Transmission coefficient | EPFL Graph Search

The transmission coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuities is considered. A transmission coefficient describes the amplitude, intensity, or total power of a transmitted wave relative to an incident wave. Overview Different fields of application have different definitions for the term. All the meanings are very similar in concept: In chemistry, the transmission coefficient refers to a chemical reaction overcoming a potential barrier; in optics and telecommunications it is the amplitude of a wave transmitted through a medium or conductor to that of the incident wave; in quantum mechanics it is used to describe the behavior of waves incident on a barrier, in a way similar to optics and telecommunications. Although conceptually the same, the details in each field differ, and in some cases the terms are not an exact analogy. Chemistry In chemistry, in particular in transition state theory, th

Colliders and conformal interfaces

Marco Meineri, Antonin Tancrède Amaury Rousset

We set up a scattering experiment of matter against an impurity which separates two generic one-dimensional critical quantum systems. We compute the flux of reflected and transmitted energy, thus defining a precise measure of the transparency of the interface between the related two-dimensional conformal field theories. If the largest symmetry algebra is Virasoro, we find that the reflection and transmission coefficients are independent of the details of the initial state, and are fixed in terms of the central charges and of the two-point function of the displacement operator. The situation is more elaborate when extended symmetries are present. Positivity of the total energy flux at infinity imposes bounds on the coefficient of the two-point function of the displacement operator, which controls the free-energy cost of a small deformation of the interface. Finally, we study out-of-equilibrium steady states of a critical system connecting two reservoirs at different temperatures. In the absence of extended symmetries, our result implies that the energy flux across an impurity is proportional to the difference of the squared temperatures and controlled by the reflection coefficient.

SPRINGER2020

On design strategies for binary dielectric metasurfaces based on the Fourier modal method

Kévin Müller

Binary dielectric metasurfaces are arrays of sub-wavelength structures that act as a thin layer of artificial material. They are generally lossless and relatively simple to fabricate since only a single structuring step is required. By carefully designing the metasurface, the phase, amplitude and the polarization of the incident light can be controlled at will. In practice, fabrication constraints and the limited choice of materials reduce what can be done with metasurfaces. But a wide range of functionalities can be implemented with the proper design techniques and knowledge. This thesis contributes to both. The modes are key to understand the phenomena occurring inside a metasurface. To facilitate the analysis of the modes, the Poynting operation, which is related to the Poynting vector, is introduced. We describe how this operation can be used to reformulate the boundary condition in order to estimate the reflection and transmission coefficients with reduced knowledge on the modes involved, and to orthonormalized the modes. The Fourier modal method, which is the method used for the rigorous simulation of metasurfaces in this work, is improved in order to facilitate the access to valuable information that can be used to better understand the phenomena occurring inside a metasurface, and to speed up the design and optimization process. This method computes the eigen-modes present in the metasurface. To better analyze them, the eigen-modes are orthonormalized using the Poynting operation. We show that most of the modes can be filter out in order to simulate a metasurface with different thicknesses in milliseconds. From the analysis of the modes propagating in metasurfaces, two types of metasurface are identified: single-mode metasurfaces and multi-mode metasurfaces. For single-mode metasurfaces, we provide design techniques that translates the desired response into internal properties of the metasurfaces. For multi-mode metasurfaces, the concept of self-coupling mode is developed. We show that, based on this concept, the angular and spectral response of a metasurface can be interpolated safely with a few simulations even if high-Q resonances are present. For both types of metasurfaces, examples of design are provided. Gradient-based optimization methods allow to obtain the optimal metasurface in a few iterations, but the derivative of the merit function is necessary. The adjoint method computes the functional derivative of the merit function with respect to the permettivity and permeability. We provide the equations of the adjoint method and apply them to diffractive optical elements such that they can be used in conjunction with the Fourier modal method. This thesis contributes to design challenges for complex electromagnetic problems,and it does not only provides concepts, but also the tools to put them into operation.

EPFL2020

Energy-selective neutron imaging for materials science

Steven Luc X Peetermans

Common neutron imaging techniques study the attenuation of a neutron beam penetrating a sample of interest. The recorded radiograph shows a contrast depending on traversed material and its thickness. Tomography allows separating both and obtaining 3D spatial information about the material distribution, solving problems in numerous fields ranging from virtually separating fossils from surrounding rock to water management in fuel cells. It is nowadays routinely performed at PSI¿s neutron imaging facilities. Energy-selective neutron imaging studies the wavelength-dependency of the cross-section by using a beam of reduced wavelength bandwidth instead of averaging out the cross-section over the incident beam spectrum. The range of observed contrasts/image information is than extended and can largely be understood in the context of the Bragg law. Different types of monochromator (mechanical neutron velocity selector, double crystal monochromator, filter materials) are characterized for use in neutron imaging. In polycrystalline samples, sharp Bragg edges are observed as coherent elastic scattering at the (hkl) plane can occur for all wavelengths up to 2dhkl, after which a sharp increase in transmission intensity is observed. Much like diffraction peaks, they contain information on e.g. crystal phase or projected strain. The absence of coherent elastic scattering past the last Bragg edge (Bragg cut-off) allows for quantification. In samples with few grains or even single crystals, all orientations w.r.t. the beam are no longer present and rather than Bragg edges, the cross section now exhibits distinct peaks, the ensemble of which holds information on the crystallite¿s phase, orientation and shape. A spatial variation in contrast appears across the sample, between those grains fulfilling the Bragg condition ¿ scattering and decreasing the transmitted beam intensity ¿ and those that do not. After initial qualitative assessments, recent advances on the quantitative grain orientation mapping are made based on time-of-flight measurements of high energy resolution recorded at the ISIS pulsed neutron source. But where do these scattered neutrons go to? A new set-up was developed to permit simultaneous transmission and diffractive neutron imaging. Capturing the neutrons diffracted by a grain also yields a projection of that grain, with the position on the detector indicative of the orientation. These projections can in turn be used for algebraic reconstruction, which yields a grain volume as well. After feasibility studies on an iron single crystal cube the recent push towards polycrystalline samples will is illustrated with a neutron diffraction contrast tomography (nDCT) of a coarse-grained aluminium strain sample.

EPFL2015