RECERCAT - Articles publicats en revistes (Física Aplicada i Òptica)

Phase mask selection in wavefront coding systems: an adaptive approach

Phase mask selection in wavefront coding systems: an adaptive approach Carles, Guillem; Carnicer González, Arturo; Bosch i Puig, Salvador A method for optimizing the strength of a parametric phase mask for a wavefront coding imaging system is presented. The method is based on an optimization process that minimizes a proposed merit function. The goal is to achieve modulation transfer function invariance while quantitatively maintaining nal image delity. A parametric lter that copes with the noise present in the captured images is used to obtain the nal images, and this lter is optimized. The whole process results in optimum phase mask strength and optimal parameters for the restoration lter. The results for a particular optical system are presented and tested experimentally in the labo- ratory. The experimental results show good agreement with the simulations, indicating that the procedure is useful.

Understanding Optical Trapping Phenomena: a Simulation for Undergraduates

Understanding Optical Trapping Phenomena: a Simulation for Undergraduates Mas Soler, Josep; Farré Flaquer, Arnau; Cuadros, Jordi; Juvells Prades, Ignacio; Carnicer González, Arturo Optical trapping is an attractive and multidisciplinary topic that has become the center of attention to a large number of researchers. Moreover, it is a suitable subject for advanced students that requires a knowledge of a wide range of topics. As a result, it has been incorporated into some syllabuses of both undergraduate and graduate programs. In this paper, basic concepts in laser trapping theory are reviewed. To provide a better understanding of the underlying concepts for students, a Java application for simulating the behavior of a dielectric particle trapped in a highly focused beam has been developed. The program illustrates a wide range of theoretical results and features, such as the calculation of the force exerted by a beam in the Mie and Rayleigh regimes or the calibration of the trap stiffness. Some examples that are ready to be used in the classroom or in the computer lab are also supplied.

Transverse and longitudinal components of the propagating and evanescent waves associated to radially-polarized nonparaxial fields

Transverse and longitudinal components of the propagating and evanescent waves associated to radially-polarized nonparaxial fields Martínez-Herrero, Rosario; Mejías, Pedro M.; Juvells Prades, Ignacio; Carnicer González, Arturo A comparison is established between the contributions of transverse and longitudinal components of both the propagating and the evanescent waves associated to freely propagating radially polarized nonparaxial beams. Attention is focused on those fields that remain radially polarized upon propagation. In terms of the plane-wave angular spectrum of these fields, analytical expressions are given for determining both the spatial shape of the above components and their relative weight integrated over the whole transverse plane. The results are applied to two kinds of doughnut-like beams with radial polarization, and we compare the behavior of such fields at two transverse planes.

Evanescent field of vectorial highly non-paraxial beams

Evanescent field of vectorial highly non-paraxial beams Martínez-Herrero, Rosario; Mejías, Pedro M.; Carnicer González, Arturo In terms of the Fourier spectrum, a simple but general analytical expression is given for the evanescent field associated to a certain kind of non-paraxial exact solutions of the Maxwell equations. This expression enables one to compare the relative weight of the evanescent wave with regard to the propagating field. In addition, in those cases in which the evanescent term is significant, the magnitude of the field components across the transverse profile (including the evanescent features) can be determined. These results are applied to some illustrative examples.

Particle shape and orientation in laser diffraction and static image analysis size distribution analysis of micrometer sized rectangular particles

Particle shape and orientation in laser diffraction and static image analysis size distribution analysis of micrometer sized rectangular particles Tinke, A. P.; Carnicer González, Arturo; Govoreanu, R.; Scheltjens, G.; Lauwerysen, L.; Mertens, N.; Vanhoutte, K.; Brewster, M. E. Laser diffraction (LD) and static image analysis (SIA) of rectangular particles [United States Pharmacopeia, USP30-NF25, General Chapter <776>, Optical Miroscopy.] have been systematically studied. To rule out sample dispersion and particle orientation as the root cause of differences in size distribution profiles, we immobilize powder samples on a glass plate by means of a dry disperser. For a defined region of the glass plate, we measure the diffraction pattern as induced by the dispersed particles, and the 2D dimensions of the individual particles using LD and optical microscopy, respectively. We demonstrate a correlation between LD and SIA, with the scattering intensity of the individual particles as the dominant factor. In theory, the scattering intensity is related to the square of the projected area of both spherical and rectangular particles. In traditional LD the size distribution profile is dominated by the maximum projected area of the particles (A). The diffraction diameters of a rectangular particle with length L and breadth B as measured by the LD instrument approximately correspond to spheres of diameter ØL and ØB respectively. Differences in the scattering intensity between spherical and rectangular particles suggest that the contribution made to the overall LD volume probability distribution by each rectangular particle is proportional to A2/L and A2/B. Accordingly, for rectangular particles the scattering intensity weighted diffraction diameter (SIWDD) explains an overestimation of their shortest dimension and an underestimation of their longest dimension. This study analyzes various samples of particles whose length ranges from approximately 10 to 1000 μm. The correlation we demonstrate between LD and SIA can be used to improve validation of LD methods based on SIA data for a variety of pharmaceutical powders all with a different rectangular particle size and shape.

On the longitudinal component of paraxial fields

On the longitudinal component of paraxial fields Carnicer González, Arturo; Juvells Prades, Ignacio; Maluenda Niubó, David; Martínez-Herrero, Rosario; Mejías, Pedro M. The analysis of paraxial Gaussian beams features in most undergraduate courses in laser physics, advanced optics and photonics. These beams provide a simple model of the field generated in the resonant cavities of lasers, thus constituting a basic element for understanding laser theory. Usually, uniformly polarized beams are considered in the analytical calculations, with the electric field vibrating at normal planes to the propagation direction. However, such paraxial fields do not verify the Maxwell equations. In this paper we discuss how to overcome this apparent contradiction and evaluate the longitudinal component that any paraxial Gaussian beam should exhibit. Despite the fact that the assumption of a purely transverse paraxial field is useful and accurate, the inclusion of the above issue in the program helps students to clarify the importance of the electromagnetic nature of light, thus providing a more complete understanding of the paraxial approach.

Design of Optical Systems With Extended Depth of Field: an Educational Approach to Wavefront Coding Techniques

Design of Optical Systems With Extended Depth of Field: an Educational Approach to Wavefront Coding Techniques Ferrán Sánchez, Carme; Bosch i Puig, Salvador; Carnicer González, Arturo A practical activity designed to introduce wavefront coding techniques as a method to extend the depth of field in optical systems is presented. The activity is suitable for advanced undergraduate students since it combines different topics in optical engineering such as optical system design, aberration theory, Fourier optics, and digital image processing. This paper provides the theoretical background and technical information for performing the experiment. The proposed activity requires students able to develop a wide range of skills since they are expected to deal with optical components, including spatial light modulators, and develop scripts to perform some calculations.

A digital holography technique for generating beams with arbitrary polarization and shape

A digital holography technique for generating beams with arbitrary polarization and shape Maluenda Niubó, David; Juvells Prades, Ignacio; Martínez-Herrero, Rosario; Carnicer González, Arturo A method for generating beams with arbitrary polarization and shape is proposed. Our design requires the use of a Mach-Zehnder set-up combined with translucent liquid crystal displays in each arm of the interferometer; in this way, independent manipulation of each transverse beam components is possible. The target of this communication is to develop a numerical procedure for calculating the holograms required for dynamically encode any amplitude value and polarization state in each point of the wavefront. Several examples demonstrating the capabilities of the method are provided.

Reconfigurable beams with arbitrary polarization and shape distributions at a given plane

Reconfigurable beams with arbitrary polarization and shape distributions at a given plane Maluenda Niubó, David; Juvells Prades, Ignacio; Martínez-Herrero, Rosario; Carnicer González, Arturo Methods for generating beams with arbitrary polarization based on the use of liquid crystal displays have recently attracted interest from a wide range of sources. In this paper we present a technique for generating beams with arbitrary polarization and shape distributions at a given plane using a Mach-Zehnder setup. The transverse components of the incident beam are processed independently by means of spatial light modulators placed in each path of the interferometer. The modulators display computer generated holograms designed to dynamically encode any amplitude value and polarization state for each point of the wavefront in a given plane. The steps required to design such beams are described in detail. Several beams performing different polarization and intensity landscapes have been experimentally implemented. The results obtained demonstrate the capability of the proposed technique to tailor the amplitude and polarization of the beam simultaneously.

Behavior of propagating and evanescent components in azimuthally polarized non-paraxial fields

Behavior of propagating and evanescent components in azimuthally polarized non-paraxial fields Martínez-Herrero, Rosario; Mejías, Pedro M.; Juvells Prades, Ignacio; Carnicer González, Arturo The contribution of the propagating and the evanescent waves associated with freely propagating non-paraxial light fields whose transverse component is azimuthally polarized at some plane is investigated. Analytic expressions are derived for describing both the spatial shape and the relative weight of the propagating and the evanescent components integrated over the transverse plane. The analysis is carried out within the framework of the plane-wave angular spectrum approach. These results are used to illustrate the behavior of a kind of donut-like beams with transverse azimuthal polarization at some plane.

Heteroepitaxial growth of MgO(111) thin films on Al2O3(0001): Evidence of a wurtzite to rocksalt transformation

Heteroepitaxial growth of MgO(111) thin films on Al2O3(0001): Evidence of a wurtzite to rocksalt transformation Martínez Boubeta, José Carlos; Botana, Antía S.; Pardo, Víctor; Baldomir, Daniel; Antony, Aldrin; Bertomeu i Balagueró, Joan; Rebled Corsellas, José Manuel; López-Conesa, Lluís; Estradé Albiol, Sònia; Peiró Martínez, Francisca We report on a growth study of MgO films deposited on Al2O3(0001) substrates by magnetron sputtering. The films exhibited a preferred rocksalt MgO(111) orientation. Surprisingly, depending on the O2 gas flow ratio, a structure of graphiticlike wurtzite MgO(0001) has been revealed. The observed Mg-O perpendicular bond length reduction is accompanied by an atomically flat surface morphology for the development of MgO(111) films; the transition to the bulk rocksalt structure occurs in the 3
6 nm coverage range. Previously, relaxation of the electrostatic instability of MgO(111) films accompanied by an in-plane lattice increase has been suggested theoretically [Phys. Rev. Lett. 98, 205701 (2007)]. Here, relying on ab initio calculations, we infer that Mg vacancies facilitate the lattice match with the substrate. This mechanism suggests methods to engineer oxide heterostructures.

The Electronic Structure of Co-Sputtered Zinc Indium Tin Oxide Thin Films

The Electronic Structure of Co-Sputtered Zinc Indium Tin Oxide Thin Films Carreras Seguí, Paz; Gutmann, Sebastian; Antony, Aldrin; Bertomeu i Balagueró, Joan; Schlaf, Rudy Zinc indium tin oxide (ZITO) transparent conductive oxide layers were deposited via radio frequency (RF) magnetron co-sputtering at room temperature. A series of samples with gradually varying zinc content was investigated. The samples were characterized with x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) to determine the electronic structure of the surface. Valence and conduction bands maxima (VBM, CBM), and work function were determined. The experiments indicate that increasing Zn content results in films with a higher defect rate at the surface leading to the formation of a degenerately doped surface layer if the Zn content surpasses 50%. Furthermore, the experiments demonstrate that ZITO is susceptible to ultraviolet light induced work function reduction, similar to what was earlier observed on ITO and TiO2 films.

Relaxation and derelaxation of pure and hydrogenated amorphous silicon during thermal annealing experiments

Relaxation and derelaxation of pure and hydrogenated amorphous silicon during thermal annealing experiments Kail, F.; Farjas Silva, Jordi; Roura Grabulosa, Pere; Secouard, C.; Nos Aguilà, Oriol; Bertomeu i Balagueró, Joan; Alzina Sureda, Francesc; Roca i Cabarrocas, P. (Pere) The structural relaxation of pure amorphous silicon a-Si and hydrogenated amorphous silicon a-Si:H materials, that occurs during thermal annealing experiments, has been analyzed by Raman spectroscopy and differential scanning calorimetry. Unlike a-Si, the heat evolved from a-Si:H cannot be explained by relaxation of the Si
Si network strain but it reveals a derelaxation of the bond angle strain. Since the state of relaxation after annealing is very similar for pure and hydrogenated materials, our results give strong experimental support to the predicted configurational gap between a-Si and crystalline silicon.

New features of the layer-by-layer deposition of microcrystalline silicon films revealed by spectroscopic ellipsometry and high resolution transmission electron microscopy

New features of the layer-by-layer deposition of microcrystalline silicon films revealed by spectroscopic ellipsometry and high resolution transmission electron microscopy Roca i Cabarrocas, P. (Pere); Hamma, S.; Hadjadj, A.; Bertomeu i Balagueró, Joan; Andreu i Batallé, Jordi Spectroscopic ellipsometry and high resolution transmission electron microscopy have been used to characterize microcrystalline silicon films. We obtain an excellent agreement between the multilayer model used in the analysis of the optical data and the microscopy measurements. Moreover, thanks to the high resolution achieved in the microscopy measurements and to the improved optical models, two new features of the layer-by-layer deposition of microcrystalline silicon have been detected: i) the microcrystalline films present large crystals extending from the a-Si:H substrate to the film surface, despite the sequential process in the layer-by-layer deposition; and ii) a porous layer exists between the amorphous silicon substrate and the microcrystalline silicon film.

Internal stress and strain in heavily boron-doped diamond films grown by microwave plasma and hot filament chemical vapor deposition

Internal stress and strain in heavily boron-doped diamond films grown by microwave plasma and hot filament chemical vapor deposition Wang, W.L.; Polo Trasancos, Ma del Carmen; Sánchez, G.; Cifre, J.; Esteve Pujol, Joan The internal stress and strain in boron‐doped diamond films grown by microwave plasma chemical vapor deposition (MWCVD) and hot filament CVD (HFCVD) were studied as a function of boron concentration. The total stress (thermal+intrinsic) was tensile, and the stress and strain increased with boron concentration. The stress and the strain measured in HFCVD samples were greater than those of MWCVD samples at the same boron concentration. The intrinsic tensile stress, 0.84 GPa, calculated by the grain boundary relaxation model, was in good agreement with the experimental value when the boron concentration in the films was below 0.3 at.%. At boron concentrations above 0.3 at.%, the tensile stress was mainly caused by high defect density, and induced by a node‐blocked sliding effect at the grain boundary

Effect of substrate temperature on deposition rate of rf plasma-deposited hydrogenated amorphous silicon thin films

Effect of substrate temperature on deposition rate of rf plasma-deposited hydrogenated amorphous silicon thin films Andújar Bella, José Luis; Bertran Serra, Enric; Canillas i Biosca, Adolf; Campmany i Guillot, Josep, 1966-; Morenza Gil, José Luis We present a study about the influence of substrate temperature on deposition rate of hydrogenated amorphous silicon thin films prepared by rf glow discharge decomposition of pure silane gas in a capacitively coupled plasma reactor. Two different behaviors are observed depending on deposition pressure conditions. At high pressure (30 Pa) the influence of substrate temperature on deposition rate is mainly through a modification of gas density, in such a way that the substrate temperature of deposition rate is similar to pressure dependence at constant temperature. On the contrary, at low pressure (3 Pa), a gas density effect cannot account for the observed increase of deposition rate as substrate temperature rises above 450 K with an activation energy of 1.1 kcal/mole. In accordance with laser‐induced fluorescence measurements reported in the literature, this rise has been ascribed to an increase of secondary electron emission from the growing film surface as a result of molecular hydrogen desorption.

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

Ellipsometric study of a-Si:H thin films deposited by square wave modulated rf glow discharge Lloret, A.; Bertran Serra, Enric; Andújar Bella, José Luis; Canillas i Biosca, Adolf; Morenza Gil, José Luis< 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 relatively abundant in high rf power discharges and cause significant effects on the deposited layers through polymers, clusters, and powder formation.

Optical absorption and graphitic clusters of hydrogenated amorphophous carbon thin films

Optical absorption and graphitic clusters of hydrogenated amorphophous carbon thin films Pascual Miralles, Esther; Serra Rodríguez, Carmen; Bertran Serra, Enric The optical absorption of hydrogenated amorphous carbon films (a‐C:H) was measured by spectroscopic ellipsometry. The a‐C:H films were deposited at different substrate temperatures by rf‐plasma of methane. A volume distribution of graphitic cluster size was assumed to reproduce the experimental spectra of the absorption coefficient. The changes in the absorption coefficient and the optical gap, induced by deposition temperature, have been interpreted in terms of changes in the graphitic cluster size of the network. The increase in the deposition temperature produces an increase in the size of the graphitic clusters.

Properties of amorphous silicon thin films grown in square wave modulated silane rf discharges.

Properties of amorphous silicon thin films grown in square wave modulated silane rf discharges. Andújar Bella, José Luis; Bertran Serra, Enric; Canillas i Biosca, Adolf; Campmany i Guillot, Josep, 1966-; Serra-Miralles, J.; Roch i Cunill, Carles; Lloret, A. Hydrogenated amorphous silicon (a‐Si:H) thin films have been obtained from pure SiH4 rf discharges by using the square wave modulation (SQWM) method. Film properties have been studied by means of spectroellipsometry, thermal desorption spectrometry, photothermal deflection spectroscopy and electrical conductivity measurements, as a function of the modulation frequency of the rf power amplitude (0.2&br&4000 Hz). The films deposited at frequencies about 1 kHz show the best structural and optoelectronic characteristics. Based upon the experimental results, a qualitative model is presented, which points up the importance of plasma negative ions in the deposition of a‐Si:H from SQWM rf discharges through their influence on powder particle formation.

In situ spectroellipsometric study of the nucleation and growth of amorphous silicon

In situ spectroellipsometric study of the nucleation and growth of amorphous silicon Canillas i Biosca, Adolf; Bertran Serra, Enric; Andújar Bella, José Luis; Drevillon, B. A detailed in situ spectroellipsometric analysis of the nucleation and growth of hydrogenated amorphous silicon (a:Si:H) is presented. Photoelectronic quality a‐Si:H films are deposited by plasma‐enhanced chemical vapor deposition on smooth metal (NiCr alloy) and crystalline silicon (c‐Si) substrates. The deposition of a‐Si:H is analyzed from the first monolayer up to a final thickness of 1.2 μm. In order to perform an improved analysis, real time ellipsometric trajectories are recorded, using fixed preparation conditions, at various photon energies ranging from 2.2 to 3.6 eV. The advantage of using such a spectroscopic experimental procedure is underlined. New insights into the nucleation and growth mechanisms of a‐Si:H are obtained. The nucleation mechanism on metal and c‐Si substrates is very accurately described assuming a columnar microstructural development during the early stage of the growth. Then, as a consequence of the incomplete coalescence of the initial nuclei, a surface roughness at the 10&br&15 Å scale is identified during the further growth of a‐Si:H on both substrates. The bulk a‐Si:H grows homogeneously beneath the surface roughness. Finally, an increase of the surface roughness is evidenced during the long term growth of a‐Si:H. However, the nature of the substrate influenced the film growth. In particular, the film thickness involved in the nucleation‐coalescence phase is found lower in the case of c‐Si (67±8 Å) as compared to NiCr (118±22 Å). Likewise films deposited on c‐Si present a smaller surface roughness even if thick samples are considered (&1 μm). More generally, the present study illustrates the capability of in situ spectroellipsometry to precisely analyze fundamental processes in thin‐film growth, but also to monitor the preparation of complex structures on a few monolayers scale.

On the Hall effect in polycrystalline semiconductors

On the Hall effect in polycrystalline semiconductors García-Cuenca Varona, María Victoria; Morenza Gil, José Luis; Codina i Vidal, Josep Ma. (Josep Maria), 1927- Some problems involved in the interpretation of Hall‐effect measurements in polycrystalline semiconductors have not been resolved, especially when the contribution of the boundaries is appreciable. Using the Herring theory of transport properties in inhomogeneous semiconductors, we present an alternative interpretation to that previously proposed. This model permits the calculation of the Hall coefficient under general conditions.

Electron tunneling in heavily In-doped polycrystalline CdS films

Electron tunneling in heavily In-doped polycrystalline CdS films García-Cuenca Varona, María Victoria; Morenza Gil, José Luis; Esteve Pujol, Joan The electrical properties of heavily In‐doped polycrystalline CdS films have been studied as a function of the doping level. The films were prepared by vacuum coevaporation of CdS and In. Conductivity and Hall measurements were performed over the temperature range 77&br&400 K. The conductivity decreases weakly with the temperature and shows a tendency towards saturation at low temperatures. A simple relationship σ=σ0(1+βT2) is found in the low‐temperature range. The temperature dependence of the mobility is similar to that of the conductivity since the Hall coefficient is found to be a constant in the whole temperature range. We interpret the experimental results in terms of a modified version of grain‐boundary trapping Seto"s model, taking into account thermionic emission and tunneling of carriers through the potential barriers. The barriers are found to be high and narrow, and tunneling becomes the predominating transport mechanism.

Particle agglomeration study in rf silane plasmas: In situ study by polarization-sensitive laser light scattering

Particle agglomeration study in rf silane plasmas: In situ study by polarization-sensitive laser light scattering Courteille, C.; Hollenstein, Ch.; Dorier, J. L.; Gay, P.; Schwarzenbach, W.; Howling, A. A.; Bertrán Serra, Enric; Viera Mármol, Gregorio; Martins, R.; Macarico, A. To determine self‐consistently the time evolution of particle size and their number density in situ multi‐angle polarization‐sensitive laser light scattering was used. Cross‐polarization intensities (incident and scattered light intensities with opposite polarization) measured at 135° and ex situ transmission electronic microscopy analysis demonstrate the existence of nonspherical agglomerates during the early phase of agglomeration. Later in the particle time development both techniques reveal spherical particles again. The presence of strong cross‐polarization intensities is accompanied by low‐frequency instabilities detected on the scattered light intensities and plasma emission. It is found that the particle radius and particle number density during the agglomeration phase can be well described by the Brownian free molecule coagulation model. Application of this neutral particle coagulation model is justified by calculation of the particle charge whereby it is shown that particles of a few tens of nanometer can be considered as neutral under our experimental conditions. The measured particle dispersion can be well described by a Brownian free molecule coagulation model including a log‐normal particle size distribution.

Microstructure of highly oriented, hexagonal, boron nitride thin films grown on crystalline silicon by radio frequency plasma-assisted chemical vapor deposition

Microstructure of highly oriented, hexagonal, boron nitride thin films grown on crystalline silicon by radio frequency plasma-assisted chemical vapor deposition Andújar Bella, José Luis; Bertrán Serra, Enric; Manniete, Y. We present a high‐resolution electron microscopy study of the microstructure of boron nitride thin films grown on silicon (100) by radio‐frequency plasma‐assisted chemical vapor deposition using B2H6 (1% in H2) and NH3 gases. Well‐adhered boron nitride films grown on the grounded electrode show a highly oriented hexagonal structure with the c‐axis parallel to the substrate surface throughout the film, without any interfacial amorphous layer. We ascribed this textured growth to an etching effect of atomic hydrogen present in the gas discharge. In contrast, films grown on the powered electrode, with compressive stress induced by ion bombardment, show a multilayered structure as observed by other authors, composed of an amorphous layer, a hexagonal layer with the c‐axis parallel to the substrate surface and another layer oriented at random

Spectroscopic ellipsometry study of the In1-x Gax Asy P1-y / InP Heterojunctions grown by metalorganic chemical-vapor deposition

Spectroscopic ellipsometry study of the In1-x Gax Asy P1-y / InP Heterojunctions grown by metalorganic chemical-vapor deposition Drévillon, B.; Bertran Serra, Enric; Alnot, P.; Olivier, J.; Razeghi, M. The dielectric functions of InP, In0.53Ga0.47As, and In0.75Ga0.25As0.5P0.5 epitaxial layers have been measured using a polarization modulation spectroscopic ellipsometer in the 1.5 to 5.3 eV region. The oxide removal procedure has been carefully checked by comparing spectroscopic ellipsometry and x ray photoelectron spectroscopy measurements. These reference data have been used to investigate the structural nature of metalorganic chemical vapor deposition grown In0.53Ga0.47As/InP and In0.75Ga0.25As0.5P0.5/InP heterojunctions, currently used for photodiodes and laser diodes. The sharpness of the interfaces has been systematically compared for the two types of heterojunctions: In1 xGaxAsy/InP and InP/In1 xGaxAsyP1 y. The sharpest interface is obtained for InP growth on In0.75Ga0.25As0.5P0.5 where the interface region is estimated to be (10±10) Å thick. The importance of performing in situ SE measurements is emphasized.

Optical trapping: a review of essential concepts

Optical trapping: a review of essential concepts Verdeny, Ione; Farré, Arnau; Mas Soler, Josep; López Quesada, Carol; Martín Badosa, Estela; Montes Usategui, Mario Optical tweezers are an innovative technique for the non-contact, all-optical manipulation of small material samples, which has extraordinarily expanded and evolved since its inception in the mid-80s of the last century. Nowadays, the potential of optical tweezers has been clearly proven and a wide range of applications both from the physical and biological sciences have solidly emerged, turning the early ideas and techniques into a powerful paradigm for experimentation in the micro- and nanoworld. This review aims at highlighting the fundamental concepts that are essential for a thorough understanding of optical trapping, making emphasis on both its manipulation and measurement capabilities, as well as on the vast array of important biological applications appeared in the last years.

Nonlinear filtering in object and Fourier space in a joint transform optical correlator: comparison and experimental realization

Nonlinear filtering in object and Fourier space in a joint transform optical correlator: comparison and experimental realization Vallmitjana i Rico, Santiago; Carnicer González, Arturo; Martín Badosa, Estela; Juvells Prades, Ignacio The use of different kinds of nonlinear filtering in a joint transform correlator are studied and compared. The study is divided into two parts, one corresponding to object space and the second to the Fourier domain of the joint power spectrum. In the first part, phase and inverse filters are computed; their inverse Fourier transforms are also computed, thereby becoming the reference in the object space. In the Fourier space, the binarization of the power spectrum is realized and compared with a new procedure for removing the spatial envelope. All cases are simulated and experimentally implemented by a compact joint transform correlator.

Application of fuzzy-rule-based postprocessing to correlation methods in pattern recognition

Application of fuzzy-rule-based postprocessing to correlation methods in pattern recognition Wolf, T.; Gutmann, B.; Weber, H.; Ferré Borrull, Josep; Bosch i Puig, Salvador; Vallmitjana i Rico, Santiago One of the most important problems in optical pattern recognition by correlation is the appearance of sidelobes in the correlation plane, which causes false alarms. We present a method that eliminate sidelobes of up to a given height if certain conditions are satisfied. The method can be applied to any generalized synthetic discriminant function filter and is capable of rejecting lateral peaks that are even higher than the central correlation. Satisfactory results were obtained in both computer simulations and optical implementation.

Sidelobe elimination for generalized synthetic discriminant functions by a two-filter correlation and subsequent postprocessing of the intensity distributions

Sidelobe elimination for generalized synthetic discriminant functions by a two-filter correlation and subsequent postprocessing of the intensity distributions Montes Usategui, Mario; Campos Coloma, Juan; Juvells Prades, Ignacio; Vallmitjana i Rico, Santiago One of the most important problems in optical pattern recognition by correlation is the appearance of sidelobes in the correlation plane, which causes false alarms. We present a method that eliminate sidelobes of up to a given height if certain conditions are satisfied. The method can be applied to any generalized synthetic discriminant function filter and is capable of rejecting lateral peaks that are even higher than the central correlation. Satisfactory results were obtained in both computer simulations and optical implementation.

Adding functionalities to precomputed holograms with random mask multiplexing in holographic optical tweezers

Adding functionalities to precomputed holograms with random mask multiplexing in holographic optical tweezers Mas Soler, Josep; Roth, Michele S.; Montes Usategui, Mario; Martín Badosa, Estela In this study, we present a method designed to generate dynamic holograms in holographic optical tweezers. The approach combines our random mask encoding method with iterative high-efficiency algorithms. This hybrid method can be used to dynamically modify precalculated holograms, giving them new functionalities¿temporarily or permanently¿with a low computational cost. This allows the easy addition or removal of a single trap or the independent control of groups of traps for manipulating a variety of rigid structures in real time.