Institut Langevin, ESPCI Paris - PSL
Tuesday, March 29, 2022
5:15 p.m. — room Jaures (29 rue d’Ulm, 75005 Paris)
The introduction of the concept of time crystal in 2013 by the Nobel Prize winner, Frank Wilczek, has prompted the emergence of a new and fast-growing field of research focused on time-varying materials. In such media, the wave velocity can be varied through the entire medium in times that are short in comparison to the wave period. These media provide new approaches and concepts for wave manipulation, some of which will be presented in this seminar. I will show how it is possible to achieve a time reversal of the waves by suddenly modifying their propagation speed. This concept of instantaneous temporal mirror will be illustrated by experiments performed on waves at the surface of a bath. We will see that it is also possible to realize phase conjugate mirrors that can refocus the waves on the sources that created them. I will also discuss how the frequency of a wave packet can be changed, without the use of non-linear processes, by means of a cascade of spatial and temporal interfaces. Finally, I will discuss how temporal modulations of a medium can dynamically stabilize unstable modes. This last point will be illustrated by experiments showing the stabilization of levitating liquid layers.