Vertical "III-V" V-Shaped Nanomembranes Epitaxially Grown on a Patterned Si[001] Substrate and Their Enhanced Light Scattering

We report on a new form of III-IV compound semiconductor nanostructures growing epitaxially as vertical V-shaped nanomembranes on Si(001) and study their light-scattering properties. Precise position control of the InAs nanostructures in regular arrays is demonstrated by bottom-up synthesis using molecular beam epitaxy in nanoscale apertures on a SiO2 mask. The InAs V-shaped nanomembranes are found to originate from the two opposite facets of a rectangular pyramidal island nucleus and extend along two opposite < 111 > B directions, forming flat {110} walls. Dark-field scattering experiments, in combination with light-scattering theory, show the presence of distinctive shape-dependent optical resonances significantly enhancing the local intensity of incident electromagnetic fields over tunable spectral regions. These new nanostructures could have interesting potential in nanosensors, infrared light emitters, and nonlinear optical elements.

Comparison of scattering and reflection SFG: a question of phase-matching

Hilton Barbosa De Aguiar, Kailash Chandra Jena, Sylvie Roke, Rüdiger Scheu

We present a comparison between sum frequency scattering (SFS) and reflection mode sum frequency generation (R-SFG). We have used scattering theory to describe both scattering experiments as well as reflection mode experiments. The interfacial vibrational spectrum of nanoscopic oil droplets dispersed in water was probed with SFS as well as with R-SFG. Spectra recorded in phase-matched R-SFG mode and spectra recorded with SFS from the same sample are different, which shows that different interfaces are measured. Scattering spectra at different scattering angles agree with nonlinear light scattering theory. We further present experiments with polymer films aimed at quantifying the comparative strength of R-SFG and SFS experiments.

2012

Optical spectral probing for epithelial tissue characterisation

Philippe Thueler

In this thesis, a new method has been developed, which determines the optical coefficients spectra (absorption and scattering) of small volumes of tissue (1 mm3) by the measurement of spatially resolved reflectance between 480 and 950 nm. Firstly, an algorithm has been developed to determine the optical coefficients form reflectance measurements: It is based on the optical propagation model introduced by Bevilacqua [Bevilacqua 99 JOSA]. This model takes the first two moments of the phase function into account through the γ parameter, allowing to accurately simulate the propagation of photons close to the source by the Monte Carlo method. These simulations are then fitted to the reflectance to obtain the optical coefficients. To obtain any value of absorption, scattering or γ parameter, the simulations are interpolated by 3D B-splines. A complete robustness to noise study of the algorithm has been presented. A theoretical study of the γ parameter in tissue over broad wavelengths has been reported. Tissue was simulated with Mie theory for a fractal distribution of sphere diameters. For such a distribution, the γ spectrum is almost flat, instead of being wavy like in the case of mono-disperse sphere diameter. The correlation of the mean value of γ with the fractal power (or dimension) of the distribution was made and showed an analytical form of hyperbolic tangent. It allowed deepening the interpretation of the γ parameter as being an indicator of the relative number of small scatterers (much smaller than the wavelength, e.g. of Rayleigh type) in comparison of bigger ones (their size being comparable to the wavelength). The assessment of the set-up and the algorithm was made by measuring the reflectance of a solution containing 5 different diameters of polystyrene spheres simulating the scattering and Nigrosin colorant as absorber. It showed a good agreement between measurements and prediction. In order to built a better comprehension of absorption and scattering at the tissue scale, a first part of in vivo investigations has been made on an animal model. The skin of the mouse back was altered with Freund's adjuvant and TPA and the optical coefficients were measured on treated and control sites. The effect of alteration (epidermis and dermis thickening, oil vesicle appearing, inflammatory cells infiltration, etc.) was clearly correlated with optical coefficients variations. The scattering spectrum always showed the better contrast. The absorption spectrum, especially in the haemoglobin peak region (between 550 and 600 nm) also showed significant information, in terms of oxy and deoxy-haemoglobin concentration and saturation. Sensitivity and specificity of the "diagnosis" was evaluated. The study of stomach epithelium in vivo on 29 human subjects was made using a small optical fibre probe (

EPFL2001

Optical Layers for Thin-film Silicon Solar Cells

Peter Cuony

In this work we develop and analyze optical layers for use in Micromorph solar cells, a tandem configuration with an amorphous silicon top cell and a microcrystalline silicon bottom cell. The morphology of the front electrode has a decisive role in maximizing the efficiency of a solar cell. To reach a better understanding of the requirements for the front electrode surface, we present a wide range of morphologies that can be obtained with as-grown rough zinc oxide (ZnO) and post-deposition argon plasma surface treatments. We correlate the morphological parameters to light scattering in transmission and reflection, and identify the inclination angles of ZnO pyramids as the most pertinent parameter for thin-film silicon solar cells. We show that there are no reflection losses at the interface between as-grown rough ZnO and silicon, and we quantify the reflection losses for smoother interfaces. With titanium dioxide, produced by reactive magnetron sputtering, we demonstrate that for flat interfaces reflection losses of up to 7.4% can be canceled at 550 nm wavelength. We show that in microcrystalline silicon cells p-type silicon sub-oxide (SiOx) can also be used to reduce reflection losses, and at the same time can act as field-creating window layer. We develop a wide range of mixed-phase SiOx layers and with energy-filtered transmission electron microscopy we reveal the filamentous nanostructure of films produced with high hydrogen dilution of the precursor gas mixture. A partial decoupling of electrical and optical parameters is achieved for such films, where the transparency and the refractive index are determined mostly by the silicon oxide matrix, while sufficient transverse conductivity for the use in solar cells is maintained by few-nanometer-wide silicon filaments. In thin-film silicon solar cells, p-type SiOx~0.5 shows the best results as a window layer with enhanced transparency, while strongly phosphorous-doped n-type SiOx~1 is best suited for use as an intermediate reflecting layer in tandem cells due to a low refractive index of 1.8 with sufficient transverse conductivity. The tunable resistance of doped SiOx layers can be used at various locations in a Micromorph cell to quench shunts or bad diodes in spatially non-uniform devices. Implementation of SiOx with these three functionalities in Micromorph cells has significantly contributed to increasing the best initial efficiency from 11.8% in 2006 to 13.5% in 2010. While rough ZnO front electrodes and SiOx-based intermediate reflecting layers both help to improve Micromorph cell efficiencies, they render the devices more sensitive to water-vapor-induced degradation. With lock-in thermography measurements we demonstrate that water contamination affects mainly the cell borders resulting in non-uniform distribution of the local open circuit voltage. Finally we show how free-carrier absorption and surface plasmon polariton absorption can reduce the reflectivity of the back reflector, and we present preliminary results aimed toward the realization of an absorption-free back reflector making use of polished ZnO and sputtered silver layers.