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We propose a new methodology to measure both the dielectric constant and thickness of dielectric coatings on a flat conducting substrate from two capacitance measurements. We discuss the principles of the method and demonstrate its feasibility using 2-D numerical simulations. We present a proof of principles experiment confirming the viability of the method in practice. Finally, we propose and analyze a multisphere electrode geometry

We explore using measurements of the effective refractive index of a metallic nanofluid to estimate the size of the particles in it. We assume the nanofluid consists of spherical metallic nanoparticles suspended in a transparent base liquid and discuss a way of measuring the real and imaginary parts of the effective refractive index for concentrated nanofluids to about 1% in particles’ volume concentration. Specifically, we consider the case of copper nanoparticles suspended in water. We propose an unambiguous effective optical parameter as a candidate to evidence the particle size, potentially in real time. Limitations due to dependent scattering effects in concentrated nanofluids are briefly stated.

In this work we study the sensitivity and detection limits of optical reflectivity measurements to inspect the deposition of metallic nanoparticles on a at surface. We use a theoretical model for the coherent reflectance of a disordered monolayer of particles to calculate the reflectivity of an air-glass interface with silver and gold nanoparticles deposited randomly on it as a function of the angle of incidence and wavelength of light. Assuming reasonable noise scenarios, we estimate the minimum detectable surface coverage fractions by the nanoparticles.

Indium-doped zinc oxide (ZnO:In) thin films were grown on glass substrates using the chemical spray technique. The effects of the acetic acid content in the starting solution ( c AA ), as well as the substrate temperature ( T S ), were studied. Our results demonstrate that when c AA is extremely low, the resistivity values of the zinc oxide (ZnO) thin films become relatively high (in the order of 4 × 10 − 2 Ω cm). When the c AA is increased at a fixed temperature, the resistivity of the films decreases, reaching values as low as 4 × 10 − 3 Ω cm for thin films deposited at 525 °C. The electron mobility could also increase to a maximum value of 10.5 cm 2 /(V s) for films deposited at 500 °C. We also observed an enhancement in the electrical transport properties of the films by varying T S ; the lowest resistivity values occurred in films deposited at T S between 475 and 525 °C. In addition, the relative intensity of the diffraction peaks associated with the crystallographic planes is strongly affected by the c AA concentration. X-ray diffraction studies reveal the polycrystalline nature of the films exhibiting a hexagonal wurtzite type, with a preferential orientation of the film depending on the acetic acid concentration. Film morphology was also affected by varying c AA , as grains with distinct geometrical shapes were observed. Finally, the optical transmittance of all these films was found to be higher than 85%.

In this work we show that the thermal lens effect can be applied to highly scattering and weakly absorbing materials. We apply the thermal lens effect and the z-scan technique to estimate the effective absorption coefficient of a suspension of TiO 2 particles with a mean diameter of 220 nm at two wavelengths: 488 nm and 514 nm. From the effective absorption coefficient we estimate the absorption cross section of the particles.

The problem of extraction of information from the capacitance measurement between a Gaussian rough surface and a controlled electrode is studied with 2D numerical simulations. It is shown that with a plane electrode it is possible to obtain information on the standard deviation of the height, but not on the correlation length. With a corrugated, periodic electrode, on the other hand, information on the correlation length can be obtained without much difficulty, but the determination of the rms value of the height variations of the surface is limited due to poor depth resolution. Considerations of the results of numerical calculations of Gaussian randomly rough surfaces with a plane electrode and with a corrugated electrode suggest a simple method of obtaining the statistics of this type of surface.

We consider the coherent reflection of light from a dilute polydisperse random system of particles. We focus our attention on the effects of the variation of the size distribution function induced near a flat solid interface. We also present a comparison of the theory with experimental results of the coherent reflectance of light in an internal reflection configuration. Introduction The coherent reflection and transmission of light in random systems of particles is potentially a useful tool to characterize highly scattering particles, colloids and composite materials. By coherent we mean the wave corresponding to the average electromagnetic field which is obtained by averaging over all possible configurations of the system (configurational average). The coherent transmission of electromagnetic waves through random systems of particles has been studied by several authors since long ago (1). More recently, the coherent reflection of an electromagnetic wave from a half space of randomly located particles has been studied using a very intuitive approach (3), which turned out to be closely related to the so called effective-field approximation (4) and valid only for a dilute suspension of particles. We will call this approach the coherent-scattering model. For relatively dense systems a more elaborate procedure, called the quasi-crystalline approximation (2), has been also developed. This latter approximation is rather intensive numerically, and since we are exploring here the importance of surface effects on the reflectance, we will restrict ourselves to dilute colloidal systems where the inversion of experimental data can be easily done. We should also add that we have performed already experiments to test and to validate the coherent-scattering model finding that it can reproduce well the experimental data (5). However, when dealing with polydisperse systems of particles in the presence of a flat interface between the matrix material and a homogeneous medium, the mere presence of the interface changes the density and size distribution over a region of the order of the width of the size distribution function. This region of variable density and size distribution may be called the surface region. In this paper we propose an iterative procedure to take into account in the reflectance calculations the presence of the surface region and compare its effects with the ones of a simpler approximation that we call the "sharp-surface" approximation. Finally, we also present a comparison between reflectance calculations using the coherent-scattering model and experimental data of light reflectance from a polydisperse colloidal suspension of latex particles, in an internal-reflection configuration.

Chemically deposited semiconductor polyaniline (PANI) thin films have been exposed to detect nitrogen dioxide gas. Because of the high electron receptor behavior, NO 2 molecules oxidize the conjugated polymer during the sensing process, leading to the modification of the electronic structure and consequently the change of the optical absorbance of PANI as a function of the toxic gas concentration. NO 2 -induced optical transmittance changes of the polymer films can be varied between 5 and 80% for a NO 2 gas concentration of 3–50 ppm. By using a parametric method, the heat of adsorption ( Q ) as well as the activation energy of adsorption ( E A ) for NO 2 molecules on PANI surface are obtained from the corresponding adsorption kinetic curves. It is found that Q is of the order of 0.50–0.58 eV, and E A of 0.28–0.50 eV, depending on the type of the PANI films and the NO 2 gas concentration. We show that the same kinetic expression can also be obtained from a reaction-diffusion coupled problem reported in literature by assuming a couple of the relations between the thermodynamic variables and kinetic reaction rates of the two models. The desorption process of NO 2 from PANI samples is kinetically irreversible in a dried air or N 2 during the experimental time, and slowly reversible if they have been kept in ambient air with a relative humidity greater than zero.

We analyze the coherent reflection of a half-space of a composite material consisting of an homogeneous matrix with spherical inclusions. We pay attention to the case where the radius of the particles is comparable to the wavelength of the incident radiation. We consider a simple model for the interface of the composite and present numerical calculations to illustrate the contribution of the embedded particles to the optical reflectance and discuss surface effects.

Based on a recent effective medium theory, applicable to the case of spherical particles of radius comparable or larger than the wavelength, we compare the behavior of the effective optical coefficients of random systems of transparent-dielectric and metallic particles as a function of the particles radius. We show numerical calculations and discuss the appearance of negative imaginary parts in either the effective electric or magnetic susceptibility in a system of Mie particles.

We calculate the coherent reflectance of light from a half-space with randomly located spherical particles using the effective field approximation to solve the integral equation obeyed by the scattered fields. We obtain the effective refractive index and the coherent reflection coefficient to first order in the density of particles. Then we use basic principles of continuum electrodynamics to derive expressions for the effective permittivity and permeability tensors of the half-space. The results are not restricted to particles of radius small compared to the wavelength of the incident radiation. It is shown that an effective magnetic permeability arises, even in the case when the particles are non-magnetic. The present work provides a standard framework to pursue higher order approximations for denser media.

Polyaniline (PANI) shows electrical as well as optical property changes when it is in contact with oxidant/reductor gas. In this work, the optical transmittance changes in PANI coatings originated by their exposure to the ammonia gas were detected by a nulling optical transmittance bridge. The adsorption and desorption curves were carefully analysed using Langmuir kinetic theory for the adsorption of ammonia gas on PANI single and PANI–PMMA composite coatings. Because of the inhomogeneous nature of the PANI film surface adsorption isotherms were the best described by Freundlich model. From the numerical fitting equations it is suggested that there may be two energetically different adsorption sites on the PANI surface for NH 3 molecules, which corroborates with the no-uniform positive charge distribution on the nitrogen atoms of the PANI backbone chains. The activation energy of adsorption and the heat of adsorption were estimated to be about 0.3–0.5 eV at a NH 3 gas concentration of 10–1000 ppm.

The main objective of this paper is to endorse a recently derived theoretical model for the coherent reflectance and transmittance from a surface supporting a disordered monolayer of large and tenuous particles by comparison with experimental measurements. The model is based on the so-called anomalous-diffraction approximation and is assumed to be valid for small and moderate angles of incidence. We prepared disordered monolayers of spherical polystyrene particles of 1.8 μm diameter and of human red blood cells on glass microscope slides. In both cases, particles were immersed in a liquid of refractive index close to that of the particle. We measured the relative reflectivity and transmissivity of the samples versus the wavelength of light at normal incidence and the reflectivity and transmissivity versus the angle of incidence at a fixed wavelength, and compared with predictions of the anomalous-diffraction approximation model. For the polystyrene particle samples, we also compare the results with another available theoretical model developed some years ago to deal with disordered monolayers of highly scattering particles. In the case of red blood cell monolayers, we also present measurements with a hemolyzed sample and a disordered multilayer film for comparison. We find that the anomalous diffraction model can be fitted very well to the experimental curves, in some cases, even for high angles of incidence.

We model theoretically the voltage response to an acoustic pulse of a multilayer system forming a low noise capacitive sensor including a Polyvinylidene Fluoride piezoelectric film. First we model a generic piezoelectric detector consisting of a piezoelectric film between two metallic electrodes that are the responsible to convert the acoustic signal into a voltage signal. Then we calculate the pressure-to-voltage transfer function for a N-layer piezo-electric capacitor detector, allowing to study the effects of the electrode and protective layers thickness in typical layered piezoelectric sensors. The derived transfer function, when multiplied by the Fourier transform of the incident acoustic pulse, gives the voltage electric response in the frequency domain. An important concern regarding the transfer function is that it may have zeros at specific frequencies, and thus inverting the voltage Fourier transform of the pulse to recover the pressure signal in the time domain is not always, in principle, possible. Our formulas can be used to predict the existence and locations of such zeroes. We illustrate the use of the transfer function by predicting the electric signal generated at a multilayer piezoelectric sensor to an ultrasonic pulse generated photoacoustically by a laser pulse at a three media system with impedance mismatch. This theoretical calculations are compared with our own experimental measurements.

We present a brief overview of using the effective refractive index of colloidal suspensions for characterizing nanoparticles. We focus our analysis on the so-called nanofluids consisting of nanoparticles suspended in a homogenous liquid matrix. Particular attention is paid to the role of the real part of the effective refractive index on sizing the nanoparticles. We then discuss possible ways to measure the real part of the effective refractive index of nanofluids and precautions needed.

Recently a multiple-scattering model for the reflectivity of a disordered monolayer of scattering particles on a flat surface was put forth [J. Opt. Soc. Am.29, 1161 (2012)]. The approximate theoretical model provides relatively simple formulas for the reflectivity, but it was developed for a monodisperse monolayer. Here we extend the model to the case of a polydisperse monolayer and derive the appropriate formulas to calculate the optical transmissivity of the monolayer supported by a flat interface. A second objective of this paper is to test the approximate theoretical model against experimental data with highly scattering particles. We prepared monolayers of three different surface coverage fractions of polydisperse alumina and titanium dioxide particles deposited randomly on a glass slide. We measured their optical reflectivity and transmissivity versus the angle of incidence using a laser with a wavelength of 670 nm. Using the nominal values for the particles' most probable radius and refractive index, we fitted the theoretical model to the experimental curves and found that it reproduces very well the experimental curves. Interestingly, a dip in the reflectivity curves at large angles of incidence is present for the alumina monolayers but not in the titanium dioxide monolayers. The dip corresponds to a maximum in the scattering efficiency by the alumina monolayers. The theoretical model reproduces very well this behavior.

We investigate the spectral response of capacitive sensors with 28 μm thick Polyvinylidene Fluoride (PVDF) films operating in the piezoelectric mode. We present spectra of signals obtained from laser-induced photoacoustic emissions in several materials. We examine the sensor response to direct laser pulses and to ultrasonic signals generated by laser pulses interacting with polyvinyl alcohol (PVA) phantoms, neoprene slabs and a composite of PVA phantom with a hidden slab of neoprene. We exhibit the sensor's sensitivity to the phantom thickness, affecting the amplitude and bandwidth of the ultrasonic output signal. The sensors fabricated and tested under water achieved an operational frequency bandwidth ranging from 1 to 50 MHz.

Polyaniline (PANI) is a π-conjugated polymer that shows electric and optical property changes, when it is in contact with oxidant/reductor chemicals. In particular, the interaction between the conductive form of PANI, the emeraldine salt (ES), and the NH 3 gas results in a decrease of the polaron densities inside the band gap of the organic material. In this work, the optical-transmittance changes detected in PANI coatings as a function of the sensing ammonia gas concentrations are studied by using a nulling optical-transmittance bridge with a 632 nm laser (He–Ne) source, which allows a sensitive monitoring of optical property changes. It is observed that the conductive PANI–polymethyl methacrylate (PMMA) composite coatings are sensitive for very low concentrations of HN 3 (from ∼10 to 4000 ppm, depending on the PANI films’ electrical conductivity), and their optical response is reversible as the reduction gas is removed by N 2 .

We give a simple proof that the electrical capacitance is proportional to an isotropic scaling factor regardless of the shape of the electrodes and of the spatial distribution of electric permittivity around them.

The problem of the formation of capacitance images of periodic rough surfaces with a pointer as one of the electrodes of the capacitor is investigated and the lateral resolution limits are found in terms of the radius of curvature of the Gaussian-shaped pointer. The lateral resolution limit is found to be appreciably less than the radius of curvature at the tip of the pointer. It is found that a simple `local-height' approximation to the capacitance gives good values for image contrast even though this method does not predict the absolute capacitance values accurately. It is also shown that the overall shape of the pointer does not influence the contrast of the capacitance images if the pointer is smooth and rounded. Results of a perturbative solution which includes a spectral window function are presented and compared with numerical results, and good agreement is obtained for cases when the surface is not too rough. Finally, results are presented for a flat-topped pointer and a hole pointer.

We present a formal comparison between dc and ac nulling optical bridges. We consider their performance in monitoring over long periods of time (several minutes), small and slow (in a time scale of seconds) changes in transmittance of an optical component. We consider two fundamentally different ac optical bridges (OB), an amplitude modulated optical bridge (AM-OB), and a switching optical bridge (Sw-OB). For each OB, we derive a general expression for the minimum detectable change in transmittance (|ΔT|min), taking into account all the important noises. It is found that under optimum conditions the dc- and AM-OB have similar detection limits imposed by the 1/f noise of the photodetectors. It is shown that the Sw-OB can in principle overcome the 1/f noise of the detector and approach the shot-noise limit; however, it is sensitive to switching device imperfections (to first order in smallness), which could easily prevent achieving a detection limit below the 1/f noise. It is also shown that the Sw-OB has i...

We consider the electrical capacitance of a parallel-plate capacitor with one rough electrode. The roughness is characterized by its amplitude and its wavenumber scale; the capacitance depends on both of these variables as well as the form of the roughness. We calculate the capacitance in the four limit cases of the two variables mentioned above. On the basis of these limit cases, we approximate the capacitance with elementary functions for the case of any surface roughness. We compare the capacitances calculated in this approach with precise numerical solutions for different forms of roughness.

The nonlinear interaction of noncollinear acoustical waves is considered. Conditions of the resonant backscattering of one wave from the lattice produced by the other two are formulated in analogy with the four-wave mixing known in optics. The efficiency of the phase-matched interaction of acoustical waves is calculated in the resonant approximation for a gas media. Such approximation is constructed on the basis of the expansion of the sound equations preserving up to cubic terms. The amplitude of the backscattered wave is expressed as the product of the efficiency, the amplitudes of three waves, the wave number of the backscattered wave, and the size of the region of interaction. Such backscattering is proposed as an acoustical remote probe. The distance to the interaction region and the amplitude of initial waves are limited by nonlinear degradation of waves due to the second-order nonlinearity. For acoustical waves with wave number 10 m−1, sources of size 1 m, and about 100 m to the interaction region,...

We discuss the possibility of approaching a shot-noise-limited measurement of the difference reflectance due to analyte absorption or adsorption on the surface of a thin film, and estimate the resolution for a variety of cases. When using a 1 mW optical beam, the limit of resolution will commonly correspond to a small or very small fraction of a monolayer of the analyte.

The optical beam deflection method using the knife edge detection technique is one of the simplest optical sensing methods. We investigate theoretically the limit to the resolution when sensing the displacements of a mechanical cantilever using the knife edge detection technique implemented with a quasimonochromatic thermal-like optical source. Simple formulas for the minimum detectable displacement of the cantilever’s front end are derived assuming all nonoptical noises negligible. It is shown that superluminiscent diodes can yield a minimum detectable displacement of the cantilever about 5 dBs above the shot noise for 1 mW of optical power. It is also shown that in many cases of practical interest the source induced noise is smaller than the thermal-vibration noise of the cantilever. Thus, in many cases, using an inexpensive thermal-like source with the knife edge detection technique can be as effective as using a high-coherence shot-noise limited source.

We derive a system of integral equations for the surface charge distribution on the electrodes of a parallel-plate capacitor for which the profile of one electrode is rough. We show that the solution to this system of equations is tractable using a perturbation technique assuming small surface heights compared with the mean plate separation. The accuracies of the first- and second-order perturbative approximations and of the local height approximation are evaluated for a few examples with a one-dimensional rough surface by comparing them with exact numerical results obtained by solving the boundary integral equations directly using a numerical procedure. Some general guidelines for when the first- and second-order approximations will be accurate are given. It is shown that the perturbative formulation provides approximations with a regime of validity that may extend over a larger region than the regime of validity of the local height approximation. These results could be useful for capacitive-sensor design purposes or in modelling solid-state electronic devices.

We present the characterization of polymeric particles in nanofluids with a diameter of tens of nanometers or less. We determine the nanoparticle’s refractive index from extinction measurements and their size using a conventional refractometer.

The theoretical performances of a split detector and a lateral effect detector used as a position sensitive device in the optical beam deflection method are compared. It is shown that the limit to the resolution in both cases is essentially equal. The possibility of improving the resolution in angle deflection by reflecting the beam from a cylindrical mirror, as was recently proposed by A. Yarai, Y. Fukunaga, K. Sakamoto, and T. Nakanishi [Jpn. J. Appl. Phys. 33, 3251 (1994)] is also explored. It is shown that using a cylindrical mirror does not result in a smaller theoretical limit to the resolution. However, it does improve the resolution in some cases and may have practical uses.

We present a new analytical approach for scattering from perfectly conducting surfaces that are rough in one dimension. The method is based on expanding the Helmholtz equation using a set of local basis functions. The transformed equations contain the field and its normal derivatives at the surface as source functions. The first order approximate solution, sought for slightly rough surfaces, is obtained using the local tangent plane approximation at the surface. It is shown analytically that the new method reduces to the first order perturbation method in the proper limit. A systematic series solution is proposed using perturbation theory in conjunction with the present formulation. The second order result is given and analyzed. The results of the present theory are not reciprocal and a reciprocity rule is formulated to overcome this disadvantage. We compare numerically the bistatic scattering cross section for Gaussian-randomly rough surfaces predicted by the first order solution with numerical results a...

In this paper we generalize the local spectral expansion method (previously developed for perfectly conducting rough surfaces) to the scattering from dielectric-dielectric interfaces rough in one dimension. The method for perfectly conducting surfaces is recovered from the present formulation in the limit when one of the dielectric constants increases to infinity. We obtain the first order approximation using the tangent plane approximation. It is shown analytically that the present approximation reduces to first order perturbation theory in the appropriate limit. On the other hand, since the only approximation in our formulation is the tangent plane approximation, the present results are expected a priori to coincide with the Kirchhoff approximation when this is accurate. A numerical comparison for Gaussian random rough surfaces with other first order theories is presented.

We propose a model for scattering from one-dimensional, perfectly conducting, slightly rough surfaces. A possible method for solving the scattering equations is examined which, with some assumptions, suggests the final result. The approximation is relatively simple and is comparable in computational effort with most first-order theories. We compare the bistatic scattering cross section for TE waves predicted by the present model for Gaussian randomly rough surfaces with numerical simulations and with some first-order theories. The comparison shows that the model is remarkably accurate for slightly rough surfaces and TE polarization.

The immense computational power of today's processors cannot be fully utilized unless appropriate sensors and actuators are developed to link them with the outside world. A method is presented here to compare different sensors with each other and to evaluate their relative merits. On the basis of physical considerations, sensors are decomposed into two essential building blocks: a sensor shell and a sensing mean. A figure of merit is defined and 11 different types of sensors, having the same sensing shells, are analyzed and compared with each other.

We studied the limits of optical reflectance to detect plasmonic nanoparticles (NPs) embedded in air and supported by a glass substrate, in an internal reflection configuration. We used a recently derived multiple-scattering model for the coherent reflectance of a disordered monolayer of particles supported on a flat surface to calculate the sensitivity of optical reflectance variations to the presence of plasmonic NPs. We considered gold and silver NPs with radii up to 50 nm and studied the sensitivity as a function of the angle of incidence, wavelength, particle size, and polarization of light. Using our own measurements of noise, we estimated the minimum detectable surface-coverage by the particles. The highest sensitivity is found around the critical angle between the glass substrate and air, for transverse-magnetic polarization, and at wavelengths of light near the plasmon resonance of the particles. We provide estimates of the minimum number of particles per unit area detectable from reflectivity variations and set the basis for single particle detection.

We study the reflection of diffuse light from 1D randomly rough dielectric interfaces. Results for the reflectance under diffuse illumination are obtained by rigorous numerical simulations and then contrasted with those obtained for flat surfaces. We also explore the possibility of using perturbation theories and conclude that they are limited for this type of study. Numerical techniques based on Kirchhoff approximation and reduced Rayleigh equations yield better results. We find that, depending on the refractive index contrast and nature of the irregularities, the roughness can increase or decrease the diffuse reflectance of the surface.

The use of a multimode optical fiber in sensing displacements of the order of 0.1Ahas been reported. It has been shown that for very small movements induced at the center of the free-standing portion of the fiber, the behavior of the sensor is linear. The movement is caused by the vibration of an actuator that is in contact with the center of the free-standing portion of the fiber. The authors, however, caution that the expected nonlinearity of the sensor upon large vibrations restricts the general applicability in displacement sensing. In this paper, we use the same sensor assembly and show how a related nonlinear behavior can be taken care of by a neural network, pointing a way to obviate this limitation. Thus, the use of a neural network in an integrated-optics sensor for the first time has been a very useful start, showing that a sensor to measure displacements of 0.1Aresolution is now feasible. The speckle configuration depends not only on the amplitude of vibration of the actuator, but also on the mean position of the fiber. It is found that the output of a photodetector, placed so as to measure a small region of the speckle configuration, follows a nonlinear function as the mean position of the fiber is varied. We have trained a neural network to learn this nonlinear function. The neural network trained for displacement values within a 1.5 μm range is found to generalize with less than ± 0.015 μm error in the input range. It is found that a functional-link net with 11 functional links and 4 neurons can be trained for a convergence criterion of 5 × 10 −6 in less than 1000 iterations. The error in the individual targeted output during training is less than ±0.01 μm.

A new mode of operation for a high-multimode optical-fiber sensor based on the detection of modal interference is described. The main mechanism on which the sensor is based is coupling between different modes due to curvature induced upon bending the fiber. In contrast to previous work, the variation of the length and the average longitudinal strain of the fiber under such curvatures may be neglected in this work. A sensor of this kind has been implemented and shows a resolution of 10 −11 m in displacement and 10 −10 N in force. Such a high resolution in this type of sensor has not been reported previously. The ‘spring’ constant of the sensor is relatively small (5.6 N/m) compared to that of similar sensors. A theoretical analysis of the transduction mechanism is given and some guidelines to understanding its performance are discussed. The potential for this sensor to perform as the sensing element of an atomic force microscope is discussed.

We present the viability of obtaining the particle size and surface coverage in a monolayer of polystyrene particles adsorbed on a glass surface from optical coherent reflectance data around the critical angle in an internal reflection configuration. We have found that fitting a CSM to optical reflectivity curves in an internal reflection configuration around the critical angle with a dilute random monolayer of particles adsorbed on the surface can in fact provide the particle's radius and surface coverage once the particles are sufficiently large., Conference Date: Sunday 24 May 2009 Conference Location: Quebec, Canada Conference Title: Photonics North 2009 Conference Chairs: Real Vallee

We review the main concepts of the effective‐medium approach for the calculation of the optical properties of colloidal systems composed by small colloidal particles and then pose the corresponding problem for turbid colloids, that is, systems with colloidal particles as big as the incident wavelength. Several attempts to properly define and to calculate the effective electromagnetic response of turbid colloidal systems are presented with special emphasis on its non‐local (spatially dispersive) character and consequently on the problem of how to measure the corresponding effective index of refraction. For this purpose an alternative spectroscopy based on light refraction is proposed.

We derive a perturbative series solution to the capacitance between two parallel electrodes with irregular profiles. The coefficients in the series are calculated using fast Fourier transform algorithms resulting in a very fast method. The applicability of the perturbative series solution is extended by introducing a spectral window function which can make the series converge in cases where the standard series does not converge. We show that the filtered perturbative solution is applicable to surface profiles with surprisingly large features. However, limitations on its applicability to surfaces with high spatial frequencies remain. Perturbation theory could be a powerful tool for simulating surface-profile images obtained by scanning a capacitive probe.

We describe a fast method for measuring the refractive-index dispersion of transparent and absorbing liquid solutions. The method is based on measuring the reflectance spectra of an optical beam of white light in an internal reflection configuration near the critical angle defined by the refractive indices of the incident medium (a glass prism) and that of the solvent, which is assumed to be transparent. From a few reflectance spectra taken near the critical angle with the pure solvent and the solution, the dispersion of the contribution to the refractive index of the solute (the sample) can be obtained. We present results with solutions of Rodamine 6G dissolved in distilled water and in methanol showing the feasibility of the proposed method.

"We discuss and analyze a practical methodology for the determination of the dielectric constant of a macroscopic solid sample in a nondestructive way. The technique consists in measuring the capacitance between a pointer electrode and the dielectric surface as a function of the separation distance in a scale comparable to the radius of curvature of the tip's apex. The changes in capacitance that must be measured will commonly be in the atto-farad scale and require specialized instrumentation which we also describe here. The technique requires two calibration standards and the sample needs to have a portion of its surface flat and some minimum dimensions, but otherwise it can have an arbitrary shape. We used a simple model based on the method of images to explain the methodology and present experimental results with the proposed methodology."

Using a non-local effective-medium approach, we analyze the refraction of light in a colloidal medium. We discuss the theoretical grounds and all the necessary precautions to design and perform experiments to measure the effective refractive index in dilute colloids. As an application, we show that it is possible to retrieve the size of small dielectric particles in a colloid by measuring the complex effective refractive index and the volume fraction occupied by the particles.

We present a passive optical ranging method consisting of three photometric measurements from the light reflected by an object. The measurements are taken from a single position using a CCD camera and different apertures.

We investigate the effect of the divergence of a Gaussian laser beam on the resonance curve and the sensitivity of optical sensors based on surface-plasmon resonance (SPR). For He–Ne laser beams it is found that, for beams with a waist radius of less than 300 µm, the SPR-curve characteristics differ appreciably from the case in which a plane wave is considered. Simple expressions for the sensitivity of (bio)chemical sensors are given. A simple Lorentzian model is used to estimate the maximum possible sensitivity when a multilayer system is used to enhance the resonance peak. It was found that the sensitivity can reach its highest value when the width of the SPR curve is equal to the laser-beam divergence. The results could be particularly important when a SPR curve is used to measure the absolute value of the refractive index of a sample or the dielectric constant and the thickness of a metal layer.

We propose a new model for the design of optical‐integrated (OI) sensor, composed of a surface waveguide covered with a sensitive material (SM) deposited at the surface in the form of thin films. Commonly these last are metallic oxides which becomes highly absorbent in the presence of the gas to be detected. In this way, the optical signal propagating in the waveguide will be attenuated proportionally to the amount of the gas in the surrounding environment. When the sensing is carried out, commonly it is supposed that completely all the volume of the SM increase its absorbance, however this assumption causes that the transmission losses be overestimated; our model, according to the adsorption kinetics of gases, considers the growth of the adsorber layer over the SM as a fourth layer where the optical changes happen. We use this model to design an ammonia NH3 sensor considering tungsten trioxide WO3 as SM. Our model approaches in 102 dB/cm the calculus of the losses, in agreement with that observed in expe...

In this paper, we develop a method to determine refractive index changes and its dispersion of diluted solutions by analyzing the refraction of light by a hollow prism containing the medium to be characterized. The system uses a double prism scheme to compensate the dispersion of the medium and the output beam is analyzed with a spectrophotometer resulting in a fast measurement with a simple setup. The system was calibrated using solutions of sucrose in water. We present experimental results of the dispersion of the specific refractive index increment (dn/dC) of sucrose. Finally we discuss the limitations of the method to the characterization of absorbing media.

In this paper we present a passive optical ranging method that consists of taking several photometric measurements from the light radiated by an object and deriving the range from these measurements. This passive ranging device uses an iris of radius a, a lens of radius larger than a, and a photodetector of radius p

A colloid is defined as a system in which a dispersed phase is immersed in a homogeneous one. Here we model a colloidal system as a collection of a large number of identical spheres embedded at random in an otherwise homogeneous matrix. Recently developed effective-medium approach is used for the calculation of electromagnetic response in this model.