A. Canillas

Effective dielectric function of mixtures of three or more materials: a numerical procedure for computations

A numerical procedure for solving the mathematical equations that define the effective medium for a mixture of materials is presented. The basic idea is to transform the inherent complex root-finding problem into a minimization task in real space so that the method can be implemented through a standard two-dimensional minimization algorithm. This leads to general, simple and useful procedures for ellipsometric and/or spectrophotometric data analysis in the optical characterization of materials and surfaces.

Emergence of Supramolecular Chirality by Flows

Hydrodynamic forces in stirred solutions induce chirality in some supramolecular species of J-aggregates, as detected at the level of the electronic transition. However, the mechanism that explains the phenomenon remains to be elucidated, although the basic effect of hydrodynamic gradients of the shear rate is most probably the folding or bending of the nanoparticles in solution. Herein, we demonstrate a correlation between chiral flows in different regions of circular and square stirred cuvettes and the emergence of true circular dichroism (CD). The results show that chaotic flows lead to a racemic mixture of chiral shaped supramolecular species, and vortical flows to scalemic mixtures. In a magnetically stirred flask the descending and ascending flows are of different chiral sign and the CD reading depends on the weighting of these two flows of inverse chiral sign. The effect of the gradient of shear rates of the flows leading to chiral shape objects depends on the shape of the cuvette, which suggests that the flask shape and the controlled addition of reagents in defined regions of the stirred solutions may exert a control in self-assembly processes.

On the Mechano‐Chiral Effect of Vortical Flows on the Dichroic Spectra of 5‐Phenyl‐10,15,20‐tris(4‐sulfonatophenyl)porphyrin J‐Aggregates

Phase-modulated ellipsometry of the J-aggregates of the title porphyrin shows that the material gives a true CD signal. This confirms that there is a real chiral transfer by mechanical forces, mediated by shear gradient flows, from the macroscopic to the electronic transition level. Dislocations in the structure of the aggregate could justify the formation of chirality at the level of the electronic transitions once the mesophases can be sculptured by hydrodynamic gradient flows.

Chiral sign selection on the J‐aggregates of diprotonated tetrakis‐(4‐sulfonatophenyl)porphyrin by traces of unidentified chiral contaminants present in the ultra‐pure water used as solvent

Traces of biological contaminants that cannot be detected, but are expected to be present, in ultra-pure water suffice to select the emerging chiral sign in the spontaneous mirror symmetry breaking that takes place during the formation of the J-aggregates of the amphiphilic diprotonated tetrakis-(4-sulfonatophenyl)porphyrin (H(4)TPPS(4)(2-)). This is demonstrated by competition experiments with a chiral cationic surfactant. The sensitivity of the detection depends on the hierarchical control of the H(4)TPPS(4)(2-) self-aggregation.

Ellipsometric study of a‐Si:H thin films deposited by square wave modulated rf glow discharge

Thin films of hydrogenated amorphous silicon (a‐Si:H), deposited by square wave modulated (SQWM) rf silane discharges, have been studied through spectroscopic and real time phase modulated ellipsometry. The SQMW films obtained at low mean rf power density (19 mW/cm2) have shown smaller surface roughness than those obtained in standard continuous wave (cw) rf discharges. At higher rf powers (≥56 mW/cm2), different behaviors depending on the modulating frequency have been observed. On the one hand, at low modulating frequencies (<40 Hz), the SQWM films have shown a significant increase of porosity and surface roughness as compared to cw samples. On the other, at higher modulating frequencies, the material density and roughness have been found to be similar in SQWM and cw films. Furthermore, the deposition rate of the films show more pronounced increases with the modulating frequency as the rf power is increased. Experimental results are discussed in terms of plasma negative charged species which can be rela...

Evidence of induced chirality in stirred solutions of supramolecular nanofibers

Two-modulator generalized ellipsometry is used to determine the spectroscopic Mueller matrix of a solution of porphyrin supramolecular aggregates that have fibrous form. During the measurements the solutions were stirred in clockwise and anticlockwise directions. The pseudopolar decompostion is applied to the experimental Mueller matrices to unveil the birefringent and dichroics properties of the sample. The vortex flow in the stirred solution is found to modify the optical response of the aggregates to polarized light, and, in particular, its chiral signature is determined by the stirring direction in a totally reversible process. The data found show that chirality can be induced by stirring in solutions of supramolecular fibers and that a effective transfer of chirality from a macroscopic phenomenon to the supramolecular structures takes place.

Effects of plasma processing on the microstructural properties of silicon powders

The effects of plasma processing conditions on the microstructural properties of silicon powders are presented. Hydrogenated nanophase silicon powders were prepared using low-pressure and low-temperature square wave modulated RF plasma (13.56 MHz) using pure silane gas. Plasma parameters such as pressure, RF power, plasma modulation frequency, and gas flow rate were varied. In situ analysis by quadrupolar mass spectroscopy and ex situ analysis of the silicon powders by Fourier transform infrared spectroscopy (FTIR) and thermal desorption spectrometry of hydrogen were performed. The thermal desorption spectrometry results show the fundamental differences between the concentrations of hydrogen weakly and strongly bonded in silicon powders as compared to amorphous silicon films. The FTIR analysis also determined the microstructural characteristics of powders and hence their volume/surface ratio. This parameter was determined from the balance of Pj probabilities of having one of the Hj-Si-Si4-j bond arrangements in the powder particles. These results reveal an increase in hydrogen content and a reduction in volume/surface ratio as the modulation frequency of RF power increases. In consequence, higher compactness of silicon powders is associated with long particle residence times inside the plasma as a result of ion bombardment. TEM analysis indicated a considerable dispersion of particle size and some degree of structure of the silicon powder characterized by intergrain linkage. We point out the dominant presence of hydrogen on the particle surfaces (external voids), which may cause the high reactivity of grains, increasing the degree of intergrain linkage.

Mueller matrix microscope with a dual continuous rotating compensator setup and digital demodulation

In this paper we describe a new Mueller matrix (MM) microscope that generalizes and makes quantitative the polarized light microscopy technique. In this instrument all the elements of the MU are simultaneously determined from the analysis in the frequency domain of the time-dependent intensity of the light beam at every pixel of the camera. The variations in intensity are created by the two compensators continuously rotating at different angular frequencies. A typical measurement is completed in a little over one minute and it can be applied to any visible wavelength. Some examples are presented to demonstrate the capabilities of the instrument.

Pseudopolar decomposition of the Jones and Mueller-Jones exponential polarization matrices

We propose a new algorithm, the pseudopolar decomposition, to decompose a Jones or a Mueller-Jones matrix into a sequence of matrix factors: J congruent withJ(R)J(D)J(1C)J(2C) or M congruent withM(R)M(D)M(1C)M(2C). The matrices J(R)(M(R)) and J(D)(M(D)) parameterize, respectively, the retardation and dichroic properties of J(M) in a good approximation, while J(iC)(M(iC)) are correction factors that arise from the noncommutativity of the polarization properties. The exponential versions of the general Jones matrix are used to demonstrate the pseudopolar decomposition and to calculate each one of the matrix factors. The decomposition preserves all the polarization properties of the system on the factorized J(R)(M(R)) and J(D)(M(D)) matrix terms. The algorithm that calculates the pseudopolar decomposition for experimentally determined Mueller matrices is presented.

Reversible Mechanical Induction of Optical Activity in Solutions of Soft‐Matter Nanophases

Nanophases of J-aggregates of several achiral amphiphilic porphyrins, which have thin long acicular shapes (nanoribbons), show the immediate and reversible formation of a stationary mechano-chiral state in the solution by vortex stirring, as detected by their circular dichroic signals measured by 2-modulator generalized ellipsometry. The results suggest that when a macroscopic chiral force creates supramolecular chirality, it also creates an enantiomeric excess of screw distortions, which may be detected by their excitonic absorption. An explanation on the effect of the shear flow gradients is proposed on the basis of the orientation of the rotating particles in the vortex and the size, shape, and mechanical properties of the nanoparticles.