Thiago Menegotto

Simple modeling of plasmon resonances in Ag/SiO2 nanocomposite monolayers.

Normal incidence transmittance and reflectance spectra of sputtered nanocomposite monolayer films of Ag in SiO2, buried and unburied, showed significant redshifted plasmon resonances from 410 to 455 nm, which could be well interpreted with a simple model that starts from the Maxwell Garnett theory and the Kreibig extension of the Drude-Lorentz equation, but with a further extension related to the dipolar interaction between the metal particles distributed on a surface.

Interferometric monitoring of dip coating

Dip-coated films, which are widely used in the coating industry, are usually measured by capacitive methods with micrometric precision. For the first time to our knowledge, we have applied an interferometric determination of the evolution of thickness in real time to nonvolatile Newtonian mineral oils with several viscosities and distinct dip withdrawing speeds. The evolution of film thickness during the process depends on time as t(-1/2), in accordance with a simple model. Comparison with measured results with an uncertainty of +/- 0.007 microm) showed good agreement after the initial steps of the process had been completed.

Optically monitored dip coating as a contactless viscometry method for liquid films

Real-time interferometric monitoring of the dip coating process is applied to the study of properties of flowing liquids. Nonvolatile Newtonian oils are considered, allowing validity of a simple model after the steady state is reached where film physical thickness depends on time as t(-1/2). Measurement of two distinct mineral oil standards, under several withdrawing speeds, resulted in kinematic viscosities of 1.17+/-0.03 and 9.9+/-0.2 S (1S = 1 cm2/s). Agreement of these results with nominal values from the manufacturer suggests that interferometric monitoring of dip coating may become a valuable method for accurate, contactless viscometry of liquid films. Advantages and present limitations are discussed.

Double optical monitoring of dip coating with a time-varying refractive index

A brief overview of optical monitoring for vacuum and wet-bench film-deposition processes is presented. Interferometric and polarimetric measurements are combined with regard to simultaneous monitoring of refractive index and physical thickness in real time. Monitoring stability and accuracy are verified during dip coating with a transparent oil standard. This double optical technique is applied to dip coating with a multicomponent zirconyl chloride aqueous solution, whose resulting temporal refractive-index and physical-thickness curves indicate good reproducibility as well as significant sensitivity to changes of film-flow properties during the dip-coating process.

Anisotropic effective medium properties from interacting Ag nanoparticles in silicon dioxide

Films containing a layer of Ag nanoparticles embedded in silicon dioxide were produced by RF magnetron sputtering. Optical transmittance measurements at several angles of incidence (from normal to 75°) revealed two surface plasmon resonance (SPR) peaks, which depend on electric field direction: one in the ultraviolet and another red-shifted from the dilute Ag/SiO₂ system resonance at 410 nm. In order to investigate the origin of this anisotropic behavior, the structural properties were determined by transmission electron microscopy, revealing the bidimensional plane distribution of Ag nanoparticles with nearly spherical shape as well as the filling factor of metal in the composite. A simple model linked to these experimental parameters allowed description of the most relevant features of the SPR positions, which, depending on the field direction, were distinctly affected by the coupling of oscillations between close nanoparticles, as described by a modified Drude-Lorentz dielectric function introduced into the Maxwell-Garnett relation. This approach allowed prediction of the resonance for light at 75° incidence from the SPR position for light at normal incidence, in good agreement with experimental observation.

Direct modeling of external quantum efficiency of silicon trap detectors

It is shown the feasibility of direct fitting of external quantum efficiency for silicon trap detectors which are applied as radiometric transfer standards at several National Institutes of Metrology. The model considers the internal quantum efficiency and the reflectance of the detector, whose parameters are fitted in the measured data of external quantum efficiency. The advantage of the suggested approach is the possibility of pursuing interpolation of spectral responsivity without loss of physical meaning of the fitted parameters.

Development of a LED based standard for luminous flux

Incandescent lamps, simple artifacts with radiation spectrum very similar to a black-body emitter, are traditional standards in photometry. Nowadays leds are broadly used in lighting, with great variety of spectra, and it is convenient to use standards for photometry with spectral distribution similar to that of the measured artifact. Research and development of such standards occur in several National Metrology Institutes. In Brazil, Inmetro is working on a practical solution for providing a led based standard to be used for luminous flux measurements in the field of general lighting. This paper shows the measurements made for the developing of a prototype, that in sequence will be characterized in photometric quantities.

Characterisation of optical filters for broadband UVA radiometer

Optical filters were characterized in order to know its suitability for use in broadband UVA radiometer head for spectral irradiance measurements. The spectral transmittance, the angular dependence and the spatial uniformity of the spectral transmittance of the UVA optical filters were investigated. The temperature dependence of the transmittance was also studied.

Surface plasmon resonances of interacting metal nano-particles in thin films: extension of Maxwell Garnett theory

Effective medium theory for composite films was extended to include interaction between neighbor particles. Application to an Ag nanoparticles/silica film agreed well with measured transmittances around surface plasmon resonances.

Simple Modeling of Nanocomposite Optical Coatings with Ag in SiO 2 Monolayers

Optical properties of sputtered nanocomposites with Ag in SiO2 monolayers were compared to simple modeling based on Kreibig extension of Drude-Lorentz and Maxwell Garnett theory. Agreement was reached around the surface plasmon resonance region.