MATERIALS SCIENCE AND APPLIED PHYSICS: 2nd Mexican Meeting on Mathematical and Experimental Physics
759(2005); http://dx.doi.org/10.1063/1.1928153View Description Hide Description
The interaction of electromagnetic radiation with condensed matter bears two important aspects: A physical one, as the most significant source of information about solids; and a technological aspect — the possibility to structure materials with the help of laser light.
In this paper, investigations of several materials (dielectrics, metals, semiconductors) with laser pulses as short as 5 fs will be shown, the behavior of the solid close to the threshold of optical breakdown is explored. The results lead to important implications for the feasibility and efficiency of femtosecond machining. Due to nonlinear processes at high intensities machining precision beyond the diffraction limit can be achieved.
On the other hand, the application of UV radiation leads to new levels of precision in micromachining, as well. This will be shown for the example of corneal ablation for Laser Vision Correction.
759(2005); http://dx.doi.org/10.1063/1.1928154View Description Hide Description
Laser ablation in addition for thin film growing is also used for analytical techniques as Laser Induced Breakdown Spectroscopy and for applications as simulation of natural lightning. In this work we present several diagnostic techniques such as probe beam deflection, shadowgraphy, interferometry, pulsed laser photoacoustic and the electrical perturbation induced by laser ablation plasmas that’s gives essential information of the plasma, hot core air and shock wave expansion.
759(2005); http://dx.doi.org/10.1063/1.1928155View Description Hide Description
Amorphous carbon (a‐C) thin films have recently attracted substantial attention due to their potential applications. The properties of these films, such as density, hardness and internal stress depend mainly on the sp2/sp3 carbon bonding ratio that is controlled by the specific deposition conditions. In the case of thin films deposited by laser ablation, these characteristics are strongly dependent on the laser energy density used during deposition and on the target to substrate distance, and these have an important influence on the energy of the plasma species and on the plasma density. In the present work the plasma plume generated during laser ablation of a carbon target was characterized by means of Optical Emission Spectroscopy and the Langmuir probe technique. The characterized plasma regimes were used for the deposition of a‐C films under different ion energies and plasma densities, so that different percentages of sp3 bonds might be formed in the deposits. The films were analyzed using Raman spectroscopy and EELS. The optical absorption of the films was studied as a function of the plasma parameters.
759(2005); http://dx.doi.org/10.1063/1.1928156View Description Hide Description
Using scanning electronic microscopy, we analyze the structural changes induced in silicon (100) wafers by focused IR (1064 nm) and UV (355 nm) nanosecond laser pulses. The experiments were performed in the laser ablation regime. When a silicon surface is irradiated by laser pulses in an O2 atmosphere conical microstructures are obtained. The changes in silicon surface morphology depend both on the incident radiation wavelength and the environmental atmosphere. We have patterned Si surfaces with a single focused laser spot and, in doing the experiments with IR or UV this reveals significant differences in the initial surface cracking and pattern formation, however the final result consist of an array of microcones when the experiment is carried out in oxygen. We employ a random scanning technique to irradiate silicon surfaces over large areas. In this form we have obtained large patterned areas.
759(2005); http://dx.doi.org/10.1063/1.1928157View Description Hide Description
Among the most important achievements of modern optics in the last decades is the optical confinement and manipulation of particles whose sizes range from tens of nanometers up to tens of microns. The two main ingredients that allow the confinement of microscopic objects are the linear momentum interchange between light and matter, and the presence of strong intensity gradients in the incident optical field. However, electromagnetic radiation may carry angular momentum as well, which can also be transferred to matter, giving rise to the rotation of the trapped objects. This is the case of the so‐called optical vortex fields, whose implementation in optical tweezers will be discussed. The general theoretical basis and complementary experimental results will be presented.
759(2005); http://dx.doi.org/10.1063/1.1928158View Description Hide Description
We study the generation of second‐harmonic radiation in the interaction of light with systems of two‐dimensional particles in random positions. The medium composing the particles is assumed to be homogeneous and isotropic, and the numerical examples are calculated employing a free‐electron model for the nonlinear polarization.
We first present calculation for the differential scattering efficiencies of isolated particles. For symmetric particles illuminated along the axis of symmetry, we find that the second harmonic scattering amplitude is antisymmetric. The result has interesting implications on the reciprocity relations for second harmonic generation. We then present results for random systems of cylindrical particles. For the fundamental frequency, the results present a clear enhanced backscattering peak due to multiple scattering effects. In contrast, the second harmonic scattering patterns contain no clear indication of the presence of such coherent effects.
759(2005); http://dx.doi.org/10.1063/1.1928159View Description Hide Description
We present a scattering theory model for the coherent reflectance of light from a turbid colloid in an internal reflection configuration. We compare with experimental measurements obtained with turbid suspensions of spherical latex particles and irregularly shaped TiO2 particles. When the colloidal particles are not small compared to the wavelength of radiation, the reflectance curve as a function of the angle of incidence predicted by the scattering theory model, differs appreciably from that predicted by Fresnel’s relations with an effective refractive index. Therefore, when the particles size is comparable or larger than the wavelength, one can not determine the effective refractive index in the usual way using a critical angle refractometer. We provide some insight to why Fresnel’s relations do not work with turbid media.
759(2005); http://dx.doi.org/10.1063/1.1928160View Description Hide Description
Preventive oncology is in need of a risk assessment technique that can identify individuals at high risk for breast cancer and has the ability to monitor the efficacy of a risk reducing intervention. Optical transillumination spectroscopy (OTS) gives information about breast tissue composition and tissue density. OTS is non‐invasive and in contrast to mammography, uses non‐ionising radiation. It is safe and can be used frequently on younger women, potentially permitting early risk detection and thus increasing the time available for risk reduction interventions to assert their influence. Before OTS can be used as a risk assessment and/or monitoring technique, its predictive ability needs to be demonstrated and maximized through the construction of various mathematical models relating OTS and breast tissue density, and hence risk.
To establish a correlation between OTS and mammographic density Principal Components Analysis (PCA), using risk classification, is calculated. The PCA scores are presented in three‐dimensional cluster plots and a plane of differentiation that separates the high and low tissue densities is used to calculate the predictive value. Stratification of PCA for measurement position on the breast in cranial‐caudal projection is introduced. Analysis of PCA scores as a function of the volunteer’s age and body mass index (BMI) is examined.
A small but significant correlation between the component scores and age or BMI is noted but the correlation is dependent on the tissue density category examined. Correction of the component scores for age and BMI is not recommended, since a priori knowledge of a women’s breast tissue density is required. Stratification for the center and distal measurement positions provide a predictive value for OTS above 96%.
759(2005); http://dx.doi.org/10.1063/1.1928161View Description Hide Description
In a previous paper [Phys. Rev. E 68, 011903 (2003)], we extracted an effective potential between pairs of alanines from an experimental correlation function, through the Ornstein‐Zernike equation and the hypernetted chain approximation. A modified version of this effective pair potential is then used in a Monte Carlo simulation. As a result, we obtain two important secondary structures, i.e., an α‐helix and a β‐ladder.
759(2005); http://dx.doi.org/10.1063/1.1928162View Description Hide Description
The family of intracellular lipid binding proteins (iLBPs) comprises a group of homologous 14–15 kDa proteins that specifically bind and facilitate the transport of fatty acids, bile acids, retinoids or eicosanoids. Members of this family include several types of fatty acid binding proteins (FABPs), ileal lipid binding protein, cellular retinoic acid binding proteins and cellular retinoid binding proteins. As a contribution to understanding the structure‐function relationship in this protein family, the solution structure and backbone dynamics of human epidermal‐type FABP (E‐FABP) determined by NMR spectroscopy are reported. Moreover, hydrogen/deuterium exchange experiments indicated a direct correlation between the stability of the hydrogen‐bonding network in the β‐sheet structure and the conformational exchange in the millisecond‐to‐microsecond time range. The features of E‐FABP backbone dynamics discussed in the present study are compared with those obtained for other phylogenetically related proteins. A strong interdependence with the overall protein stability and possibly also with the ligand‐binding affinity for members of the lipid‐binding protein family is shown.
759(2005); http://dx.doi.org/10.1063/1.1928163View Description Hide Description
Inverse micelles of surfactant AOT at low water contents are studied. From RMN measurements the molar fraction of bound water was obtained. Ultrasonic measurements, which depend on the compressibility, permit to determine the internal radius (water radius) of the micelles. With both results, the size of the bound water layer in this system was found around 0.3 nm. Correlated changes in the FT‐IR absorption spectra give us information about the interaction between water and the polar head of AOT. Multilamellar vesicles, at different concentrations, of phospholipid DPPC, studied by DSC and FT‐IR present a sub‐zero transition near −40°C. This transition is attributed to the interstitial water, i.e. the water confined between two DPPC bilayers. These results emphasize the peculiar properties of the bound water in such systems.
Glucose Oxidase Adsorption on Sequential Adsorbed Polyelectrolyte Films Studied by Spectroscopic Techniques759(2005); http://dx.doi.org/10.1063/1.1928164View Description Hide Description
The adsorption of Glucose Oxidase (GOX) on layers of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) deposited on Sequentially Adsorbed Polyelectrolyte Films (SAPFs) were studied by three different spectroscopic techniques. These techniques are: Optical Wave Light Spectroscopy (OWLS) to measure surface density; Fluorescence Resonance Energy Transfer (FRET) to verify the adsorption of GOX on the surface; and Fourier Transform Infrared Spectroscopy in Attenuated Total Reflection mode (FTIR‐HATR) to inspect local structure of polyelectrolytes and GOX. Two positive and two negative polyelectrolytes are used: Cationic poly(ethyleneimine) (PEI) and poly(allylamine hydrochloride) (PAH) and anionic poly(sodium 4‐styrene sulfonate) (PSS) and poly(acrylic acid) (PAA). These spectroscopic techniques do not require any labeling for GOX or SAPFs, specifically GOX and PSS are naturally fluorescent and are used as a couple donor‐acceptor for the FRET technique. The SAPFs are formed by a (PEI)‐(PSS/PAH)2 film followed by (PAA/PAH)n bilayers. GOX is finally deposited on top of SAPFs at different values of n (n=1..5). Our results show that GOX is adsorbed on positive ended SAPFs forming a monolayer. Contrary, GOX adsorption is not observed on negative ended film polyelectrolyte. GOX stability was tested adding a positive and a negative polyelectrolyte after GOX adsorption. Protein is partially removed by PAH and PAA, with lesser force by PAA.
759(2005); http://dx.doi.org/10.1063/1.1928165View Description Hide Description
In this report we review the recent work on the phenomena arising from the effects of flow on the thermodynamic state of complex fluids, with particular attention to the experimental and theoretical developments on the so‐called “shear banding” flow. In the last part of this report we exhibit the consistency of the Extended Irreversible Thermodynamics formulation and recent work on the hydrodynamics of the coupling of a rheological constitutive equation and non‐Fickian diffusion. In the same line of thought, the modeling of the flow of wormlike micellar systems is also examined.
759(2005); http://dx.doi.org/10.1063/1.1928166View Description Hide Description
One of the most important applications of the stochastic matrix method is the evaluation of the probability of forming a ring for B 2 O 3, as a description of the growth process of a solid. In a recent work Dagdug and García‐Colín (L. Dagdug and L. S. García‐Colín, J. Phys.:Condens. Matter 11, 2193 (1999)) generalized this method for the same substance introducing a probabilistic cut temperature finding a theoretical VFT‐like equation for the average relaxation time (or viscosity) of the strong glass forming liquid. To carry out their purpose they take the average relaxation time as inversely proportional to the probability of forming a ring, calculated for a large number of steps of growth. They also use the temperature derivative method to recognize the functional dependence of the relaxation time. Dagdug et. al. (L. Dagdug, L. S. García‐Colín and P. Goldstein, Rev. Mex Phys, 2004 (in press)) used the configurational entropy to calculate the isoentropic tempearture for this boron oxide glass using the probability of find boroxol rings and dendrites. With the prediction of the VFT‐like equation and the calculation of the isoentropic temperature, in this work, we give a completely theoretical framework to understand the viscosity processes in strong glasses.