Concept# Onde longitudinale

Résumé

En physique ondulatoire, une est une onde dont la perturbation du milieu se fait dans la même direction que sa propagation. Lorsque ces directions sont orthogonales, l'onde est dite transversale.
Exemples
Les ondes sonores sont des ondes sphériques longitudinales. La grandeur qui varie est la pression, et l'air est le milieu perturbé.
Les ondes sismiques P sont également sphériques et longitudinales. Elles sont créées par des séismes ou des explosions.
Voir aussi

- Onde électromagnétique

Source officielle

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Publications associées

Chargement

Personnes associées

Chargement

Unités associées

Chargement

Concepts associés

Chargement

Cours associés

Chargement

Séances de cours associées

Chargement

Publications associées (46)

Chargement

Chargement

Chargement

Concepts associés (26)

Son (physique)

vignette|upright=1|Propagation d'ondes sphériques de pression dans un fluide.
Le son est une vibration mécanique d'un fluide, qui se propage sous forme dondes longitudinales grâce à la déformation él

Onde

vignette|Propagation d'une onde.
Une onde est la propagation d'une perturbation produisant sur son passage une variation réversible des propriétés physiques locales du milieu. Elle se déplace avec un

Longueur d'onde

La longueur d’onde est une grandeur physique caractéristique d'une onde monochromatique dans un milieu homogène, définie comme la distance séparant deux maxima consécutifs de l'amplitude.
La longue

Cours associés (54)

Personnes associées (7)

PHYS-114: General physics: electromagnetism

The course first develops the basic laws of electricity and magnetism and illustrates the use in understanding various electromagnetic phenomena.

PHYS-753: Dynamics of astrophysical fluids and plasmas

The dynamics of ordinary matter in the Universe follows the laws of (magneto)hydrodynamics. In this course, the system of equations that describes astrophysical fluids will be discussed on the basis of selected astrophysical examples, from the physics of stars, to galaxies and the early Universe.

EE-445: Microwaves, the basics of wireless communications

This course is an introduction to microwaves and microwave passive circuits. A special attention is given to the introduction of the notion of distributed circuits and to the scattering matrix

Collective spin excitations can propagate in magnetically ordered materials in the form of waves. These so-called spin waves (SWs) or magnons are promising for low-power beyond-CMOS information processing, which does not rely anymore on the lossy movement of electric charges. SWs in the few GHz frequency regime possess nanoscale wavelengths about five orders of magnitude smaller than electromagnetic waves of the same frequency. This property makes SWs ideally suited for application in microwave technology, essential for on-chip processing of wireless telecommunication signals. In this thesis, three crucial challenges relevant for the technological application of SWs are addressed:
First, to functionalize SWs and exploit their small wavelengths, it is necessary to control them at the nanoscale. Here, periodically nanostructured materials, denoted magnonic crystals, are promising, as they allow to tailor the band structure of SWs. We report on SWs propagating in a prototypical one-dimensional magnonic crystal consisting of dipolarly coupled magnetic nanostripes. The remanent magnetization of individual stripes was designed to be bistable along the long axis. By magnetizing an individual stripe in opposite direction to the others, we created a magnetic defect. We measured by means of all-electrical spin wave spectroscopy and Brillouin light scattering microscopy phase and amplitude of SWs trespassing the defect. We found that SW phases and amplitudes were modified at the nanoscale, and phase shifts could be tuned by an applied bias magnetic field. Using micromagnetic simulations, we identified specific bias fields for which phase shifts of Pi are achieved without suppressing SW amplitudes. This result is highly relevant for the implementation of logic gates based on interference of phase-controlled SWs. We further measured propagation of short-waved SWs in an antiferromagnetically ordered one-dimensional magnonic crystal, where every second stripe was magnetized in opposite direction. We found a band gap closing at the Brillouin zone boundary when no magnetic bias field was applied. Our observations are promising for reprogrammable microwave filters capable of adjusting stop- and passband.
Second, we address how long-waved electromagnetic waves can be coupled efficiently to nanoscale SWs. We demonstrate by space- and time-resolved scanning X-ray transmission measurements, that excited nanogratings allow to transfer their reciprocal lattice vector and multiple of it to an underlying magnetic thin film, in which nanoscale propagating SWs are launched. Additionally, we discovered a second method for short-waved SW generation based on magnetic microwave guides. This approach is easy to fabricate and relies on the adaption of the SW wavelength to a changing effective magnetic field. Efficient coupling of electromagnetic waves to nanoscale SWs promises an unprecedented miniaturization of microwave components.
Third, we found that the magnetization direction of bistable nanomagnets can be switched by propagating SWs in an underlying magnetic thin film when a threshold amplitude is reached. This discovery is promising for the realization of a non-volatile magnonic memory, which stores SW amplitudes. A possible application are SW logic gates, which encode the outcome of a logic operation in the output SW amplitude. Magnonic memory would allow for storing these amplitudes directly, without requiring lossy conversion into the electrical domain.

Séances de cours associées (115)

Unités associées (7)

Printed circuits in bounded media encompass a wide range of practical structures such as discontinuities in waveguides, planar circuits embedded in shielded multilayered media or even two-dimensional printed periodic structures. The Electromagnetic (EM) modeling of printed circuits in layered bounded media is performed via an Integral Equation (IE) technique. Green's functions (GFs) are specially defined to satisfy both the Boundary Conditions (BCs) imposed by the layered media and by the transverse boundary enclosing the entire structure. Finally, a system of IEs on the equivalent sources can be solved numerically by means of the Method of Moments (MoM). Each of the problems enumerated above has already been solved by other authors using IE-MoM techniques. Nevertheless, our formulation introduces a unified approach applicable to all the aforementioned problems. Due to the symmetry presented by a bounded layered media, the GF problem can be reduced into a two-dimensional transverse boundary problem and a one-dimensional transmission line problem in the normal direction. Both problems can be treated independently. This thesis proposes and fully develops an efficient technique that encompasses different laterally bounded multilayered problems with a seamless transition between them. The method is based on a modal representation of the transverse boundary problem and on the expansion of the equivalent surface currents by zero-curl & constant-charge Basis Functions (BFs). It offers a unified and versatile approach that, on one hand eliminates redundancy in the formulation and on the other hand simplifies each particular problem to the evaluation of constant coefficients or basic line integrals. Analytical solutions can be found for the combination of linear subsectional basis functions in rectangular and circular Perfect Electric Conductor (PEC) boundaries as well as for periodic lattices. This thesis then solves the problem of transmission line model in the longitudinal direction by proposing an efficient algorithm that guarantees numerical stability under a variety of known critical conditions where other already known formulations fail. In addition, it introduces alternate equivalent expressions of this formulation that allow new interpretations of the problem. Due to its practical interest, the method is applied for the EM modeling of multilayered boxed printed circuits. This motivated the implementation of a dedicated software tool for the efficient analysis of these topologies including losses. Extensive numerical experiments have been carried out to assess the validity of the aforementioned theory and some properties of test-structures (losses, mesh, etc).

This project has been done on the development of a monolithic fiber-based electric field sensor, with a focus on establishing a reliable, cost-effective, and scalable production cycle to be implemented initially at a pilot and consequently at an industrial scale. The small scale and the low cost of these electrically passive sensors would enable their application where applying an external electric current is either forbidden (e.g. by the ATEX directive), or would distort the electric field and thus limit the possibility of monitoring sensitive assets. This electric field sensor works on the principles of electrostatic induction and translates the electric field to the displacement of an array of mirrors and holes suspended over a second array of mirrors. By shining the rays of light onto the suspended array, it would be possible to monitor the variations of the reflected light due to the displacement of the suspended array over the second array which is deposited on a transparent substrate. At the first phase of the project, by utilizing the COMSOL Multiphysics simulation software, the optimal parameters of the electric sensor were simulated. Based on the results of the simulations, a general process flow was drafted and received the approval from the experts at the EPFL Center of Micro/Nano Technology (CMi). Based on the finite element simulations, in order to maximize the displacement of the sensor along the electric field, it has been shown that reducing the thickness of the sensor would significantly increase the longitudinal displacement of the sensor. Additionally, in order to further decrease the stiffness of the springs connecting the suspended mass to the frames, the structural material of the sensor was selected to be Copper, which also has a high electrical permittivity and low fabrication cost. Next, the micro fabrications based on the optimized design and the process flow were carried out at the CMi ISO 5 cleanroom facilities. In order to adapt the process flows to the capabilities and the availability of the machines, several process flows have been tested and the result of which have all been documented in this report. Moreover, regarding the limited range of the available choices for having a process flow which is compatible with the CMi facilities, the recommendations for further simplifying the process and decreasing the final production costs are mentioned in this report.

2018