Miroir de Bragg | EPFL Graph Search

Le miroir de Bragg, mis au point par William Lawrence Bragg (lauréat du prix Nobel de physique de 1915), est une succession de surfaces planes transparentes d’indices de réfraction différents. Il permet de réfléchir, grâce à des phénomènes d’interférences constructives, la quasi-totalité de l’énergie incidente à une longueur d'onde donnée. Ceci est possible à condition que l’onde incidente soit proche de l’incidence normale. Aucun autre miroir ne peut égaler ce résultat (les pertes diélectriques étant plus faibles que les pertes métalliques pour les longueurs d’onde optiques). Principe Une structure simple de miroir de Bragg est un empilement de plusieurs couches alternant deux indices de réfraction différents. Si l'on considère deux matériaux, l’un d’indice de réfraction faible n_1et l’autre d’indice plus fort n_2, on obtient une réflectivité maximale pour une longueur d’onde dite de Bragg

Evaluation of Cost-Effective Technologies for Highly Efficient Silicon-Based Solar Cells

Luca Gautero

The work underscores the feasibility of highly efficient silicon solar cell structures manufactured with high throughput machines. The main challenge consists in the implementation of more performant structures of intrinsic higher complexity. These structures are meant to be fabricated within similar constraints on time and on cost as for conventional devices of industrial manufacture. Achievements in this direction foster larger economical deployment of this kind of renewable energy. The structure chosen for the implementation of "high efficiency" concepts requires an advanced machining of the silicon substrate. Etching techniques, dielectric coating preparations, and metal to semiconductor contact formation were evaluated for their integration in a complete silicon solar cell fabrication sequence. These sequences were later tested to gain knowledge regarding the effects of the aforementioned advanced processing. Prototypes were created using the fabrication sequence. They were analysed to acquire a further understanding of the advanced processing influences. Statistical interpretation of the data obtained was used to support physical interpretation of the observed phenomena. For one particular implementation of a surface passivation (floating junction passivation) an attempt of modelling aiming to unveil its specific dynamics is reported. A resulting solar cell concept compatible with high throughput equipment achieved a certified efficiency of 18% (VOC = 635 mV, JSC = 37.3 mA/cm2, FF = 76 %) on substrate with a reduced thickness (120 µm). The resulting sequence prepares the back surface with a thermal oxidation for passivation purposes and with a laser technique to locally contact the bulk. The thickness is compatible with the intent of cost reduction through the decrease in material consumption. Other approaches performed on the same substrate achieved 17.1% efficiency (VOC = 617 mV, JSC = 36.1 mA/cm2, FF = 75 %). In this case the passivation was achieved with deposited dielectric layers on the back surface. Furthermore, the investigated dynamics of the floating junction passivation allows for better insight into its traits. The proposed solution has an interesting ratio of efficiency versus costs. The general consequence is a higher market appeal of this particular renewable source on the energy market. The study on the floating junction passivation allows a better exploitation of this particular implementation towards more performant silicon solar cells.

EPFL2010

Advanced optical components for mid-infrared applications

Grégoire Maxime Smolik

The mid-infrared wavelength range, defined from 2.5 um to 25 um is of a high scientific and technological interest. One of its main application field is in spectroscopy, since these wavelengths coincidewith the fundamental rotational-vibrational modes of many molecules. While the typical optical sensing setup is composed of active elements - detectors and radiation sources- it also requires critical passive components such as lenses, filters and an interface with the medium to be probed. To this day, most of the scientific work in mid-infrared has been mostly focused on active elements such as the quantum cascade laser. While very modern detectors and lasers are now available, the experimental setups still rely on classical and bulky passive optics. The work presented in this thesis explores modern optical technologies for the realization of novel passive mid-infrared optical components. The fabrication of an external cavity tunable quantum cascade laser emitting from 7.6 um to 8.1 um is presented. This thesis describes the fundamentals of sub-wavelength structures for optical phase generation. Such structures allow local modifications of the effective refractive index of a dielectric material. Using these concepts, original designs of polarizers, anti-reflective and diffractive micro-optical elements are proposed for high-index dielectrics such as silicon or germanium. In particular, the fabrication and characterization of a novel binary grating in silicon that combines anti-reflective and diffractive functions around 7.85 um is presented. This component is characterized using the developed tunable light source setup. Another type of optical component is presented, based one dimensional photonic crystals. It consists of a ZnSe-YbF3 multi-layer deposited on top of a CaF2 substrate. This element allows to observe the first occurrence of Bloch surface waves in the mid-infrared, using the same tunable laser setup. The evanescent field of a surface-propagating wave can be used to probe the surrounding medium. Such structures are promising novel types of substrates for surface-based sensing applications. In addition, a quantum cascade laser-based setup for the detection of cocaine in liquid is presented. This experiment illustrates the great potential of quantum cascade lasers for compact chemical sensing setups.

EPFL2017

Ultra-thin dielectric layers on metal substrates studied by scanning tunneling microscopy and scanning tunneling spectroscopy

In this thesis, ultra-thin dielectric layers on metal substrates are studied mainly by scanning tunneling microscopy and scanning tunneling spectroscopy. In chapter 2, field emission resonances, i.e. bound states in the potential well between sample and tip with an applied voltage, are used to determine the local work function on a sample with patches of different structural composition. To do so, a simple theoretical model is developed and used to simulate the peak positions of the measured dI/dV spectra. In the present case the sample is a Ag(100) surface covered by up to three monolayers of NaCl. The presented method is also applicable to other systems suitable for STM measurements. Additionally, the surface potential is locally probed in the dipole layer region between domains of different work function. In chapter 3, new molecular structures on silver surfaces are presented, that are formed by a mixture of gases. This mixture contains the small molecules H2, H2O, CO, CO2, and N2. No final conclusion can be drawn on the chemical nature of these structures due to the impossibility of the scanning tunneling microscope to determine chemical species. But already the presence of these structures indicates that there is a multitude of unknown interactions, that are possible between a transition metal surface and even smallest molecules. In chapter 4, the moiré pattern is described that a BC3 film creates on a NbB2(0001) surface. As the growth of the BC3 sheet is incommensurate, a moiré pattern is formed that modulates the apparent height of the atoms on the STM images, i.e. the local density of states. The BC3 layer exhibits a completely different behavior than the closely related graphite layer, because it is transparent for the tunnel current. In the present case, the BC3 layer is visible on the STM images only by its moiré effect.

EPFL2007