Gesuri Morales-Luna

Experimental Test of Reflectivity Formulas for Turbid Colloids: Beyond the Fresnel Reflection Amplitudes

We compare light reflectivity measurements as a function of the angle of incidence for an interface between an optical glass and a turbid suspension of small particles, with theoretical predictions for the coherent reflectance calculated with different available theoretical models. The comparisons are made only in a small range of angles of incidence around the critical angle of the interface between the glass and the matrix of the colloidal suspensions. The experimental setup and its calibration procedure are discussed. We considered two Fresnel-based approximations and another two based on a multiple-scattering approach, and we present results for monodisperse latex colloidal suspensions of polymeric spherical particles in water with particle diameters of 120 and 520 nm, polydisperse titanium dioxide (rutile) particles suspensions in water with a most probable diameter of 404 nm, and suspensions of copper particles in water with diameters of 500 nm. The comparisons between experiment and theory are made without fitting any parameters.

Optical reflectivity as an inspection tool for metallic nanoparticles deposited randomly on a flat substrate

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.

Sensitivity of optical reflectance to the deposition of plasmonic nanoparticles and limits of detection

Abstract. 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.

Feasibility of sizing metallic nanoparticles in concentrated suspensions from effective optical properties

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.

Feasibility of measuring the effective refractive index of blood from backscattered light near the critical angle

A recently devised method to measure the effective refractive index of highly turbid media, based on the transmission of backscattered light near the critical angle, is used to measure the refractive index of human blood.