Control of the diffracted response of a metallic wire array with double period: experimental demonstration.

In recent papers, it has been theoretically shown that by using dual-period wire gratings, it is possible to control the relative efficiencies of the diffracted orders, regardless of the wires' material, incident polarization and wavelength. In this Letter, we experimentally demonstrate, for the first time, that by appropriately choosing the geometrical parameters of a nanometric periodic structure, it is possible to control the optical response in the visible range. We show examples of nanostructures designed to cancel out or to intensify a particular diffraction order. Such nanostructures allow a broad control over the directionality and the intensity of the diffracted light, which makes them useful for applications such as highly directional optical nanoantennas and photonic multiplexers.

Optical analysis of the fine crystalline structure of artificial opal films.

Herein, we present a detailed analysis of the structure of artificial opal films. We demonstrate that, rather than the generally assumed face centered cubic lattice of spheres, opal films are better approximated by rhombohedral assemblies of distorted colloids. Detailed analysis of the optical response in a very wide spectral range (0.4 < or = a/lambda < or = 2, where a is the conventional lattice constant), as well as at perpendicular and off-normal directions, unambiguously shows that the interparticle distance coincides very approximately with the expected diameter only along directions contained in the same close-packed plane but differs significantly in directions oblique to the [111] one. A full description of the real and reciprocal lattices of actual opal films is provided, as well as of the photonic band structure of the proposed arrangement. The implications of this distortion in the optical response of the lattice are discussed.

Core and shell sizing of small silver-coated nanospheres by optical extinction spectroscopy.

Silver metal nanoparticles (Nps) are extensively used in different areas of research and technology due to their interesting optical, thermal and electric properties, especially for bare core and core-shell nanostructures with sizes smaller than 10 nm. Since these properties are core-shell size-dependent, size measurement is important in manipulating their potential functionalization and applications. Bare and coated small silver Nps fabricated by physical and chemical methods present specific characteristics in their extinction spectra that are potentially useful for sizing purposes. This work presents a novel procedure to size mean core radius smaller than 10 nm and mean shell thickness of silver core-shell Nps based on a comparative study of the characteristics in their optical extinction spectra in different media as a function of core radii, shell thickness and coating refractive index. From the regularities derived from these relationships, it can be concluded that plasmon full width at half-maximum (FWHM) is sensitive to core size but not to coating thickness, while plasmon resonance wavelength (PRW) is related to shell thickness and mostly independent of core radius. These facts, which allow sizing simultaneously both mean core radius and shell thickness, can also be used to size bare silver Nps as a special case of core-shell Nps with zero shell thickness. The proposed method was applied to size experimental samples and the results show good agreement with conventional TEM microscopy.

Excitation mechanism for the Lambda doubling in the 0-0 UV laser band of N(2).

The gain of the Lambda-doublet terms in the 0-0 UV laser band of N(2) is calculated through an excitation model assuming low energy electron impact with ground state molecules. The results are used to explain the observation of the A doubling for axial and transverse discharge lasers.

Cascade population mechanism in nitrogen lasers.

The way in which a cascade mechanism from the C3Pi(u) state affects the population of the v = 0 level of the B3Pi(g) state in a N2 laser is studied. The spectra of the 0-1 IR laser band are fit by using simple theoretical calculations that account for the population of the three laser levels involved, C3Pi(u) B3Pi(g) and A3Sigma(u)+ and two excitation mechanisms, direct electron impact and cascade. The inhibition of the 0-0 UV laser band showed a strong influence on the 0-1 IR laser band output.