Thibault Congy

Contact Information

Laboratoire Physique Théorique et Modèles Statistiques LPTMS
Université Paris Sud, CNRS, UMR 8626, 91405 Orsay (France)

Mail: thibault.congy at
Phone: +33 (0)1 69 15 74 75

ResearcherID   P-3541-2016

Ma tronche
Ma tronche

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Research activity

keywords: quantum fluids, nonlinear optics, shock wave, instability

Caustic For my PhD project, I studied self-accelerating Airy beams with the experimental group of S. Barad (Tel Aviv University). These nondiffracting auto - accelerating waves have received considerable attention in recent years. We showed that they can form spontaneously as a laser beam propagates in a defocusing nonlinear medium, inside a cylindrical channel with a reflective boundary. The beam forms a ring-shaped optical caustic, which, following reflection from the boundary, converges to a focal point. By means of a semi-classical treatment, we have demonstrated that the radially symmetric wave has an Airy-function profile.
Modulational instability I have also been interested in nonlinear effects in two-component Bose-Einstein condensates in one dimension. Using a muliple scale expansion, we have shown that these condensates experience phenomena similar to those encoutered in fluid mechanics or nonlinear optics. Nonlinear density excitations are well described by a KdV-type equation. In the presence of spin-orbit coupling, excitations of the polarization experience modulational instabilities also known as the Benjamin-Feir instability.
Shock wave In the limit where intra-species and inter-species interaction constants are very close, the dynamics of the density and the polarization waves decouple. The polarization wave-dynamics is governed by the dissipationless Landau-Lifshitz equation. Dispersive shock waves (DSW) can be observed in this system. Thanks to the Whitham theory of waves modulation, we have described the DSW within the Gurevitch-Pitaevskii scheme. The DSW can be seen as a nonlinear periodic wave for which its parameters (amplitude, velocity, ...) vary slightly over one period of space and time.
In 2013 I did a 3 months internship at Trento. Under the supervision of F. Dalfovo, I studied the stability of solitons in two-dimensional Bose-Einstein condensate. Grey solitons in condensate with repulsive interractions undergo a dynamical instability for long wavelength transverse excitations. This phenomenon is called « snake oscillations ».
Numerical simulation of the 2D Gross-Pitaevkii equation for an initial 1D dark-soliton profile (k represents the wavenumber of the transverse excitation)
k=30 (WEBM GIF)   k=60 (WEBM GIF)

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Solution of the Riemann problem for polarization waves in a two-component Bose-Einstein condensate
S. K. Ivanov, A. M. Kamchatnov, T. Congy and N. Pavloff  arXiv: 1709.04193

Evolution of initial discontinuities in the Riemann problem for the Kaup-Boussinesq equation with positive dispersion
T. Congy, S. K. Ivanov, A. M. Kamchatnov and N. Pavloff
Chaos 27, 083107 (2017)   PDF   doi: 10.1063/1.4997052

Dispersive hydrodynamics of nonlinear polarization waves in two-component Bose-Einstein condensates
T. Congy, A. M. Kamchatnov and N. Pavloff
SciPost Phys. 1, 006 (2016)   PDF   doi: 10.21468/SciPostPhys.1.1.006

Nonlinear waves in coherently coupled Bose-Einstein condensates
T. Congy, A. M. Kamchatnov and N. Pavloff
Phys. Rev. A 93, 043613 (2016)   PDF   doi: 10.1103/PhysRevA.93.043613

Spontaneously formed autofocusing caustics in a confined self-defocusing medium
M. Karpov, T. Congy, Y. Sivan, V. Fleurov, N. Pavloff and S. Bar-Ad
Optica 2, 1053 (2015)   PDF   doi: 10.1364/OPTICA.2.001053


The Riemann problem for nonlinear polarization waves in two-component Bose-Einstein condensates
T. Congy, S. K. Ivanov, A. M. Kamchatnov and N. Pavloff
Rencontre du Non-Linéaire 2017   PDF   ISBN: 978-2-9538596-6-9

Autofocusing of cylindrical caustics self-generated in a defocusing nonlinear medium
M. Karpov, Y. Sivan, V. Fleurov, T. Congy, N. Pavloff and S. Bar-Ad
Nonlinear Optics, NLO 2015  PDF   doi: 10.1364/NLO.2015.NF2A.3

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Solver for 1D Partial Derivative Equations.
Language: C++    src: t33b0/pde_rk

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Université Paris-Sud