arXiv subjects

David Colas

Publications and source records attributed to David Colas.

12 recordsLinked to original sources

arXiv2026

Pulse, polarization and topology shaping of polariton fuids

Here we present different approaches to ultrafast pulse and polarization shaping, based on a ``quantum fluid'' platform of polaritons. Indeed we exploit the normal modes of two dimensional polariton fluids made of strong coupled quantum well excitons and microcavity photons, by rooting different polarization and topological states into their sub-picosecond Rabi oscillations. Coherent control of two resonant excitation pulses allows us to prepare the desired state of the polariton, taking benefit from its four-component features given by the combination of the two normal modes with the two degrees of polarization. An ultrafast imaging based on the digital off-axis holography technique is implemented to study the polariton complex wavefunction with time and space resolution. We show in order coherent control of the polariton state on the Bloch sphere, an ultrafast polarization sweeping of the Poincar\'{e} sphere, and the dynamical twist of full Poincar\'{e} states such as the skyrmion on the sphere itself. Finally, we realize a new kind of ultrafast swirling vortices by adding the angular momentum degree of freedom to the two-pulse scheme. These oscillating topology states are characterized by one or more inner phase singularities tubes which spirals around the axis of propagation. The mechanism is devised in the splitting of the vortex into the upper and lower polaritons, resulting in an oscillatory exchange of energy and angular momentum and in the emitted time and space structured photonic packets.

cond-mat.other↗

arXiv2022

Crossover Between Quantum and Classical Waves and High Frequency Localization Landscapes

Anderson localization is a universal interference phenomenon occurring when a wave evolves through a random medium and it has been observed in a great variety of physical systems, either quantum or classical. The recently developed localization landscape theory offers a computationally affordable way to obtain useful information on the localized modes, such as their location or size. Here we examine this theory in the context of classical waves exhibiting high frequency localization and for which the original localization landscape approach is no longer informative. Using a Webster's transformation, we convert a classical wave equation into a Schr\"{o}dinger equation with the same localization properties. We then compute an adapted localization landscape to retrieve information on the original classical system. This work offers an affordable way to access key information on high-frequency mode localization.

cond-mat.dis-nn↗

arXiv2022

Topologically driven Rabi-oscillating interference dislocation

Quantum vortices are the quantized version of classical vortices. Their center is a phase singularity or vortex core around which the flow of particles as a whole circulates and is typical in superfluids, condensates and optical fields. However, the exploration of the motion of the phase singularities in coherently-coupled systems is still underway. We theoretically analyze the propagation of an interference dislocation in the regime of strong coupling between light and matter, with strong mass imbalance, corresponding to the case of microcavity exciton-polaritons. To this end, we utilize combinations of vortex and tightly focused Gaussian beams, which are introduced through resonant pulsed pumping. We show that a dislocation originates from self-interference fringes, due to the non-parabolic dispersion of polaritons combined with moving Rabi-oscillating vortices. The morphology of singularities is analyzed in the Poincar\'{e} space for the pseudospin associated to the polariton states. The resulting beam carries orbital angular momentum with decaying oscillations due to the loss of overlap between the normal modes of the polariton system.

cond-mat.quant-gas↗

arXiv2022

Coupled quantum vortex kinematics and Berry curvature in real space

The Berry curvature provides a powerful tool to unify several branches of science through their geometrical aspect: topology, energy bands, spin and vector fields. While quantum defects -- phase vortices and skyrmions -- have been in the spotlight, as rotational entities in condensates, superfluids and optics, their dynamics in multi-component fields remain little explored. Here we use two-component microcavity polaritons to imprint a dynamical pseudospin texture in the form of a double full Bloch beam, a conformal continuous vortex beyond unitary skyrmions. The Berry curvature plays a key role to link various quantum spaces available to describe such textures. It explains for instance the ultrafast spiraling in real space of two singular vortex cores, providing in particular a simple expression -- also involving the complex Rabi frequency -- for their intricate velocity. Such Berry connections open new perspectives for understanding and controlling highly-structured quantum objects, including strongly asymmetric cases or even higher multi-component fields.

physics.optics↗

arXiv2021

Shaping the topology of light with a moving Rabi-oscillating vortex

Quantum vortices are the analogue of classical vortices in optics, Bose-Einstein condensates, superfluids and superconductors, where they provide the elementary mode of rotation and orbital angular momentum. While they mediate important pair interactions and phase transitions in nonlinear fluids, their linear dynamics is useful for the shaping of complex light, as well as for topological entities in multi-component systems, such as full Bloch beams. Here, setting a quantum vortex into directional motion in an open-dissipative fluid of microcavity polaritons, we observe the self-splitting of the packet, leading to the trembling movement of its center of mass, whereas the vortex core undergoes ultrafast spiraling along diverging and converging circles, in a sub-picosecond precessing fashion. This singular dynamics is accompanied by vortex-antivortex pairs creation and annihilation, and a periodically changing topological charge. The spiraling and branching mechanics represent a direct manifestation of the underlying Bloch pseudospin space, whose mapping is shown to be rotating and splitting itself. Its reshaping is due to three simultaneous drives along the distinct directions of momentum and complex frequency, by means of the differential group velocities, Rabi frequency and dissipation rates, which are natural assets in coupled fields such as polaritons. This state, displaying linear momentum dressed with oscillating angular momentum, confirms the richness of multi-component and open quantum fluids and their innate potentiality to implement sophisticated and dynamical topological textures of light.

physics.optics↗

arXiv2020

Self-accelerating beam dynamics in the space fractional Schr\"odinger equation

Self-accelerating beams are fascinating solutions of the Schr\"odinger equation. Thanks to their particular phase engineering, they can accelerate without the need of external potentials or applied forces. Finite-energy approximations of these beams have led to many applications, spanning from particle manipulation to robust in vivo imaging. The most studied and emblematic beam, the Airy beam, has been recently investigated in the context of the fractional Schr\"odinger equation. It was notably found that the packet acceleration would decrease with the reduction of the fractional order. Here, I study the case of a general nth-order self-accelerating caustic beam in the fractional Schr\"odinger equation. Using a Madelung decomposition combined with the wavelet transform, I derive the analytical expression of the beam's acceleration. I show that the non-accelerating limit is reached for infinite phase order or when the fractional order is reduced to 1. This work provides a quantitative description of self-accelerating caustic beams' properties.

cond-mat.quant-gas↗

arXiv2019

Finite-energy accelerating beam dynamics in wavelet-based representations

Accelerating beams are wave packets which appear to spontaneously accelerate without external potentials or applied forces. Since their first physical realisation in the form of Airy beams, they have found applications on various platforms, spanning from optics to plasma physics. We investigate the dynamics of examples of finite-energy accelerating beams derived from catastrophe theory. We use a Madelung transformation in momentum-space, combined with a wavelet transformed analysis, to demonstrate that the beams' properties arise from a special type of vanishing self-interference. We identify the modes responsible for the wave packet's acceleration and we derive the general acceleration for higher-order cupsoid-related beams. We also demonstrate how bright solitons resulting from nonlinear Airy beams can be unambiguously detected using the wavelet transform. This methodology will allow for a better understanding of nontrivial wave packet dynamics.

cond-mat.quant-gas↗

arXiv2019

Formation of nonlinear X-waves in condensed matter systems

X-waves are an example of a localized wave packet solution of the homogeneous wave equation, and can potentially arise in any area of physics relating to wave phenomena, such as acoustics, electromagnetism, or quantum mechanics. They have been predicted in condensed matter systems such as atomic Bose-Einstein condensates in optical lattices, and were recently observed in exciton-polariton condensates. Here we show that polariton X-waves result from an interference between two separating wave packets that arise from the combination of a locally hyperbolic dispersion relation and nonlinear interactions. We show that similar X-wave structures could also be observed in expanding spin-orbit coupled Bose-Einstein condensates.

cond-mat.quant-gas↗

arXiv2018

Negative-mass effects in spin-orbit coupled Bose-Einstein condensates

Negative effective masses can be realised by engineering the dispersion relation in a variety of quantum systems. A recent experiment with spin-orbit coupled Bose-Einstein condensates has shown that a negative effective mass can halt the free expansion of the condensate and produce fringes in the density [M. Khamehchi et al., Phys. Rev. Lett. 118, 155301 (2017)]. Here, we show that the observed fringes are due to the negativity of only one of the two effective mass parameters that characterise the dispersion, which leads to previously predicted self-interference of the wave packet. We show how confgurations are nevertheless accessible to spin-orbit coupled Bose-Einstein condensates where both mass parameters controlling the propagation and diffusion of the condensate are negative, resulting in the novel phenomenon of counter-propagating self-interfering packets.

cond-mat.quant-gas↗

arXiv2018

Full-Bloch beams and ultrafast Rabi-rotating vortices

Strongly-coupled quantum fields, such as multi-component atomic condensates, optical fields and polaritons, are remarkable systems where the simple dynamics of coupled oscillators can meet the intricate phenomenology of quantum fluids. When the coupling between the components is coherent, not only the particles number, but also their phase texture that maps the linear and angular momentum, can be exchanged. Here, on a system of exciton-polaritons, we have realized a so-called full-Bloch beam: a configuration in which all superpositions of the upper and the lower polariton -- all quantum states of the associated Hilbert space -- are simultaneously present at different points of the physical space, evolving in time according to Rabi-oscillatory dynamics. As a result, the light emitted by the cavity displays a peculiar dynamics of spiraling vortices endowed with oscillating linear and angular momentum and exhibiting ultrafast motion of their cores with striking accelerations to arbitrary speeds. This remarkable vortex motion is shown to result from distortions of the trajectories by a homeomorphic mapping between the Rabi rotation of the full wavefunction on the Bloch sphere and Apollonian circles in the real space where the observation is made. Such full-Bloch beams offer new prospects at a fundamental level regarding their topological properties or in the interpretation of quantum mechanics, and the Rabi-rotating vortices they yield should lead to interesting applications such as ultrafast optical tweezers.

cond-mat.quant-gas↗

arXiv2015

Self-interfering wavepackets

We study the propagation of non-interacting polariton wavepackets. We show how two qualitatively different concepts of mass that arise from the peculiar polariton dispersion lead to a new type of particle-like object from non-interacting fields---much like self-accelerating beams---shaped by the Rabi coupling out of Gaussian initial states. A divergence and change of sign of the diffusive mass results in a "mass wall" on which polariton wavepackets bounce back. Together with the Rabi dynamics, this yield propagation of ultrafast subpackets and ordering of a spacetime crystal.

cond-mat.mes-hall↗

arXiv2013

VLS-HVPE growth of ultra-long and defect-free GaAs nanowires investigated by ab initio simulation coupled to near-field microscopy

High aspect ratio, rod-like and single crystal phase GaAs nanowires (NWs) were grown by gold catalyst-assisted hydride vapor phase epitaxy (HVPE). High resolution transmission electron microscopy (HRTEM) and micro-Raman spectroscopy revealed polytypism-free zinc blende NWs over lengths of several tens of micrometers for diameters ranging between 50 and 150 nm. Micro-photoluminescence studies of individual NWs showed linewidths smaller than those reported elsewhere which is consistent with the crystalline quality of the NWs. HVPE makes use of chloride growth precursors of which high decomposition frequency, after adsorption onto the catalyst particle, favors a direct and rapid introduction of the Ga atoms from the vapor phase into the catalyst liquid droplet. This yields high axial growth rate (more than 100 micron/h) of NWs. The fast diffusion of the Ga atoms in the droplet towards the interface between the liquid and the solid nanowire was investigated by using density functional theory calculations. The diffusion coefficient of Ga atoms was estimated to be 3x10-9 m2/s, which matches the experimental observations.

cond-mat.mes-hall↗