Growth dynamics of nanocolumnar thin films deposited by magnetron sputtering at oblique angles.

The morphology of numerous nanocolumnar thin films deposited by the magnetron sputtering technique at oblique geometries and at relatively low temperatures has been analyzed for materials as different as Au, Pt, Ti, Cr, TiO2, Al, HfN, Mo, V, WO3and W. Despite similar deposition conditions, two characteristic nanostructures have been identified depending on the material: a first one defined by highly tilted and symmetric nanocolumnar structures with a relatively high film density, and a second one characterized by rather vertical and asymmetric nanocolumns, with a much lower film density. With the help of a model, the two characteristic nanostructures have been linked to different growth dynamics and, specifically, to different surface relaxation mechanisms upon the incorporation of gaseous species with kinetic energies above the surface binding energy. Moreover, in the case of Ti, a smooth structural transition between the two types of growths has been found when varying the value of the power used to maintain the plasma discharge. Based on these results, the existence of different surface relaxation mechanisms is proposed, which quantitatively explains numerous experimental results under the same conceptual framework.

Kinetic energy-induced growth regimes of nanocolumnar Ti thin films deposited by evaporation and magnetron sputtering.

We experimentally analyze different growth regimes of Ti thin films associated to the existence of kinetic energy-induced relaxation mechanisms in the material's network when operating at oblique geometries. For this purpose, we have deposited different films by evaporation and magnetron sputtering under similar geometrical arrangements and at low temperatures. With the help of a well-established growth model we have found three different growth regimes: (i) low energy deposition, exemplified by the evaporation technique, carried out by species with typical energies in the thermal range, where the morphology and density of the film can be explained by solely considering surface shadowing processes, (ii) magnetron sputtering under weak plasma conditions, where the film growth is mediated by surface shadowing mechanisms and kinetic-energy-induced relaxation processes, and (iii) magnetron sputtering under intense plasma conditions, where the film growth is highly influenced by the plasma, and whose morphology is defined by nanocolumns with similar tilt than evaporated films, but with much higher density. The existence of these three regimes explains the variety of morphologies of nanocolumnar Ti thin films grown at oblique angles under similar conditions in the literature.

Structural control in porous/compact multilayer systems grown by magnetron sputtering.

In this work we analyze a phenomenon that takes place when growing magnetron sputtered porous/compact multilayer systems by alternating the oblique angle and the classical configuration geometries. We show that the compact layers develop numerous fissures rooted in the porous structures of the film below, in a phenomenon that amplifies when increasing the number of stacked layers. We demonstrate that these fissures emerge during growth due to the high roughness of the porous layers and the coarsening of a discontinuous interfacial region. To minimize this phenomenon, we have grown thin interlayers between porous and compact films under the impingement of energetic plasma ions, responsible for smoothing out the interfaces and inhibiting the formation of structural fissures. This method has been tested in practical situations for compact TiO2/porous SiO2 multilayer systems, although it can be extrapolated to other materials and conditions.

Nanocolumnar association and domain formation in porous thin films grown by evaporation at oblique angles.

Porous thin films grown at oblique angles by evaporation techniques are formed by tilted nanocolumnar structures which, depending on the material type and growth conditions, associate along certain preferential directions, giving rise to large domains. This arrangement, commonly denoted as bundling association, is investigated in the present work by performing fundamental experiments and growth simulations. It is proved that trapping processes of vapor species at the film surface, together with the shadowing mechanism, mediate the anisotropic widening of the nanocolumns and promote their preferential coalescence along certain directions, giving rise to domains with different shape and size. The role of these two processes is thoroughly studied in connection with the formation of these domains in materials as different as SiO2 and TiO2.

Assessment of the characterization of nonabsorbing nanoparticles in suspension from effective optical properties.

We analyze a method recently proposed to retrieve the size, refractive index, and concentration of particles in nonabsorbing nanofluids from measurements of the complex effective refractive index of two dilutions of the nanofluid [Opt. Lett.39, 559 (2014)]. The method uses simple formulas to retrieve the particles' parameters. First, we discuss precautions needed with the new method when inferring the refractive index of the particles from measurements of the imaginary part of the effective refractive index of two dilutions of the original nanofluid. Then we analyze the use of this methodology to obtain some average radius in the size of polydisperse suspensions and in the case of suspensions of weakly absorbing particles. We also perform an error analysis considering fixed errors in measurements and calculating the errors in the retrieved size, refractive index, and concentration of particles. Finally, we characterize experimentally nanofluids of polymeric particles fabricated of poly(methyl-methacrylate) (PMMA) and polystyrene (PS) for which we achieved an uncertainty of 5×10(-3) and 1×10(-2) in the determination of the particle refractive index, respectively, and a precision better than 3% in the determination of their radii.

Frequency analysis for an extended photoacoustic transport model.

In photoacoustic imaging, the signal attenuation is a well-known source of artifacts over the image reconstruction. It is recognized that this is caused by optical absorption effects and by the ultrasound broadband scattering. However, the sound dispersion is generally neglected, although it appears notably in thick or heterogeneous tissues. In the present Letter, we give an experimental example in which both attenuation and sound dispersion are dealt with as relevant features to be taken into consideration. An analytic perspective of these perturbations leads us to a waveform transport-model extension that provides a linear description of the induced acoustic effects. We find a near match between the theoretical predictions and the experimental results in the frequency domain. These outcomes approximate projection data that represent forward solutions in photoacoustic image reconstruction.

Refractive index measurement of turbid media by transmission of backscattered light near the critical angle.

We investigate experimentally the determination of the effective refractive index (RI) of a turbid particle suspension from the angle dependence of light scattered by the particles and then transmitted into a transparent prism of higher RI. We assembled a versatile experimental device that may be recognized as an Abbe-type refractometer in which the sample is illuminated from the prism side and use it to measure the intensity profile of diffuse light refracted into the prism around the critical angle. By fitting a recently proposed theoretical model we extract the complex RI of turbid suspensions of particles from the measured intensity profiles. We show that the real part of the effective RI is readily obtained with good precision regardless of how the sample is illuminated, whereas obtaining the imaginary part is done with less precision but nevertheless useful measurements can be obtained. The effective RI obtained with this method compares very well with the so-called van de Hulst effective RI and the one derived from Keller's model of the effective propagation constant.

Insights into the dependent-scattering contributions to the extinction coefficient in highly scattering suspensions.

We study theoretically the extinction of collimated light in random systems of highly scattering particles embedded in nonabsorbing media. We aim to provide rough guidelines on the behavior of the extinction coefficient in the so-called dependent-scattering regime. We base our analysis on Keller's second order perturbative approximation to the effective propagation constant. To gain physical insight, we also analyze a simple model based on the physical notion that particles in a dense system scatter light in an effective medium.

Spectroscopic refractometer for transparent and absorbing liquids by reflection of white light near the critical angle.

We propose and evaluate a spectroscopic refractometer device to measure the refractive index dispersion of transparent and absorbing solutions. The angle-dependent reflectivity of a white beam of light in an internal reflection configuration around the critical angle is spectrally analyzed. The refractive index in a wavelength range from 400 nm to 900 nm is obtained from the angle-reflectivity curve around the critical angle at each wavelength. The device does not use angle scanning mechanisms, decreasing considerably the complexity of the instrument in comparison to previous proposals. As a result, the measurements are obtained relatively fast. Nevertheless, a good experimental resolution in refractive index of about Δn ≈ 10(-4) at all the wavelengths is achieved in the case of transparent solutions. The calibration procedure of the device is discussed in detail. We also present measurements of the refractive index dispersion of rhodamine 6G-methanol solutions, which has a strong absorption band in the visible spectra.

Obtaining the dielectric constant of solids from capacitance measurements with a pointer electrode.

We analyze the determination of the dielectric constant of macroscopic solid samples in a nondestructive way from measurements of the capacitance between a pointer electrode and the sample's surface. We assembled an experimental device and found that an accuracy of 1% or better can be attained with common laboratory instrumentation.

Device for characterization of thermal effusivity of liquids using photothermal beam deflection.

We propose and study a novel optoelectronic device for thermal characterization of materials. It is based on monitoring the photothermal deflection of a laser beam within a slab of a thermo-optic material in thermal contact with the sample under study. An optical angle sensor is used to measure the laser deflection providing a simple and experimental arrangement. We demonstrate its principle and a simple procedure to measure thermal effusivity of liquids. The proposed device could be implemented into a compact sensor head for remote measurements using electrical and fiber optic links.

Reflectivity of a gaussian beam near the critical angle with external optically absorbing media.

One can use the angle-modulated reflectance of a gaussian beam near the critical angle to sense with high resolution the index of refraction of the external medium. We analyze in detail the reflectivity of a gaussian beam near the critical angle and its dependence on the optical absorption of the external medium. The given formulation is relatively simple and is useful in discerning the effects of the various parameters involved on the reflectivity and its differentials with respect to the angle of incidence. The results presented can be readily used for the quantitative design of novel sensors based on modulated reflectance near the critical angle. We provide a simple algebraic expression for the loss of sensitivity of modulated reflectance near the critical angle as the sample's absorption coefficient increases. We find that, in a typical case, the sensitivity has decreased to approximately half its value for transparent samples when the absorption coefficient has increased to 25 cm(-1). We conclude that modulated reflectance near the critical angle remains a competitive technique for monitoring the index of refraction of an external medium with an absorption coefficient of as much as 120 cm(-1). We compared experimentally obtained curves of the first differential of the reflectivity with respect to the angle of incidence with theory and found good agreement.

Sensitivity of optical sensors based on laser-excited surface-plasmon waves.

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 microm, 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.

Measuring optical power transmission near the critical angle for sensing beam deflection.

We investigate an internal-transmission method for measuring microdeflections of an optical beam as a potential tool for the development of new compact and stable optical sensors. We calculate the detection limits of the internal-transmission method when an ideal coherent optical source and an ideal quasi-monochromatic thermallike source are used. The proposed method is compared with an internal-reflection method previously studied. It is found theoretically and verified experimentally that the transmission method may have better resolution than the reflection method. We also compare the calculated sensitivity as a function of the angle of incidence with experimental results for both methods.

Detection limits of an internal-reflection sensor for the optical beam deflection method.

The theoretical detection limit on angle deflection measurement when the quasi-critical internal-reflection method is used is calculated and compared with the more common method of using a bicell position-sensitive detector. Simple formulas for the sensitivity and resolution when the system is shot noise limited are given. It is shown that, even though the bicell detector is potentially much more sensitive for wide and well collimated beams, under typical laboratory restrictions, the internal reflection method may be more sensitive and have better resolution. It is argued that the internal-reflection method may be a tool in developing compact sensors based on the optical beam deflection method.