Light refocusing with up-scalable resonant waveguide gratings in confocal prolate spheroid arrangements

Resonant waveguide gratings (RWGs) are thin-film structures, where coupled modesinterfere with the diffracted incoming wave and produce strong angular and spectral filtering. The combination of two finite-length and impedance matched RWGs allows the creation of a passive beam steering element, which is compatible with up-scalable fabrication processes. Here, we propose a design method to create large patterns of such elements able to filter, steer, and focus the light from one point source to another. The method is based on ellipsoidal mirrors to choose a system of confocal prolate spheroids where the two focal points are the source point and observation point, respectively. It allows finding the proper orientation and position of each RWG element of the pattern, such that the phase is constructively preserved at the observation point. The design techniques presented here could be implemented in a variety of systems, where large-scale patterns are needed, such as optical security, multifocal or monochromatic lenses, biosensors, and see-through optical combiners for near-eye displays.

Integrated microfluidic system for non-invasive electrophysiological measurements on Xenopus oocytes

Elodie Deborah Dahan

We developed a new non-invasive integrated microsystem for electrophysiological measurements on Xenopus laevis oocytes. The Xenopus oocyte is a well-known expression system for various kinds of ion channels that are of potential interest for drug screening. In the traditional "Two Electrode Voltage Clamp" (TEVC) method, delicate micromanipulation is required to impale an oocyte with two microelectrodes. In our system, a non-invasive electrical access to the cytoplasm is provided by permeabilizing the cell membrane with an ionophore (e.g. nystatin). Unlike for the classical patch-clamp or "macropatch" techniques, this method does not require removal of the fibrous vitelline membrane. Cell handling is significantly simplified, resulting in more robust recordings with increased throughput. Moreover, because only part of the oocyte surface is exposed to reagents, the required volume of reagent solutions could be reduced by an order of magnitude compared to the TEVC method. In a second part of this work, a rapid fluidic exchange system was implemented on-chip to allow recording of fast kinetic events of exogenous ion channels expressed in the cell membrane. Reducing fluidic exchange times of extracellular reagent solutions is a great challenge with these large millimeter-size cells. Fluidic switching is obtained by shifting the laminar flow interface in a perfusion channel under the cell by means of integrated poly-dimethylsiloxane (PDMS) microvalves. Reagent solution exchange times down to 20 ms have been achieved. An on-chip purging system allows to perform complex pharmacological protocols, making the system suitable for screening of ion channel ligand libraries. The fabrication process for this disposable microchip, based on PDMS micromolding, is cost-efficient and simple. Moreover, an innovative integration of agar gel-based reference electrodes was developed. A conductive liquid junction is injected by capillary force filling of suitable microchannels, overcoming the problems encountered with the integration of Ag/AgCl thin film or wire reference electrodes. We tested this new microdevice by performing electrophysiological measurements on oocytes expressing the human Epithelial Sodium Channel (hENaC). Transmembrane currents could be recorded with a large signal-to-noise ratio. The performance of the integrated rapid fluidic exchange system was demonstrated by investigating the self-inhibition of hENaC. Our results show that the response time of this ion channel to a specific reactant is about one order of magnitude faster than estimated with the traditional TEVC technique. We conclude that this new microdevice has high potential for improved electrophysiological investigations with oocytes in the field of pharmacology and toxicology.

EPFL2008

Advances in the characterization of nanowire photovoltaic devices

Dmitry Mikulik

III-V nanowires (NWs) have a great potential for solar energy applications due to their diameter-dependent optical properties, which may enhance absorption of light. In addition, core-shell radial p-i-n structures, in which the direction of light absorption is orthogonal to the carrier collection, can provide efficient carrier collection. The main goal of this thesis is the experimental study of the challenges of NW-based solar cells, related to materials and device fabrication. In the first part of the thesis, we present an analysis of where the electrical losses can be originated. By applying an equivalent circuit analysis approach, we classified them into three main groups: (i) the non-uniformity of NWs which may result in a reduction of the parallel resistance, (i) potential barriers originated at the different materials interfaces in the solar cell structure may result in an increase of the series resistance or addition of a second diode and (iii) surface recombination resulting in the reduction of the open-circuit voltage. In this thesis, we propose separate strategies to characterize and tackle these factors. The electric scheme of a NW-based solar cell consists of an ensemble of p-n junctions connected in parallel. We show how conductive-probe atomic force microscopy, C-AFM, is an essential tool for the characterization and optimization of these parallel-connected NW devices. We demonstrate topography and current mapping of the NW arrays, combined with current-voltage (IV) measurements of the individual NW junctions from the ensemble. Our results provide discussion elements on some of the factors limiting the performance of a NW-based solar cell, such as uniformity and photosensitivity of the individual NW p-n junctions within the array, and thereby a path for their improvement. Besides parallel losses due to uniformity issues, barriers in the carrier collection through the various heterointerfaces composing the device is discussed. To analyze it, we illuminate GaAs NW-based solar cells at different levels of light intensity and extract IV characteristics. This analysis helps to separately study the NW p-n junction response and the series resistance. The high series resistance of the NW-ensemble device can be attributed to the following interfaces: 1) GaAs-ITO, forming a photoactive Schottky diode, which suppresses the p-n junction at high concentrations of light, and 2) Si-GaAs heterojunction, disturbing the flow of majority carriers. Finally, the characterization of surface passivation in high-aspect-ratio nano/micro structures is addressed by electrochemical impedance spectroscopy (EIS). The method is applied to Si micropillars, as a proof-of-concept prior to the application to III-V nanowires. We tested structures passivated by a dielectric layer. The effect of different surface treatments on the interface state density were quantified by the analysis of the capacitance-voltage and conductance-voltage characteristics. This method allows the electrical measurements on rough vertical surfaces, which would otherwise suffer from high gate leakage currents if tested using solid-state metal-insulator-semiconductor scheme. The results and characterization methods, demonstrated in this work, contribute to the overall efforts of the scientific community on how to reveal the main engineering challenges in NW-based solar cells. It thus paves the way to approach the fundamental conversion efficiencies predicted by theory.

EPFL2018

Advances in the characterization of nanowire photovoltaic devices

Dmitry Mikulik

EPFL2018

Integrated microfluidic system for non-invasive electrophysiological measurements on Xenopus oocytes

Elodie Deborah Dahan

EPFL2008