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

Photothermal optomechanics

International audienc

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

International audienc

Physical mechanisms of coherent acoustic phonons generation by ultrafast laser action

International audienceIn this review we address the microscopic mechanisms that are involved in the photogeneration processes of GHz–THz coherent acoustic phonons (CAP) induced by an ultrafast laser pulse. Understanding and describing the underlying physics is necessary indeed for improving the future sources of coherent acoustic phonons useful for the non-destructive testing optoacoustic techniques. Getting more physical insights on these processes also opens new perspectives for the emerging field of the opto-mechanics where lattice motions (surface and/or interfaces ultrafast displacements, nanostructures resonances) are controlled by light. We will then remind the basics of electron–phonon and photon-phonon couplings by discussing the deformation potential mechanism, the thermoelasticity, the inverse piezoelectric effect and the electrostriction in condensed matter. Metals, semiconductors and oxide materials will be discussed. The contribution of all these mechanisms in the photogeneration process of sound will be illustrated over several examples coming from the rich literature

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

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

Time-domain Brillouin scattering is an all-optical experimental technique based on ultrafast lasers applied for generation and detection of coherent acoustic pulses on time durations of picoseconds and length scales of nanometers. In transparent materials scattering of the probe laser beam by the coherent phonons permits imaging of sample inhomogeneity. The transient optical reflectivity of the sample recorded by the probe beam as the acoustic nanopulse propagates in space contains information on the acoustical, optical, and acousto-optical parameters of the material under study. The experimental method is based on a heterodyning where weak light pulses scattered by the coherent acoustic phonons interfere at the photodetector with probe light pulses of significantly higher amplitude reflected from various interfaces of the sample. The time-domain Brillouin scattering imaging is based on Brillouin scattering and has the potential to provide all the information that researchers in material science, physics, chemistry, biology etc., get with classic frequency-domain Brillouin scattering of light. It can be viewed as a replacement for Brillouin scattering and Brillouin microscopy in all investigations where nanoscale spatial resolution is either required or advantageous. Here we review applications of time-domain Brillouin scattering for imaging of nanoporous films, ion-implanted semiconductors and dielectrics, texture in polycrystalline materials and inside vegetable and animal cells, and for monitoring the transformation of nanosound caused by nonlinearity and focusing. We also discuss the perspectives and the challenges for the future