Research Areas 

  • Experimental physics in Wave Propagation in Complex Media
  • Light-Matter Interaction
  • Multiple Scattering, Anderson Localization
  • Nonlinear and Active Random Media
  • Random Lasers
  • Nonlinear Scattering
  • Instabilities
  • Speckle Statistics
  • Optical Singularities
  • Optics
  • Acoustics
  • Microwaves
Wave propagation in disordered media is a field of research with both fundamental important topics as well as a large range of applications related to imaging through opaque media, an important issue in medical imaging.
                                                              Metamaterials with Flexural Waves METAMATERIALS WITH FLEXURAL WAVES By replacing atoms with man-made elementary structures, new artificial materials may provide novel functionalities. These so-called metamaterials may bend waves in an unusual manner (“negative refraction”), hide objects (“cloaking”), focus light below the diffraction limit

Latest Publications.

Pulse dynamics of flexural waves in transformed plates
Kun Tang, Chenni Xu, Sébastien Guenneau and Patrick Sebbah
Advanced Functional Materials 31, 2009266 (2021)

Dynamics of pulse propagation in the waveshifter.
(a) Sketch of the coordinate mapping, (b) Top view
of the waveshifter. (c) Experimental demonstration
and (d) numerical simulation.

Dynamics of pulse propagation in the rotator.
(a) 3d-printer elastic rotator. (b) Absence of temporal
dispersion. (c) 30o rotation experienced by the elastic
wave within the rotator and wave front recovery
outside the rotator.

Coordinate-transformation-inspired optical devices have been mostly examined in the continuous-wave regime: the performance of an invisibility cloak, which has been demonstrated for monochromatic excitation, is likely to deteriorate for short pulses.
Here, pulse dynamics of flexural waves propagating in transformed plates is investigated. A practical realization of a waveshifter and a rotator for flexural waves based on the coordinate transformation method is proposed. Time-resolved measurements reveal how the waveshifter deviates a short pulse from its initial trajectory, with no reflection at the bend and no spatial and temporal distortion of the pulse.
  Extending the strategy to cylindrical coordinates, a wave rotator is designed. It is demonstrated experimentally how a pulsed plane wave is twisted inside the rotator, while its wavefront is recovered behind the rotator and the pulse shape is preserved, with no extra time delay. The realization of the dynamical mirage effect is proposed, where an obstacle appears oriented in a deceptive direction.

Observations of symmetry induced topological mode steering in a reconfigurable elastic plate
K. Tang, M. Makwana, R. V. Craster, P. Sebbah
Phys. Rev. B 102, 214103 (2020)

We investigate vibrational interface modes on elastic plates, to observe distinctive valley-Hall edge states. We have designed a reconfigurable system using clamped holes to create strong scatterers and interfaces that strongly confine vibration. At the physical scales we use here we can scan the field at points throughout the entire plate and extract the mode shapes. Our experimental observations of topological valley transport for sharp and gentle bends and the topological mode coupling around the gentle bend for flexural waves (see Figure), highlight key differences between sharp and gentle bends and the importance of the relative orientations of inclusion sets on either side of the interface. The direct visualization of the mode patterns through spatial scanning of the wavefield, linked with the underlying principles at the junction cells, sheds light on the transport of energy around bends in partitioned media. This important new observation, also confirmed here by theory and simulation, exposes the coupling required for geometrically distinct modes to move from one interface to another.

Gentle bend and sharp bend geometry in an elastic plate.
Experiment demonstration and numerical simulation of edge states steering

Localized modes revealed in Random Lasers
Bhupesh Kumar, Ran Homri, Priyanka, Santosh Maurya, Mélanie Lebental, and Patrick Sebbah
Reference: Optica 8(8), 1033-1039 (2021)

A collection of obstacles randomly positioned can be sufficient to trap light, without the need for an optical cavity. Add gain/amplification and you obtain -at no cost- a mirrorless laser, often dubbed “random laser”.
Here, we have used this concept to demonstrate disorder-induced localization, a rather difficult wave phenomenon to observe, but also one of the most striking and puzzling manifestations of wave interference, predicted by the Nobel Prize winner P.W. Anderson for electrons and later generalized to light waves. Because this faint phenomenon is amplified in our “plastic microlaser”, we directly observe laser light built up in confined scattering region, each confined mode corresponding to a different emission color/wavelength.
To observe these localized lasing modes individually, we propose a new method to disentangle interacting modes and suppress mutual competition for gain. To do so, a non-uniform gain distribution is created that optimally select one mode and extinguish the other.
Once mode competition is suppressed and a laser mode is optimized, we are able to boost laser power-efficiency, and unleash the “optimally-outcoupled lasing modes”, i.e. the laser modes with the strongest emission for the smallest energy cost.

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