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Couplage spin-electron-phonon-photon-structure

Couplage spin-electron-phonon-photon-structure

Magnétisme dans les architectures magnétiques

Spin disorder versus Exchange bias coupling in magnetic nanoparticles with complex architecture

An important relevant features of the size effect in magnetic nanoparticles (NPs) is the occurrence of non-collinear spin structures (spin canting, magnetic frustration, spin disorder). Indeed, the non-collinear spin structures could strongly modify the magnetic properties of the magnetic NPs, for example as it was shown recently by us which found a strong correlation between complex spin disorder and exchange bias (EB) features in iron oxide hollow NPs. Although the macroscopic model of EB effect existed for nearly six decades, the microscopic origin of this phenomenon is still requiring further investigation in some specific systems at nanoscales. Even in the case of very well-studied and investigated nanomaterials such as spinel ferrites, a full understanding of the correlation between spin structure, surface/interface effect and EB coupling is still lacking. In this context, we propose to investigate this phenomenon in systems presenting high spin disorder like Hollow NPs with different sizes and shell thickness and extended this study to a system of shell/shell that consists of two magnetic phases: a ferrimagnetic phase and an antiferromagnetic one. The idea is to discriminate the EB resulting from surface spin disorder from that resulting from spins interface coupling between two different magnetic phases. Figure below show the Mössbauer spectra for hollow (g-Fe2O4) and shell/shell (g-Fe2O4/NiO) NPs with same external diameter and thickness (15 nm and 7.5 nm). As a preliminary result, we observe that the spin disorder is bigger in the shell/shell than in the hollow one. On the other hand the EB observed in shell/shell NPs is bigger than that observed in hollow NPs. This raises the question about the effect of the antiferromagnetic phase on the spin disorder and the EB coupling in this kind of nanoscale systems ? Finally, In order to avoid the effect of dipolar interaction and study its effects on spin disorder and EB we propose to disperse these systems in non magnetic matrix.

In-field Mössbauer spectra of magnetic NPs with different architectures: γ-Fe2O3, (hollow) and γ-Fe2O3/NiO (Shell/shell)

Multiferroic Materials

Multiferroic materials constitute a class of multifunctional materials presenting in the same time coupled properties in terms of ferromagnetic-ferroelectric and ferroelastic order. On a fundamental scale, the nature of the interactions and in particular the magnetoelastic coupling mechanism is not fully understood despite the amount of experiments. On a technological level, this class of material is interesting in the field of information storage or for microelectromechanical systems development. Among multiferroic materials, BiFeO3 is one is one of the few which presents in the same time both ferroelectric and magnetic order above 300K. However despite this unique property, BiFeO3 also presents a magnetic helicoïdal structure of Fe3+ magnetic moments, this magnetic structure leads to weak magnetoelectric coupling. A drastic increase of magnetoelectric coupling can be obtained through the breakdown of helicoïdal spirale and the formation of an antiferromagnetic linear structure of type G leading to a non-zero mean magnetization.

Multiples ways can be followed to breakdown this helicoidal structure among them:

  1. Reducing the size of the material dimension to a value lower than the cycloid period (~64 nm). This can be obtained by a polyol method for synthetizing BiFeO3 nanoparticles.
  2. Substituting Bi3+ and Fe3+ by Ti4+ and Zr4+ using a ceramic synthesis in order to partially break helicoïdal structure of Fe3+ magnetic moments


MEB picture of BiFeO3 particles obtained with polyol processModification of helicoïdal magnetic order and apparition of ferromagnetic order

Classical Magnetic models : towards a multi-scale approach

Phenomenological magnetic models are able to bridge a gap between the microscopic magnetic properties and local organization of magnetic sites from a first part and the overall magnetic properties of bulk or nanosized materials from a second part.

In the case of Heisenberg model, the magnetic Hamiltonian is governed by few critical parameters, namely the magnetic moments of sites, the exchange energy between neighbors, the volume anisotropy constant  and for finite size objects, the surface anisotropy constant and interface anisotropy constants.

Accuracy of the results emerging from the classical model is then intimately related to the theoretical or experimental determination of the underlying magnetic parameters and microscopic site organization of the system. This level of knowledge is not necessarily fulfilled in the case of complex structures or unusual compounds. Using a numeric multi-scale approach coupling ab-initio, molecular dynamics and Heisenberg simulations, access to a finer level of description. From one part the use of molecular dynamics simulation, permits to relax the structure of nano-sized or bulk systems and then to reproduce the magnetic sites position and distribution in a more realistic way. From another side, adjustments of ab-initio calculations and Heisenberg model on a large statistical sample of representative local environments allow us to determine the  microscopic magnetic parameters.


Left: Influence of dopamine functionalization on magnetic properties. Right: Influence of structure relaxation on magnetic properties

Related papers
  • Brymora and F. Calvayrac, Surface anisotropy of iron oxide nanoparticles and slabs from first principles: Influence of coatings and ligands as a test of the Heisenberg model, Journal of Magnetism and Magnetic Materials, 2017, 434, 14–22.
  • Sayed, Y. Labaye, R. Sayed Hassan, F. El Haj Hassan, N. Yaacoub and J. M. Greneche, Size and thickness effect on magnetic structures of maghemite hollow magnetic nanoparticles, Journal of Nanoparticle Research, 2016, 18, 279.
  • Nehme, Y. Labaye, R. Sayed Hassan, N. Yaacoub and J. M. Greneche, Modeling of hysteresis loops by Monte Carlo simulation, AIP Advances, 2015, 5, 127124.

Rare Earth free permanent magnets

Among currently investigated rare-earth-free magnets, ferromagnetic τ-MnAl is a potential candidate having promising intrinsic magnetic properties.  Mn(Fe)AlC was synthesized by mechanical alloying method. Effects of carbon on microstructure and magnetic properties were systematically investigated. It was found that high purity of τ-MnAl(C) could be obtained at 2 at.% C doping, showing clearly stabilizing effect of carbon. Mn54.2Al43.8C2 has the best magnetic properties: magnetization at 2T M2T = 414 kA.m-1, remanent magnetization Mr = 237 kA.m-1, coercivity HC = 229 k.Am-1, and |BH|max = 11.2 kJ.m-3. HC increased inversely with the crystallite size of τ phase and proportionally with C content. Moreover, first principle calculation showed both stabilizing effect and preferable interstitial positions of carbon in tetragonal   τ-MnAl. Mn51-xFexAl47C2 (x= 0.25, 0.5, 1, 2, 4, 6) alloys were also synthesized by mechanical alloying method, showing the high purity of τ phase up to 2 at.% Fe doping. Adding of Fe on MnAl(C) reduced both magnetization and TC but likely increased slightly HC. 57Fe Mössbauer spectrometry at 300K was used to probe local environment in ε-, τ-, β-, and γ2-MnFeAl(C). In which, γ2-, ε-, and β-MnFeAl(C) exhibited a quadrupolar structure while τ -Mn50.5Fe0.5Al47C2 spectrum showed a rather complex magnetic hyperfine splitting. The interaction between Fe and Mn examined by in-field Mössbauer measurement at 10 K and 8 T showed a non-collinear magnetic structure between Fe and Mn with different canting angles at different sites.  Hyperfine field of MnFeAl alloy calculated by Wien2k supported both magnetic properties and Mössbauer results.

Changes in magnetic moment with substitution. Mössbauer spectra showing ferrimagnetic order. Magnetization curve of substituted phases

Related papers
  • Tang Nguyen, F. Calvayrac, A. Bajorek and N. Randrianantoandro, Mechanical alloying and theoretical studies of MnAl(C) magnets, Journal of Magnetism and Magnetic Materials, 2018, 462, 96–104

Exploring the magnetic disorder in Ultrahin Iron Oxide Hollow Nanoparticules

Magnetic nanoparticles are of special interest in the field of storage information and nanomedicine (hyperthermia, medical imagery, …). Due to the nanometric size of the particles they can be guided using a magnetic field. Unfortunately, reducing the size of the nanomagnets leads to magnetic instability incompatible to technological applications. Modifying the shape of the nanoparticles (core-shell particles, Hollow particles, …) lead to an exaltation of surface/interface effects and an enhancement of the magnetic anisotropy and the apparition of dynamical effect (superparamagnetic). As an example, by use of in-field Mössbauer spectrometry, we have shown the existence of complex non-collinear magnetic structure in iron oxide hollow nanoparticles. Indeed, this structure consists in a ferrimagnetic layer of few atomic planes confined between two layers with canted structure resulting from two antiferromagnetic coupled speromagnetic structures. Such a magnetic structure leads to an increase of the magnetic coupling between the interfacial moments of the magnetic phases. The exchange bias coupling finally tends to increase the magnetic anisotropy of the nanoparticles and in definitive the magnetic stability of the nanostructure.


Properties of Hollow magnetic nanoparticle.

Related papers
  • Sayed, N. Yaacoub, Y. Labaye, R. S. Hassan, G. Singh, P. A. Kumar, J. M. Greneche, R. Mathieu, G. C. Hadjipanayis, E. Agostinelli and D. Peddis, Surface Effects in Ultrathin Iron Oxide Hollow Nanoparticles: Exploring Magnetic Disorder at the Nanoscale, J. Phys. Chem. C, 2018, 122, 7516–7524.
  • Muscas, N. Yaacoub, G. Concas, F. Sayed, R. Sayed Hassan, J. M. Greneche, C. Cannas, A. Musinu, V. Foglietti, S. Casciardi, C. Sangregorio and D. Peddis, Evolution of the magnetic structure with chemical composition in spinel iron oxide nanoparticles, Nanoscale, 2015, 7, 13576–13585.
  • Gaudisson, R. Sayed-Hassan, N. Yaacoub, G. Franceschin, S. Nowak, J.-M. Grenèche, N. Menguy, P. Sainctavit and S. Ammar, On the exact crystal structure of exchange-biased Fe 3 O 4 –CoO nanoaggregates produced by seed-mediated growth in polyol, CrystEngComm, 2016, 18, 3799–3807.
  • Sayed, Y. Labaye, R. Sayed Hassan, F. El Haj Hassan, N. Yaacoub and J. M. Greneche, Size and thickness effect on magnetic structures of maghemite hollow magnetic nanoparticles, Journal of Nanoparticle Research, 2016, 18, 279.
  • Prado, N. Daffé, A. Michel, T. Georgelin, N. Yaacoub, J.-M. Grenèche, F. Choueikani, E. Otero, P. Ohresser, M.-A. Arrio, C. Cartier-dit-Moulin, P. Sainctavit, B. Fleury, V. Dupuis, L. Lisnard and J. Fresnais, Enhancing the magnetic anisotropy of maghemite nanoparticles via the surface coordination of molecular complexes, Nature Communications, 2015, 6, 10139.

Dynamique ultra-rapide photo-induite et optique non linéaire

Simulation of ultrafast phenomena

  • Brice Arnaud
  • Florent Calvayrac

Electron and phonon dynamics in photoexcited solids

Schematic view of relaxation processes in photoexcited silicon

Pump probe experiments like optical pump probe experiments, time resolved X-ray diffraction, or time resolved photoemission experiments are currently used to study electron and phonon dynamics in nanostructures on time scales ranging from a few femtoseconds to a few picoseconds.  Among the aforementioned techniques, optical pump probe experiments allow to study the coherent phonon generation mechanisms or even the possibility to demagnetize a sample with an ultrashort laser pulse. It is often difficult to interpret experimental results without resorting to models whose parameters can be inferred from ab-initio calculations. There are few calculations because of the need to describe non-equilibrium phenomenon occurring on different time scales and length scales. Nonetheless, ab-initio calculations combined with models already shed new light on ultrafast physics, especially on the coherent phonon generation, on the non thermal melting processes or on the energy transfer from the electronic degrees of freedom to the the vibrational degrees of freedoms.


Related papers
  • Isabel González Vallejo,  Geoffrey Gallé, Brice Arnaud, Shelley A. Scott,  Max G. Lagally,  Davide Boschetto, Pierre-Eugene Coulon,  Giancarlo Rizza,  Florent Houdellier,  David Le Bolloc’h,  and Jerome Faure, “Observation of large multiple scattering effects in ultrafast electron diffraction”, Phys. Rev. B 97, 054302 (2018).

TDDFT approaches to compute excited states of finite systems

The PW-TELEMAN project aims at developing an open source (under GPL3) and easy accessible package of real-time TDDFT libraries and codes, based on programs developed over the past 20 years in a Toulouse-Erlangen collaboration which spread to Le Mans and China via former students. At the time being no available code truly accounts for a complete non-adiabatic electron-ions coupling allowing a full follow-up of a whole dynamical scenario from early excitation (fs or sub-fs) to long time response (ps) of a given physical system. To the best of our knowledge, the Toulouse-Erlangen package is one of the single ones allowing such an investigation in a fully time-resolved manner. Still this package requires a strong effort of standardization and documentation and in order to make it more efficient and usable by external groups. More optimization is highly desirable to exploit the latest computer technology such as GPGPU and so to access the much more demanding tasks in organic molecules. It is the goal of the project to succeed in these two complementing directions.

The variety of potential applications is obvious. We mention in particular the very practical aspects concerning irradiation of biological molecules and radiation damage in materials. The plan is to develop our project in two complementing directions: first, implementation and tests of formal developments such as approximations to TDSIC/OEP methods, coupled with an exploration of various dynamical scenarios of increasing complexity (requiring ongoing optimization), starting with free molecules and clusters, turning to systems such as a chromophore cluster embedded in a matrix, ending with a coupled quantum mechanics/molecular mechanics hierarchical modeling of a biophysical problem made possible due to the expected increase in numerical performance and on the other hand standardization, documentation, and publication of a toolbox of routines which will be easily usable by other groups.


Benchmark of PW-Teleman code, Efficiency versus core number for differents architecture optimisation

Related papers
  • Calvayrac, Kullback-Leibler divergence as an estimate of reproducibility of numerical results, in: IEEE, 2015: pp. 1-5. DOI: 10.1109/NTMS.2015.7266501

Electron and phonon ultrafast dynamic in solid exhibiting exotic coupling between charge, spin and structure

  • Vincent Juvé
  • Gwenaëlle Vaudel
  • Thomas Pezeril
  • Mathieu Edely,
  • Brice Arnaud,
  • Rémi Busselez,
  • Pascal Ruello.
  • Coll. B. Dkhil, C. Paillard, SPMS Centrale Supelec, J. Szade, K. Balin, Institute of Physics, Katowice (Poland). Laboratoire International Associé France-Japon (LIA-IM-LED)

Ultrafast photostriction in ferroelectrics and multiferroics

The control of the photoinduced coherent acoustic phonon in ferroelectric and multiferroic compounds have been investigated in order to investigate the electron-phonon coupling and the coherent acoustic phonon detection processes. This research has permitted to reveal an original ultrafast optical-light mode conversion in BiFeO3 and LiNbO3. We have shown that it was possible to switch the light polarization from the ordinary to the extraordinary component (and vice-versa) with coherent acoustic phonon and this, up to hundreds of GHz (10ps). This mechanism is based on the modulation of the birefringence of the uniaxial crystal (BiFeO3, LiNbO3) induced, through the acousto-optic effect, by the coherent acoustic phonons (left panel in the figure below) [1]. This work has been selected by INP CNRS as a “highlight” [2]. This research is still going on with a focus on new possible way to generate acoustic phonon [3] (transformation of light energy into mechanical energy) with an ANR project UP-DOWN (IMMM leader) that has started in October 2018. The extension of the coupling between coherent acoustic phonons with the magnetic order and in particular with antiferromagnons will be investigated in the ANR SANTA (SPEC-CEA Leader) that has started in October 2018 as well.


Ultrafast light-mode conversion in birefringent ferrolectric materialsUltrafast electron and phonon dynamics in Topological insulators

Related papers
  • Ultrafast acousto-optic mode conversion in optically birefringent ferroelectrics, Mariusz Lejman, Gwenaelle Vaudel, Ingrid C Infante, Ievgeniia Chaban, Thomas Pezeril, Mathieu Edely, Guillaume F Nataf, Mael Guennou, Jens Kreisel, Vitalyi E Gusev, Brahim Dkhil, Pascal Ruello, Nature comm. 7, 12345 (2016)
  • Photothermal optomechanics, P Ruello, Nature Photonics 10 (11), 692 (2016).

Ultrafast electron and phonon dynamics in topological insulators

In this work we have investigated ultrathin films of topological insulator (4-15nm). We have first studied the influence of the substrate and the microstructure (single versus polycrystalline structure on the femtosecond pulse-induced electron and phonon dynamics. While the optical phonons (zero group velocity) have frequency and damping dynamics nearly unaffected, the acoustic phonon signature (propagating wave) is clearly different revealing the importance of acoustic phonon scattering on grains boundaries and at interface between the nanometric film and the substrate [1]. In a second study, we have in particular measured the dynamic of hot carriers in Bi2Te3 compound and have shown the existence of a strong quantum confinement appearing for thickness as thin as 5-6nm. A dramatic enhancement of the hot carriers relaxation is observed (right panel in the figure above). Such quantum confinement is also revealed in a thickness dependence of the process of generation of coherent acoustic phonons and in the anomalous damping of a Raman active mode A1g mode, together with a slight A1g mode softening. These phenomena evidence a drastic evolution of the electron-phonon coupling in a highly confined system. We have proposed with a simple model, that the band gap increases with decreasing the thickness is an indication of the increase of the electron-longitudinal acoustic phonon deformation potential. A more advanced theoretical work is now required to extract a more precise microscopic picture. Nevertheless, these experimental results are important to tailor the proper thickness of such material for future integration in more complex devices where for example spin-to-charge conversion will be investigated.

Related papers
  • Ultrafast light-induced coherent optical and acoustic phonons in few quintuple layers of the topological insulator Bi2Te3, M Weis, K Balin, R Rapacz, A Nowak, M Lejman, J Szade, P Ruello, Physical Review B 92 (1), 014301 (2015)
  • Quantum size effect on charges and phonons ultrafast dynamics in atomically controlled nanolayers of topological insulators Bi2Te3, M. Weis, B Wilk, G Vaudel, K. Balin, R. Rapacz, A Bulou, B Arnaud, J Szade, P Ruello, Sci. Rep. 7, 13782 (2017)
  • French-Polish project, Bourse Ambassade de France (M. Weis), thèse co-tutelle (Le Mans, Insttut de Physique Katowice).

Ultrafast terrahertz spectroscopy

  • Gwenaëlle Vaudel
  • Vasily Temnov
  • Pascal Ruello
  • Thomas Pezeril
  • Vincent Juvé.
  • Coll. JY CHauleau, M. Viret, CEA-SPEC, J. Szade Institute of Physics Katowice (Poland), Laboratoire International Associé CNRS, France-Japon (LIA-IM-LED)

THz polarization modulation of visible light by THz pulses

Over the last decade, ultrafast Terahertz spectroscopy has gained tremendous attention thanks to the development of high-power ultrafast laser systems, which allowed generating intense single-cycle picosecond pulses of electric field at THz frequencies. Their relatively long optical cycle period (1 ps for 1 THz) and high electric field (from hundred of kV/cm to few MV/cm) provide a new tool for studying fundamental aspects of light-matter interactions. Field-resolved detection of ultra-short THz pulses is well known since many years and the most common technique is based on free-space electro-optic sampling. This leads to the polarization change of an optical pulse, which is detected by the polarization sensitive scheme, and to the field-resolved detection of a THz pulse. Here we evidenced efficient non-resonant and noncollinear χ(2)-type type light-matter interaction in femtoseconds polarization sensitive time-resolved optical measurements. Such nonlinear optical interaction of visible light and ultra-short THz pulses leads to THz modulation of visible light polarization in bulk LiNbO3 crystal. Theoretical simulations based on the wave propagation equation capture the physical processes underlying this nonlinear effect [1].


Left: A femtosecond optical pulse and a THz pulse, delayed in time respect to each other, are overlapping in a LiNbO3 crystal. Right: Non-resonant and noncollinear χ(2)-type light-matter interaction leads to a rotation Δθ of the optical pulse polarization at THz frequencies

Related papers
  • [1] V. Juvé, G. Vaudel, Z. Ollmann, J. Hebling, V. Temnov, V. Gusev and T. Pezeril, Optics Letters 43 (2018)

Ultrafast THz emission by spin to charge current conversion (ferroic materials and topological insulators)

 Topological insulators (Bi2Se3, Bi2Te3 etc…) are a new class of material, which is stated to be at the core of the next generation of electronics devices. Their structure consists of a bulk small bandgap insulator together with spin polarized surface states, which can lead to surface spin current. Studying and controlling the surface spin current is of interest for the scientific community. Using ultrafast THz spectroscopy, with photon energy smaller than the band gap, one can try to have access to the surface states dynamics and, thus, to the spin dynamics. High quality samples, which are well characterized of Bi2Te3, are provided by the University of Silesia in Poland [1].


Left: Simplified band structure of topological insulators, which consists of a combination of a small band gap together with spin-polarized Dirac cones. Right: Relative change of the transmission induced by a THz pulse (2 meV photon energy) measured with a 3.1 eV photon energy for a Bi2Te3 sample of 15nm thickness.


We also have started to investigate the emerging field related to spin-to-chareg conversion (vice-versa). spin bursts can be produced by direct excitation of hot electrons in a ferromagnetic layer triggered by a femtosecond laser pulse. During the relaxation processes the diffusion lifetime and mobility differ for majority and minority spin carriers, resulting in the emission of angular momentum on a timescale of picoseconds. This effect, combined with Inverse Spin Hall Effect in an adjacent non-magnetic layer (Pt), was recently used to make efficient THz generation devices [2]. Based on this principle, we have already carried out preliminary measurement with CoFeB on Pt by shining femtosecond laser pulses to inject spin current bursts. As the direction of spin current is determined by the magnetization of the Ferromagnetic, the mechanism provides an easy and efficient way of controlling the direction of spin torque with a small magnetic field. This project is part of the Santa ANR (2018-2021) lead by Michel Viret (CEA/SPEC) within which IMMM is involved (starting Oct 2018).


Left: Principle of the spin to charge current conversion leading to THz emission (taken from Seifert et al Nat. Phot. 2016). Right: Ultrafast THz emission measured by electro-optic sampling after ultrafast photoexcitation in a FeCoB(5nm)/Pt(3nm) sample. Reversing the external magnetic field leads to a change of sign of the THz electric field.

Related papers
  • [1] V. Juvé et al, in preparation
  • [2] T. S. Seifert, N. M. Tran, O. Gueckstock, S. M. Rouzegar, L. Nadvornik, S. Jaiswal, G. Jakob, V. V. Temnov, M. Münzenberg, M. Wolf, M. Kläui and T. Kampfrath. J. Phys. D Appl. Phys. (2018)

Ultrafast Acoustics

  • Rémi Busselez

  • Thomas Pezeril

  • Gwenaëlle Vaudel

  • Vincent Juvé

  • Guillaume Brotons

  • Nicolas Delorme

  • Pascal Ruello

  • Coll. H. Piombini, P. Belleville CEA Le Ripault, V. Gusev, S. Raetz, N. Chigrev LAUM Le Mans Univ (Lmac Project). K. Nelson, C. Klieber (MIT, USA)

Vibrational properties of liquids and interfacial liquids

At Solid-Liquid interface, a drop of liquid film thickness towards nanoscale induces deep modifications of the liquid properties such as heat or particle transport, fluid rheology and lubrication. Among the properties impacted by the fluid size reduction and liquid-solid interface, the importance of the modifications in vibrational properties are actually scrutinized and debated. Despite this interest in interfacial liquids, experimental measures are not easily accessible at exceptions of Surface Force Apparatus and Atomic Force Microscopy techniques. We recently develop a home-built Time Dependant Brillouin Scattering permitting to reach the GHz dynamics range for liquids of thickness comprises between few microns to tenth of nanometers permitting to access to viscoelastic properties from the bulk liquids to thin films and may shed a new light on the comprehension of interfacial fluid properties.

TDBS measurements are also supported by molecular dynamics simulations which permits to bridge a gap between the length and time scales of both techniques and may relate modifications of macroscopic values such as viscoelastic properties to microscopic aspects.


TDBS spectra of glycerol at different temperaturesVelocity of longitudinal and transverse sound as a function of frequency obtained with molecular dynamics simulation of glycerol at different temperatures

Related papers
  • [1]I. Chaban, H. D. Shin, C. Klieber, R. Busselez, V. E. Gusev, K. A. Nelson and T. Pezeril, Time-domain Brillouin scattering for the determination of laser-induced temperature gradients in liquids, Review of Scientific Instruments, 2017, 88, 074904.
  • [2]R. Busselez, T. Pezeril and V. E. Gusev, Structural heterogeneities at the origin of acoustic and transport anomalies in glycerol glass-former, The Journal of Chemical Physics, 2014, 140, 234505.

Probing elasticity at the nanoscale

The ability to generate and to detect with femtosecond lasers coherent acoustic phonons with very high frequency (10-100s GHz) offers an unique possibility to directly measure the propagation speed of these acoustic phonons in nanostructures and then to investigate the elasticity [1]. Within this context, we have applied our method to probe the elasticity of different systems. First of all, we have studied assemblies of silica nanoparticles (10nm). These are the central element in advanced coating of many lenses in the French project Laser MégaJoule of CEA-DAM. The realization of coating with NPs assemblies permits to remove the coating quite often after each LaserMegaJoule Impact and to renew then the lenses. However, no advanced investigations of the mechanical properties of these coating were achieved up to now. This was our task. In this study, we have been able then to reveal the nature of the contact between nanoparticles (Van der Waals or Hydrogen, Covalent) directly by evaluating how fast the coherent acoustic phonon propagate within a thin film made of this nanoparticle (see figure below). The transformation of such bond (called hardening process) was realized by chemists of CEA. We have been then able to extract the characteristic elastic modulus of films as thin as 70nm (but the technique can be applied to thinner system) [2]. This research has been selected by CEA DAM as one of the 25 more important results in 2017 of research at CEA DAM.

We have also recently applied this method to detect a phase transition in thin films of the multiferroic compound BiFeO3. In particular, we are able to distinguish the transition from the rhombhedral to tetragonal phase by analyzing the values of the sound velocity measured in sub-micrometric films [3].


Left: principal of a pump-probe method on a thin film made of an assembly of nanoparticles. The pump laser can induced mechanical resonances of the film that in turn induce some periodic modulation in time on the optical reflectivity. The latter one is measured with the probe beam Typicall mechanical oscillations are shown on the right where the change of frequency of the two first eigenmode clearly indicate the modification of the nanoscontact strength.

Related papers
  • [1] Advances in applications of time-domain Brillouin scattering for nanoscale imaging, VE Gusev, P Ruello, Applied Physics Reviews 5 (3), 031101 (2018)
  • [2] Controlling the Nanocontact Nature and the Mechanical Properties of a Silica Nanoparticle Assembly, J Avice, C Boscher, G Vaudel, G Brotons, V Juvé, M Edely, C Méthivier, Vitalyi E Gusev, Philippe Belleville, Herve Piombini, Pascal Ruello The Journal of Physical Chemistry C 121 (42), 23769-23776 (2017)
  • [3] Evaluation of the structural phase transition in multiferroic (Bi1-x Prx)(Fe0.95 Mn0.05)O3 thin films by a multi-technique approach including picosecond laser ultrasonics,  Samuel Raetz, Alexey Lomonosov, Sergey Avanesyan, Nikolay Chigarev,  Elton de Lima Savi, Alain Bulou, Nicolas Delorme, Zheng Wen, Qiao Jin, Maju  Kuriakose, Anthony Rousseau, Gwenaëlle Vaudel, Pascal Ruello, Di Wu, Vitalyi  Gusev, in revision in Applied Sciences
  • Contrat CEA (2015-2018) avec Bourse cofinancé CEA-Région Pays de la Loire
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