Réseau de diffraction | EPFL Graph Search

Un réseau de diffraction est un dispositif optique composé d'une série de fentes parallèles (réseau en transmission), ou de rayures réfléchissantes (réseau en réflexion). Ces traits sont espacés de manière régulière, et l'espacement est appelé le « pas » du réseau. Si la distance entre plusieurs traits est de l'ordre de grandeur de la longueur de cohérence spatiale de la lumière incidente, le réseau permet d'obtenir des figures de diffraction particulières influencées par la répétition. Il s'agit donc d'un effet de diffraction lié à la répétition d'une structure optique, distinct de l'effet issu de la diffraction par une structure de taille comparable à la longueur d'onde, comme une fente de Young. Lorsque la lumière blanche est incidente sur un réseau, celui-ci décompose la lumière sous différents angles, selon ses longueurs d'onde (ou couleurs) constitutives. Ce phénomène se produit de manière similaire à un prisme (voir image). Les réseaux sont donc utilisés dans de nombreuse

Accurate, Stable and Efficient Modal Calculations of Photoelectrically Useful Absorption in Lamellar Metallic and Semiconductor Diffraction Gratings

David Isaak Gablinger

Solar energy has seen tremendous advances in the past years. For thin film photovoltaics, which use less of the expensive semiconductor materials, insufficient light absorption can be a limiting factor. It is hoped that by using diffractive optics to improve the light absorption, the cost per Watt could sink. Correspondingly, the optics of such structures need to compensate for the low absorption by high (structural) resonance, which is challenging to calculate. To estimate optimal structures, a numerical method should be able to assess feasible structures with widely varying geometries quickly. Modal methods allow for an efficient analysis of structures with varying height through the separation of eigenvalue and boundary value problem. First, the thesis aspires to further develop the modal methods for the calculation of optical properties of layered structures containing weakly absorbing metals and semiconductors. Second, the thesis aims to calculate absorption enhancements in idealized, prototypical structures by applying the newly developed methods. The calculations should only depend on material parameters and not contain additional assumptions. These absorption enhancements are not tied to a priori assumptions such as mode couplings, but they solely follow the physics of the structure investigated. The first part of the thesis is concerned with the methodical improvements. A first emphasis is put on studying peculiar properties of the eigenvalue problem, and on new developments of methods to solve it within a layer. Furthermore, it shows several variants for the numerical implementation of the eigenvalue problem. This part includes a new method to calculate the eigenvalues that can be adapted to two dimensional grating problems of arbitrary shape. The new method integrates the eigenvalue problem by making use of a two point trapezoidal formula, and satisfies the boundary condition between different materials exactly. It is energy conserving and the rate of convergence depends on the approximation order. The eigenvalues show a monotonic convergence that allows for extrapolation. The second methodical emphasis is placed on variants of the implementation of the boundary value problem that connects the grating to the incoming and outgoing plane waves. This algorithm describes the propagation of the incident energy to the semiconductor layer and the substrate by solving a non-recursive and numerically stable system of linear equations. A novel variant reduces the bandwidth of the corresponding matrix by a third. The third part of the thesis concerns calculations using the improved methods. First, the improved calculations are verified by showing that the energy conservation of the modal method, as well as the well-behavedness of the condition number of the calculation. Next, numerical results for the new methods are compared to results from the literature for analytic modal methods, and a comparison with existing software is made. Thereafter, the interface plasmons occuring for H polarization are investigated. In the last part of the thesis, calculations are made for the material specific absorption of light in metallic gratings covered by semiconductors, with a special interest in the absorption in the semiconductor. Here, the spectra for rectangular, sinusoidal gratings, and asymmetric gratings are calculated, and the absorption improvement is investigated through an analysis of the involved modes.

EPFL2016

Thermal-mechanical detector array with integrated diffraction grating

Onur Ferhanoglu

An uncooled thermal detector array with low NETD is designed and fabricated using MEMS bimaterial structures. A diffraction grating is embedded on the pixel membrane for sensing sub-nm mechanical deflections. The first order reflected light was focused on a CCD camera to monitor the entire array. Results show that it is possible to achieve

Ieee Service Center, 445 Hoes Lane, Po Box 1331, Piscataway, Nj 08855-1331 Usa2007

Accurate, Stable and Efficient Modal Calculations of Photoelectrically Useful Absorption in Lamellar Metallic and Semiconductor Diffraction Gratings

David Isaak Gablinger

EPFL2016