Exciton Control in a Room-Temperature Bulk Semiconductor with Coherent Strain Pulses

The coherent manipulation of excitons in bulk semiconductors via the lattice degrees of freedom is key to the development of acousto-optic and acousto-excitonic devices. Wide-bandgap transition metal oxides exhibit strongly bound excitons that are interesting for applications in the deep-ultraviolet, but their properties have remained elusive due to the lack of efficient generation and detection schemes in this spectral range. Here, we perform ultrafast broadband deep-ultraviolet spectroscopy on anatase TiO$_2$ single crystals at room temperature, and reveal a dramatic modulation of the exciton peak amplitude due to coherent acoustic phonons. This modulation is comparable to those of nanostructures where exciton-phonon coupling is enhanced by quantum confinement, and is accompanied by a giant exciton shift of 30-50 meV. We model these results by many-body perturbation theory and show that the deformation potential coupling within the nonlinear regime is the main mechanism for the generation and detection of the coherent acoustic phonons. Our findings pave the way to the design of exciton control schemes in the deep-ultraviolet with propagating strain pulses

Europium Doping Impact on the Properties of MBE Grown Bi2Te3 Thin Film

The impact of europium doping on the electronic and structural properties of the topological insulator Bi2Te3 is studied in this paper. The crystallographic structure studied by electron di raction and transmission microscopy confirms that grown by Molecular Beam Epitaxy (MBE) system film with the Eu content of about 3% has a trigonal structure with relatively large monocrystalline grains. The X-ray photoemission spectroscopy indicates that europium in Bi2Te3 matrix remains divalent and substitutes bismuth in a Bi2Te3 matrix. An exceptional ratio of the photoemission 4d multiplet components in Eu doped film was observed. However, some spatial inhomogeneity at the nanometer scale is revealed. Firstly, local conductivity measurements indicate that the surface conductivity is inhomogeneous and is correlated with a topographic image revealing possible coexistence of conducting surface states with insulating regions. Secondly, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) depth-profiling also shows partial chemical segregation. Such in-depth inhomogeneity has an impact on the lattice dynamics (phonon lifetime) evaluated by femtosecond spectroscopy. This unprecedented set of experimental investigations provides important insights for optimizing the process of growth of high-quality Eu-doped thin films of a Bi2Te3 topological insulator. Understanding such complex behaviors at the nanoscale level is a necessary step before considering topological insulator thin films as a component of innovative devices

Ultrafast acousto-optic mode conversion in optically birefringent ferroelectrics

The ability to generate efficient giga-terahertz coherent acoustic phonons with femtosecond laser makes acousto-optics a promising candidate for ultrafast light processing, which faces electronic device limits intrinsic to complementary metal oxide semiconductor technology. Modern acousto-optic devices, including optical mode conversion process between ordinary and extraordinary light waves (and vice versa), remain limited to the megahertz range. Here, using coherent acoustic waves generated at tens of gigahertz frequency by a femtosecond laser pulse we reveal the mode conversion process and show its efficiency in ferroelectric materials such as BiFeO3 and LiNbO3. Further to the experimental evidence, we provide a complete theoretical support to this all-optical ultrafast mechanism mediated by acousto-optic interaction. By allowing the manipulation of light polarization with gigahertz coherent acoustic phonons, our results provide a novel route for the development of next-generation photonic-based devices and highlight new capabilities in using ferroelectrics in modern photonics

Advances in applications of time-domain Brillouin scattering for nanoscale imaging

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Génération et détection par impulsion optique femtoseconde de phonons acoustiques cohérents dans le semi-conducteur piézo-électrique d arséniure de gallium

L'utilisation d'impulsions laser femtoseconde en technique pompe sonde est une technique commune en vue de générer et détecter des phonons acoustiques cohérents, dont les longueurs d'onde caractéristiques nanométriques sont adaptées à la caractérisation de nanostructures. Les caractéristiques de ces phonons cohérents peuvent être contrôlées par la connaissance du processus de transduction optoacoustique, dépendant de la longueur d'onde d'excitation optique et de l'intensité du laser. L'interaction laser-matière entre le faisceau de pompe optique et le semiconducteur conduit à l'existence de contraintes mécaniques (déformation de potentiel, thermo-élastique, électrostrictif, piézo-électrique). Les expériences et l'analyse théorique de la transduction opto-acoustique dans le GaAs, pour différents niveaux de dopage et/ou orientations cristallographiques ont été menées, et la dépendance du processus de transduction avec la fluence du rayonnement laser de pompe a été mise en évidence.The utilisation of femtoseconde laser pulses in pump probe spectroscopy techniques is an established approach to generate and to detect coherent acoustic phonons, which nanometric caracteristical wavelength is well adapted to nano-structures characterisation. Characteristics of these coherent phonons could be controlled through the opto-acoustic transformation process, which depends on optical excitation wavelength and laser intensity. The laser-matter interaction between the optical pump beam and the semi-conductor leads to the creation of different types of mechanical stress (deformation potential, thermo-elastic, electrostrictive, piezoelectric). The experiments and theoretical analysis of opto-acoustic transformation in GaAs of different doping level and crystallographic orientation had been performed. The dependence of the opto-acoustic transformation process on the fluence of pump laser radiation was revealed.LE MANS-BU Sciences (721812109) / SudocSudocFranceF

How polarons can enhance UO2 irradiation resistance?

International audienceUO2 has a specific behaviour at high temperature related to a high concentration of charged defects, polarons, affecting its thermodynamic properties. We discuss how polarons can improve the radiation resistance of UO2

Is UO2 irradiation resistance due to its high temperature behaviour?

International audienceUO2 is the main component of nuclear fuel for most of nuclear power plants. In operation in these plants, it withstands irradiation damage without major changes. Szenes proposed a simple criterion for determining an amorphisation threshold due to swift heavy ions, which works reasonably well for many ionio-covalent compounds. In the case of UO2, this criterion predicts that this threshold is reached for 8.6 keV/nm, which means well under the electronic energy loss of fission products, for example 19.7 keV/nm for 72 MeV iodine. We will address this apparent contradiction. In fact the resistance of a given material to irradiation damage depends on the way it can absorbs the deposited energy by electronic losses before reaching an energy density, generally associated with a threshold temperature that leads to metastable changes. In this paper we will discuss the equilibrium properties of UO2 and how UO2 can absorb energy before reaching this threshold. At thermodynamic equilibrium, the way by which a solid stores energy can be estimated by its heat capacity. In order to better understand UO2 resistance to irradiation, we discuss how the properties of uranium and oxygen sublattice have a drastic impact on the thermodynamic properties of UO2 compared to other fluorite compounds. We will discuss why the unusual behaviour of uranium sublattice is associated with the formation of polarons at high temperature and how this gives UO2 an enhanced irradiation resistance