Papers by Alexandre Mayer
A genetic algorithm with randomly shifted gray codes and local optimizations based on quadratic approximations of the fitness
Proceedings of the Genetic and Evolutionary Computation Conference Companion, 2017
We present a Genetic Algorithm that we developed in order to address computationally expensive op... more We present a Genetic Algorithm that we developed in order to address computationally expensive optimization problems. In order to accelerate this algorithm, we establish, generation after generation, quadratic approximations of the fitness in the close neighborhood of the best-so-far individual. We then inject in the population an individual that corresponds to the optimum of this approximation. We also introduce a modified mutation operator that acts on randomly-shifted Gray codes. We show that these techniques lead to the global optimum of typical benchmark problems in 5, 10 and 20 dimensions with a probability of success in one run of the order of 95--97% and an average number of fitness evaluations of the order of 400--750x n, where n refers to the dimension of the problem.
When nanophotonics meet thin crystalline-silicon photovoltaics
11-18 sept. 2015International audienceno abstrac
Physical and Chemical News
The transfer-matrix methodology is used to solve linear systems of differential equations, in sit... more The transfer-matrix methodology is used to solve linear systems of differential equations, in situations where the solutions of interest are in the continuous part of the energy spectrum. The technique is actually a generalization in three dimensions of methods used to obtain scattering solutions in one dimension. Using the layer-addition algorithm allows one to control the stability of the computation and describe efficiently periodic repetitions of a basic unit. The paper provides a pedagogical presentation of this technique. It also describes in details how the band structure associated with an infinite periodic medium can be extracted from the transfer matrices characterizing a single basic unit. The method is applied to the calculation of the transmission and band structure of electrons subject to cosine potentials in a cylindrical wire. The simulations show that bound states must be considered because of their impact as sharp resonances in the transmission diagram and to obtai...
Photonic structuring of perovskite solar cells using opal-like sub-units
Physics and Simulation of Optoelectronic Devices XXIX, 2021
Perovskite solar cells are called to revolutionize the field of optoelectronic due to their intri... more Perovskite solar cells are called to revolutionize the field of optoelectronic due to their intrinsic high absorption. Photonic structuration is widely reported as an efficient way to improve light harvesting. Nevertheless, little is known on the combination of photonic structuring and perovskite material. In this study, photonically-structured TiO2 is considered as photoanode layer for perovskite solar cells in a will to enhance light absorption through the excitation of quasi-guided modes within the photoactive perovskite material, while optimizing the charge collection and the global efficiency in the photovoltaic assemblies. Consequently, the photo-active layer is structured using opal-like perovskite layers (monolayers, bilayers or trilayers) made of perovskite (full or truncated) spheres, including hybrid uniform/structured layers, embedded in a TiO2 matrix. We present both numerical simulations and experimental results.
Proceedings of 1st International Electronic Conference on Applied Sciences, 2020
Conclusion: In this work based on the numerical modelling of atomistic deposition by reactive spu... more Conclusion: In this work based on the numerical modelling of atomistic deposition by reactive sputtering of AZO on a structured substrate, a full characterization process was performed to estimate the electrical and optical properties of a crystalline silicon solar cell using this AZO coating as a front transparent conductive oxide. Such study pointed the high impact of the substrate pattern on those physical properties, especially the decrease of the electrical conductivity. Moreover, a global optimization by genetic algorithm was used to enhance the efficiency of such multi-layered structured solar cells.
For many applications in image analysis, learning models that are invariant to translations and r... more For many applications in image analysis, learning models that are invariant to translations and rotations is paramount. This is the case, for example, in medical imaging where the objects of interest can appear at arbitrary positions, with arbitrary orientations. As of today, Convolutional Neural Networks (CNN) are one of the most powerful tools for image analysis. They achieve, thanks to convolutions, an invariance with respect to translations. In this work, we present a new type of convolutional layer that takes advantage of Bessel functions, well known in physics, to build Bessel-CNNs (B-CNNs) that are invariant to all the continuous set of possible rotation angles by design.
Supplementary document for Genetic-algorithm-aided ultra-broadband perfect absorbers using plasmonic metamaterials - 5438026.pdf
Supplemental document
Vietnam Journal of Science and Technology, 2018
We present a Genetic Algorithm that we developed to address optimization problems in optical engi... more We present a Genetic Algorithm that we developed to address optimization problems in optical engineering. Our objective is to determine the global optimum of a problem ideally by a single run of the genetic algorithm. We want also to achieve this objective with a reasonable use of computational resources. In order to accelerate the convergence of the algorithm, we establish generation after generation a quadratic approximation of the fitness in the close neighborhood of the best-so-far individual. We then inject in the population an individual that corresponds to the optimum of this approximation. We also use randomly-shifted Gray codes when applying mutations in order to achieve a better exploration of the parameter space. We provide automatic settings for the technical parameters of our algorithm and apply it to typical benchmark problems in 5, 10 and 20 dimensions. We show that the global optimum of these problems can be determined with a probability of success in one run of the ...
Perovskite solar cells (PSC) have been under the spotlight of the photovoltaics community since t... more Perovskite solar cells (PSC) have been under the spotlight of the photovoltaics community since the past decade due notably to high instrinsic absorption of pervoskite. However, little is known on the impact of structuring the active material using photonic crystal layers. We present here numerical simulations showing the effect of photonic crystal structuring on the integrated quantum efficiency of perovskite solar cells. The photo-active layer is structured using opal-like perovskite layers (monolayers, bilayers or trilayers) made of perovskite (full or truncated) spheres, including hybrid uniform/structured layers, embedded in a TiO 2 matrix. Fano resonances are exploited in order to enhance the absorption, especially near the electronic bandgap of perovskite material. The excitation of quasiguided modes inside the absorbing spheres increases the integrated quantum efficiency and the photonic enhancement factor. A genetic algorithm approach allows us to determine the optimum structure among more than 1.4 10 9 potential combinations. These numerical results of the benefits of photonic structuring on perovskite solar cells are also compared to experimental studies on selected configurations of perovskite solar cells.
Optimization by a genetic algorithm of pyramidal structures made of one, two or three stacks of metal/dielectric layers for a quasi-perfect broadband absorption of UV to near-infrared radiations
We use a genetic algorithm to optimize 2-D periodic arrays of truncated square-based pyramids mad... more We use a genetic algorithm to optimize 2-D periodic arrays of truncated square-based pyramids made of successive stacks of metal/dielectric layers. The objective is to achieve a quasi-perfect broadband absorption of normally incident radiations with wavelengths comprised between 420 and 1600 nm. We compare the results one can obtain by considering (i) Ni, Ti, Al or Cr for the metal, and (ii) poly(methyl methacrylate) (PMMA) or TiO2 for the dielectric. The structures considered consist of only one, two or three stacks of each metal/dielectric combination. The absorption spectrum of these structures is calculated by a Rigorous Coupled Waves Analysis method. A genetic algorithm is then used to determine optimal values for the period of the system, the lateral dimensions of each stack of metal/dielectric and the width of each dielectric. The results show that Ni/PMMA represents the best metal/dielectric combination. With an optimized structure made of only three stacks of Ni/PMMA, it is...
Recent Progress in Computational Sciences and Engineering (2 vols), 2006
We present a model for the calculation of the polarization properties of fullerenes and carbon na... more We present a model for the calculation of the polarization properties of fullerenes and carbon nanotubes. This model describes each atom by both a net electric charge and a dipole. Compared to dipole-only models, the consideration of electric charges enables one to account for the displacement of free electrons in structures subject to an external field. It also enables one to account for the accumulation of additional charges. By expressing the electrostatic interactions in terms of normalized propagators, the model achieves a better consistency as well as an improved stability. In its most elementary form, the model depends on a single parameter and provides an excellent agreement with other experimental/theoretical data. Compared to dipole-only models, the technique improves the calculation of the local fields. It also quantifies the role of free charges in the polarization of fullerenes and carbon nanotubes.
Journal of Physics: Condensed Matter, 2000
A numerical technique is presented which enables the propagation of wave functions in a three-dim... more A numerical technique is presented which enables the propagation of wave functions in a three-dimensional complex potential-energy distribution, as required for modelling electron 'absorption'. The technique is implemented in a transfer-matrix and Green's-function general procedure to simulate field-emission and electronic projection microscopy. In particular, simulated observations of a small carbon fibre by projection microscopy reveal the effects of absorption on the final images. In the situation considered, absorption is responsible for the reinforcement of the external fringes compared to those located well inside the geometrical projection. In agreement with experimental figures and two-dimensional simulations, it turns out that absorption has to be accounted for to explain the shape of projected images, even if the sample has a low level of opacity.
Applied Physics Letters, 2014
The cooling effect of field emission from an n-type semiconductor was theoretically investigated ... more The cooling effect of field emission from an n-type semiconductor was theoretically investigated in quest for a solid state cooler. The vacuum potential was exactly expressed in terms of the semiconductor cathode geometry. This leaded to the more accurate configuration-dependent calculations of the energy exchange and the cooling power. It has been shown that a sharper tip of semiconductor can yield either a larger field emission current density or a larger energy exchange, according to the applied bias. For an atomic size tip, the n-Si cathode yielded the cooling power density C ¼ 2.0, 75, and 713 W/cm 2 at temperature T ¼ 300, 600, and 900 K, respectively. This implies that an optimized configuration of an n-Si cathode produces a significant electron emission cooling, especially at high temperatures. V
Optics Express, 2021
Complete absorption of electromagnetic waves is paramount in today's applications, ranging from p... more Complete absorption of electromagnetic waves is paramount in today's applications, ranging from photovoltaics to cross-talk prevention into sensitive devices. In this context, we use a genetic algorithm (GA) strategy to optimize absorption properties of periodic arrays of truncated square-based pyramids made of alternating stacks of metal/dielectric layers. We target ultra-broadband quasi-perfect absorption of normally incident electromagnetic radiations in the visible and near-infrared ranges (wavelength comprised between 420 and 1600 nm). We compare the results one can obtain by considering one, two or three stacks of either Ni, Ti, Al, Cr, Ag, Cu, Au or W for the metal, and poly(methyl methacrylate) (PMMA) for the dielectric. More than 10 17 configurations of geometrical parameters are explored and reduced to a few optimal ones. This extensive study shows that Ni/PMMA, Ti/PMMA, Cr/PMMA and W/PMMA provide high-quality solutions with an integrated absorptance higher than 99% over the considered wavelength range, when considering realistic implementation of these ultra-broadband perfect electromagnetic absorbers. Robustness of optimal solutions with respect to geometrical parameters is investigated and local absorption maps are provided. Moreover, we confirm that these optimal solutions maintain quasi-perfect broadband absorption properties over a broad angular range when changing the inclination of the incident radiation. The study also reveals that noble metals (Au, Ag, Cu) do not provide the highest performance for the present application.
Light Scattering Spectroscopy Combined with Principal Component Analysis for Animal Species Identification in Historical Parchments
Light scattering spectroscopic data collected from historical parchments were processed according... more Light scattering spectroscopic data collected from historical parchments were processed according to different principal component analysis schemes, leading to reliable parchment optical fingerprints. This method enabled animal species identification without resorting to molecular level analysis.
Rectenna Solar Cells
We present a method for optical rectification that has been demonstrated both theoretically and e... more We present a method for optical rectification that has been demonstrated both theoretically and experimentally and can be used for the development of a practical rectification device for the electromagnetic spectrum including the visible portion. This technique for optical frequency rectification is based, not on conventional material or temperature asymmetry as used in MIM or Schottky diodes, but on a purely geometric property of the antenna tip or other sharp edges that may be incorporated on patch antennas. This "tip" or edge in conjunction with a collector anode providing connection to the external circuit constitutes a tunnel junction. Because such devices act as both the absorber of the incident radiation and the rectifier, they are referred to as "rectennas." Using current nanofabrication techniques and the selective Atomic Layer Deposition (ALD) process, junctions of
Applied Sciences
Materials consisting of single- or a few atomic layers have extraordinary physical properties, wh... more Materials consisting of single- or a few atomic layers have extraordinary physical properties, which are influenced by the structural defects. We present two calculation methods based on wave packet (WP) dynamics, where we compute the scattering of quasiparticle WPs on localized defects. The methods are tested on a graphene sheet: (1) We describe the perfect crystal lattice and the electronic structure by a local atomic pseudopotential, then calculate the Bloch eigenstates and build a localized WP from these states. The defect is represented by a local potential, then we compute the scattering by the time development of the WP. (2) We describe the perfect crystal entirely by the kinetic energy operator, then we calculate the scattering on the local defect described by the potential energy operator. The kinetic energy operator is derived from the dispersion relation, which can be obtained from any electronic structure calculation. We also verify the method by calculating Fourier tran...
Cooling effects of field emission from thermoelectric materials
2010 8th International Vacuum Electron Sources Conference and Nanocarbon
ABSTRACT This paper presents cooling effect of field emission from PbTe thermoelectric semiconduc... more ABSTRACT This paper presents cooling effect of field emission from PbTe thermoelectric semiconductor cathode. The cooling effect of field emission comes from the energy difference between the outgoing field electron and the incoming replacement electron.
We present a genetic algorithm that we developed in order to address computationally expensive op... more We present a genetic algorithm that we developed in order to address computationally expensive optimization problems in optical engineering. The idea consists of working with a population of individuals representing possible solutions to the problem. The best individuals are selected. They generate new individuals for the next generation. Random mutations in the coding of parameters are introduced. This strategy is repeated from generation to generation until the algorithm converges to the global optimum of the problem considered. For computationally expensive problems, one can analyze the data collected by the algorithm in order to infer more rapidly the final solution. The use of a mutation operator that acts on randomly-shifted Gray codes helps the genetic algorithm escape local optima and enables a wider diversity of displacements. These techniques reduce the computational cost of optical engineering problems, where the design parameters have a finite resolution and are limited ...
arXiv: Mesoscale and Nanoscale Physics, 2018
We test the consistency with which Simmons' model can predict the local current density obtai... more We test the consistency with which Simmons' model can predict the local current density obtained for at metal-vacuum-metal junctions. The image potential energy used in Simmons' original papers had a missing factor of 1=2. Besides this technical issue, Simmons' model relies on a mean-barrier approximation for electron transmission through the potential-energy barrier between the metals. In order to test Simmons' expression for the local current density when the correct image potential energy is included, we compare the results of this expression with those provided by a transfer-matrix technique. This technique is known to provide numerically exact solutions of Schrodinger's equation for this barrier model. We also consider the current densities provided by a numerical integration of the transmission probability obtained with the WKB approximation and Simmons' mean-barrier approximation. The comparison between these different models shows that Simmons' ex...
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Papers by Alexandre Mayer