Axelle Amon

Dual-polarization microchip laser at 1.53??m

Two-frequency operation of a composite Er,Yb:glass-LiTaO3 monolithic microlaser is demonstrated at 1.53 microm. The thermo-optic effect of the intracavity birefringent crystal makes it possible to control the wavelengths of the two orthogonal linearly polarized eigenstates. It provides a tunable beat note from 0 to 60 GHz. Direct evaluation of the Lamb-type coupling constant C between the two eigenstates is obtained from a single intensity noise spectrum measurement, yielding values from 0.33 to 0.86, depending on transverse and axial distributions of the eigenstates. Applications to radar and telecommunication systems are discussed.

Experimental investigation of plastic deformations before a granular avalanche

We present an experimental study of the deformation inside a granular material that is progressively tilted. We investigate the deformation before the avalanche with a spatially resolved diffusive wave spectroscopy setup. At the beginning of the inclination process, we first observe localized and isolated events in the bulk, with a density which decreases with the depth. As the angle of inclination increases, series of microfailures occur periodically in the bulk, and finally a granular avalanche takes place. The microfailures are observed only when the tilt angles are larger than a threshold angle much smaller than the granular avalanche angle. We have characterized the density of reorganizations and the localization of microfailures. We have also explored the effect of the nature of the grains, the relative humidity conditions, and the packing fraction of the sample. We discuss those observations in the framework of the plasticity of granular matter. Microfailures may then be viewed as the result of the accumulation of numerous plastic events.

Topological signature of deterministic chaos in short nonstationary signals from an optical parametric oscillator

Although deterministic chaos has been predicted to occur in the triply resonant optical parametric oscillator (TROPO) 15 years ago, experimental evidence of chaotic behavior in this system has been lacking so far, in marked contrast with most nonlinear systems, where chaos has been actively tracked and found. This situation is probably linked to the high sensitivity of the TROPO to perturbations, which adversely affects stationary operation at high power. We report the experimental observation in this system of a burst of irregular behavior of duration 80 micros. Although the system is highly nonstationary over this time interval, a topological analysis allows us to extract a clear-cut signature of deterministic chaos from a time series segment of only nine base cycles (3 micros). This result suggests that nonstationarity is not necessarily an obstacle to the characterization of chaos.

Unraveling Complex Interplay between Heat Shock Factor 1 and 2 Splicing Isoforms

Chaperone synthesis in response to proteotoxic stress is dependent on a family of transcription factors named heat shock factors (HSFs). The two main factors in this family, HSF1 and HSF2, are co-expressed in numerous tissues where they can interact and form heterotrimers in response to proteasome inhibition. HSF1 and HSF2 exhibit two alternative splicing isoforms, called α and β, which contribute to additional complexity in HSF transcriptional regulation, but remain poorly examined in the literature. In this work, we studied the transcriptional activity of HSF1 and HSF2 splicing isoforms transfected into immortalized Mouse Embryonic Fibroblasts (iMEFs) deleted for both Hsf1 and Hsf2, under normal conditions and after proteasome inhibition. We found that HSF1α is significantly more active than the β isoform after exposure to the proteasome inhibitor MG132. Furthermore, we clearly established that, while HSF2 had no transcriptional activity by itself, short β isoform of HSF2 exerts a negative role on HSF1β-dependent transactivation. To further assess the impact of HSF2β inhibition on HSF1 activity, we developed a mathematical modelling approach which revealed that the balance between each HSF isoform in the cell regulated the strength of the transcriptional response. Moreover, we found that cellular stress such as proteasome inhibition could regulate the splicing of Hsf2 mRNA. All together, our results suggest that relative amounts of each HSF1 and HSF2 isoforms quantitatively determine the cellular level of the proteotoxic stress response.

Spatially resolved measurements of micro-deformations in granular materials using diffusing wave spectroscopy

This article is a tutorial on the practical implementation of a method of measurement of minute deformations based on multiple scattering. This technique has been recently developed and has proven to give new insights into the spatial repartition of strain in a granular material. We provide here the basics to understand the method by giving a synthetic review on diffusing wave spectroscopy and multiple scattering in granular materials. We detail a simple experiment using standard lab equipment to pedagogically demonstrate the implementation of the method. Finally we give a few examples of measurements that have been obtained in other works to discuss the potential of the method.

Experimental study of shear band formation: Bifurcation and localization

We report the experimental observation of the bifurcation at the origin of localization of the deformation in a granular material submitted to uniaxial compression. We present a quantitative characterization of the heterogeneity in the strain field repartition allowing to evidence objectively the existence of a bifurcation initiating the shear bands formation process. We show that this bifurcation is supercritical and has no clear signature on the stress-strain curve. At the bifurcation, a symmetry breaking occurs characterized by the emergence of a well-defined orientation corresponding to the Mohr-Coulomb angle. Yet, plasticity is still diffuse and the shear band extension is of the order of the sample width. While loading proceeds, the shear band narrows until it reaches, after the peak of the stress-strain curve, a stationary width.

Mechanical response of granular media: New insights from Diffusing-Wave Spectroscopy

We experimentally investigate the deformation of a granular material submitted to a localised compression force. The strain field is characterised using a near field Diffusing-Wave Spectroscopy technique. The results of point load experiments on an elastic sample are first presented to illutrate the possibilities of the technique. Then, we report new results concerning the behaviour of a granular medium, showing a non-linear response with the force increment and a spatial repartition of the strain that differs from elasticity. Moreover, we analyse the partial reversibility of the deformation after a loading/unloading cycle in terms of both elastic and plastic contributions.

Mechanical fluctuations suppress the threshold of soft-glassy solids: The secular drift scenario.

We propose a dynamical mechanism leading to the fluidization by external mechanical fluctuations of soft-glassy amorphous material driven below the yield stress. The model is based on the combination of memory effect and nonlinearity, leading to an accumulation of tiny effects over a long term. We test this scenario on a granular packing driven mechanically below the Coulomb threshold. We provide evidence for an effective viscous response directly related to small stress modulations in agreement with the theoretical prediction of a generic secular drift. We propose to extend this result more generally to a large class of glassy systems.

Preface: Focus on imaging methods in granular physics

Axelle Amon, Philip Born, Karen E. Daniels, Joshua A. Dijksman, Kai Huang, David Parker, Matthias Schröter, Ralf Stannarius, and Andreas Wierschem Institut de Physique de Rennes, UMR UR1-CNRS 6251, Université de Rennes 1, 35042 Rennes, France Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luftund Raumfahrt, 51170 Cologne, Germany Department of Physics, North Carolina State University, Raleigh, NC, 27695 USA Physical Chemistry and Soft Matter, Wageningen University & Research, Wageningen, The Netherlands Experimentalphysik V, Universität Bayreuth, 95440 Bayreuth, Germany School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK Institute for Multiscale Simulation, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 91052 Erlangen, Germany. Institut für Experimentelle Physik, Otto-von-Guericke-Universität, 39106 Magdeburg, Germany Institute of Fluid Mechanics, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 91058 Erlangen, Germany

Eshelby inclusions in granular matter: theory and simulations

We present a numerical implementation of an active inclusion in a granular material submitted to a biaxial test. We discuss the dependence of the response to this perturbation on two parameters: the intragranular friction coefficient on one hand, and the degree of the loading on the other hand. We compare the numerical results to theoretical predictions taking into account the change of volume of the inclusion as well as the anisotropy of the elastic matrix.