Optical performance of a versatile illumination system for high divergence LED sources

An efficient flat-top illuminating optical system optimized for an extended light source is presented. The source is a high-brightness high divergence light emitting diode (LED), sized 1 mm × 1 mm, producing monochromatic emission (525 ± 5 nm) with viewing angle of 130°. The design is based on a rotationally symmetrical catadioptric system, developed on a geometrical optics basis, and modelled with ZEMAX^® software. The device consists of two optical systems: (i) a collimating system which, in turn, is formed by an aspheric lenses system (low numerical apertures, NA < 0.26) and two-mirror system (0.26 < NA < 0.86), and (ii) an external mirror (NA > 0.86) designed and optimized for each purpose. By itself, the collimating system works with a residual divergence of θ_C = 1.46°. The external mirror can be adequately designed to produce some given conditions. For instance, a flat-top profile is obtained in the selected focusing plane, with a maximum transversal intensity variation of 2.5% over 18 mm. In addition, when the focusing mirror is allowed to move along the optical axis in a ±1 mm range, other interesting profiles can be reached for a given working distance, therefore increasing the versatility of the system.     

Application of the polar decomposition to light scattering particle systems

We have applied the polar decomposition theorem (PDT) to the scattering matrix of simple systems such as an isolated sphere, either metallic or dielectric. The discrete dipole approximation (DDA) method has been used as an intermediate tool to calculate these matrices. We show how the parameters introduced by the PDT for describing the scattering by a single sphere can be easily interpreted. It is also shown that the PDT provides us with an alternative frame to conventional Mueller matrix analysis, by characterizing the systems by means of a group of independent parameters representing magnitudes of simple (virtual) elements. This step is necessary in order to translate other complex system matrices into manageable information.     

Monitoring small defects on surface microstructures through backscattering measurements

The detection and characterization of small defects on particles or structures of a size comparable with the incident wavelength and located on flat substrates is numerically analyzed. For metallic scattering systems, calculations based on the extinction theorem of physical optics show that geometrical information about a small Gaussian defect can be obtained by partially integrating the backscattering patterns.     

Plasmon spectroscopy of metallic nanoparticles above flat dielectric substrates

We numerically analyze the spectral properties of localized plasmon resonances in metal nanoparticles when these are above a dielectric substrate. This analysis is performed as a function of the various parameters involved in the problem (relative optical properties, particle-substrate separation, angle of incidence, etc.). It can be shown that from the spectral behavior of the resonance in the far field, information about particle near-field interactions can be obtained.     

Molecular basis of carbon dioxide transport in polycarbonate membranes

Gas transport of carbon dioxide in poly[bisphenol A carbonate-co-4,4'-(3,3,5-trimethylcyclohexylidene)diphenol carbonate] films over a wide range of pressure is described. The interpretation of the experimental results in terms of the dual mode model allowed the evaluation of the parameters of the model that govern the gas permeation process. The value of the diffusion coefficient obtained for carbon dioxide at zero concentration was 2.4 x 10(-8) cm(2) s(-1), at 303 K. This parameter was also measured by using pulsed field gradient NMR finding that its value reaches a nearly constant value of (2.7 +/- 0.9) x 10(-8) cm(2) s(-1), at 298 K, for diffusion times greater than 20 ms. Both the diffusion and solubility coefficients were also computed by using simulation methods based on the transition states theory and the Widom method, respectively. The value obtained for the diffusion coefficient was 1.8 x 10(-8) cm(2) s(-1), at 303 K, which compares very favorably with the experimental measurements. The drop of the simulated solubility coefficient with increasing pressure is sharper than that of the experimental one, at low pressures, and similar, at high pressures.     

Theoretical analysis of conformational energies,unperturbed dimensions,and dipole moments of poly(dichlorophosphazene)

A theoretical analysis of the conformational energies of poly(dichlorophosphazene) (PDCP) is presented. The results indicate that the bond pair P? N? P possesses a considerable conformational freedom, whereas the bond pair N? P? N is relatively rigid. This difference explains the low glass transition temperatures T_g and large end-to-end distances measured for polyphosphazenes. A statistical model containing four rotational isomers (ie., trans, gauche, cis, and negative gauche) is developed and used to calculate unperturbed dimensions and dipole moments of PDCP. The results, obtained at 25°C with n = 400 skeletal bonds (200 repeating units), were C_n = 〈r²〉₀/nl² = 13.5; C_T = 10³d(ln〈r²〉₀)/dT = ?3.0 K^?1; D_n = 〈μ²〉/nμ = 0.35; D_T = 10³d(ln〈μ²〉)/dT = ?3.4 K^?1. All the calculated magnitudes are extremely sensitive to the energy E_σ that controls the statistical weights of the conformations tg, tc, tg^?, gt, ct, and g^?t relative to tt for the bond pair P? N? P. A qualitative explanation for this sensitivity is discussed.     

Multiscattering effects in the far-field region for two small particles on a flat conducting substrate

Abstract

The scattered field in the far-field region for two small metallic particles on a conducting substrate is analysed as a function of both their separation and the angle of incidence. Special attention is paid to multiple scattering, which appears when the particles are very close, as well as to its related effects such as its influence on the enhanced backscattering phenomenon and depolarization of the incident beam in the plane of incidence.