study shows that, under the same setup, spatially inhomogeneous polarized beams offer a better height and composite material discrimination, in comparison to the use of linearly polarized beams. We used, as testing objects, reflective composite materials and highly reflective surface structures that have lateral dimensions up to 8 mm and depth variations from 50 μm to 3 mm. These surfaces can be related to applications in the semiconductor and metallic materials industry.</p>
In this work, we present the implementation of an experimental setup for controlling the phase gradient of arbitrary light beams, using a spatial light modulator. Simple arbitrary shapes are initially proposed based on their parametric equations, and the desired beam shape, as well as its phase behavior are interpreted through an algorithm. The analysis of the electric field distribution and its manipulation through the topological charge and the normal direction respect every position of an arbitrary shape, allow to encode in a Spatial Light Modulator the behavior of the phase of a light beam. The far field intensity profiles are captured, studied and compared to those designed. The phase of a set of generated beams is tested using a linear polarizer as an analyzer and by an optical trapping setup.
We present the methodology for the surface measurement of small testing objects with maximum depth variation of 3mm, using a polarimetric approach. The experimental setup is based on the function of a Differential Interference Contrast microscope, which works as a shearing interferometer. As it is expected, when it comes to an application for non-microscopic samples, certain modifications should be considered on the development of the measurement system. This work focuses on such details that lead to the profiling of a testing object of known dimensions. An algorithm that computes height distribution based on the polarimetric data is implemented and the resulting surface profile is analyzed. Finally, our conclusions about the requirements for an improvement of the presented measurement setup are listed.
We report on the observation of a normal streak effect on hollow micron sized spheres when illuminated by a focused Gaussian beam in a conventional optical tweezers setup. The hollow microspheres suspended in water can be optically trapped at the center of the illuminating beam. When the microsphere is illuminated off center, an emerging beam approximately perpendicular respect to the incoming beam is generated. This effect due to total internal reflections has been observed in microspheres with different external diameters, ranging from 5-20 microns. The generated normal beam is used to either pull or push other particles or objects around the microsphere or to remove particles stuck to the sphere due to radiation pressure.
Using a scanning near-field optical microscope with a metallic probe tip (Fig 1), we investigate the formation of near-field optical images. The scatter-probe is used only for converting an evanescent field to a propagating field and the detection system is in the far-field. This situation models the usual experimental set up employed in scatter-probe near-field microscopy. The calculations of the scattered intensity at constant height were based on an integral equation, method of moments approach.
Using a scanning near-field optical microscope with a metallic probe tip, we investigate the formation of near-field optical images. The scatter-probe is used only for converting an evanescent field to a propagating field and the detection system is in the far-field. This situation models the usual experimental set up employed in scatter-probe near-field microscopy. We study a 2D model of the scattering of s-polarized light, in which the object is illuminated by total internal reflection. The calculations of the scattered intensity at constant height were based on an integral equation, method of moments approach.
Following our previous study of inverse scattering with one-dimensional perfectly conducting surfaces, we consider the applicability of the proposed inverse scattering algorithm to the case of surfaces of more general materials. In particular, results corresponding to glass and silver surfaces are presented. The algorithm, based on wavefront matching principles, is used to reconstruct one-dimensional surface profiles from far-field amplitude scattering data calculated using rigorous techniques. The study is complemented by considerations of the tolerance of the algorithm to noise in the data.
An inversion algorithm for the reconstruction of surface profiles from far-field amplitude scattering data is considered. The algorithm is based on a wavefront matching principle, and is related to interferometric profiling techniques. The performance of the algorithm is studied using rigorous numerical data for the field scattered by one- dimensional surfaces.
Multiple scattering and shape-related effects are an active and important field of research in the area of diffraction and scattering of electromagnetic waves by rough surfaces. Most of the rigorous numerical techniques for dealing with this problem were limited to the treatment of single-valued surfaces. We have extended the formulation of Mendoza- Suarez and Mendez (1997) for dealing with multi-valued profile functions in order to study the scattering of reentrant surfaces or cavities in both, the near and far-field. We have evaluated the near-field in circular cavities with narrow entrances, as well as in the case of clusters of rods or cylinders. Resonant frequencies are clearly identified for these structures. We have also found that our model could be useful to predict wave-induced oscillations in harbors of arbitrary geometry. This comes form the fact that the mathematical formulation of the problem of light scattering by cavities (in the case of p polarization) is similar to the one employed in the case of harbors of arbitrary shape, when a water wave arrives at its entrance (Hwang and Tuck, 1970; Lee, 1971). The results obtained with our model are in close agreement with previously reported theories and experimental data.
An extensive set of measurements of scintillation over a 17.55 km path have been made using point sources at wavelengths of 633 nm and 10.6 micrometers , and using an extended thermal source at 3 - 5 micrometers and 8 - 12 micrometers . The basic data consists of normalized variances, probability histograms and normalized autocorrelation functions of intensity. The main aim was to product a set of data that might be used as inputs to models for scintillation. The measurements, as expected, showed a very large range of observed fluctuations, with a highest recorded normalized variance at 633 nm of approximately equals 34 and an average value of 4.8 (averaged over 130 data sets), with a standard deviation of 4.1. The probability histograms have been fitted using log- normal, exponential, log-normally modulated exponential and K distributions. As a general rule, the log-normal model gives a good fit in a large number of cases. Power spectra and correlations functions were measured and show the expected trends with wavelength, with average correlation times (defined in the text) in the range 10 msec (visible) to 68 msec (CO<SUB>2</SUB>).
An experimental study of light scattering from gold-coated ground glass and chemically etched surfaces is presented. A substantial amount of energy was detected in the cross-polarized component of the light scattered by all our samples and, with the chemically etched surfaces, the phenomenon of enhanced backscattering was observed. The surfaces were characterized with a mechanical profilometer but the estimated statistics are inconsistent with the evidence presented by the light scattering experiments. We believe that this is due to a lack of fidelity of the profilometric measurements. A computer simulation of the effects of the non-vanishing stylus tip has also been conducted to get some insight into the causes of this discrepancy.
The scattering from surfaces whose profile has a center of symmetry is considered. It has been reported that the
diffuse component displays a very narrow peak in the specular direction. We show that if the portion of the surface
illuminated has a length L. the width of this peak is of order X I L .andthat the enhancement doubles the diffuse intensity
in the specular direction. We also show that, by phase-shifting one half of the pupil, it is possible to destroy the effect
and deplete the intensityin this direction. These effects are difficult to detect experimentally and, to alleviate this situation,
we have also considered the scattering from diffusers which are symmetric in sections. They have similar scattering
characteristics to those in the previously considered case, but the enhancement is easier to detect experimentally. Some
random surfaces with symmetries have been prepared, and we present experimental results to support our conclusions.