Optical studies of rare-earth crystal-field excitations in manganites and laser materials/Études optiques du champ cristallin dans les manganites et matériaux laser

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Optical techniques such as light scattering, photoluminescence and infrared interferometry are useful to develop fundamental science and to explain microsopic interactions in solids. We use these techniques to better understand the fundamental physical properties of the following important recent high tech materials : a)- various multiferoics (rare-earth manganites and vanadates) in which magnetism and ferroelectricity, that have led to most important applications, coexist with a pronounced interplay. In these materials an electric polarization is actuated by a magnetic field and a magnetization is triggered by an electric field. They are of great application potential leading for instance to eight-state new memory devices, ferroelectric field-effect transistors and electric field controlled magnetic components. b)- solid state lasers in which interacting rare-earth ion pairs are formed (e.g. Yb-Yb ions in Yb doped NdVO4). Such pairs are at the origin of intrinsic bistability for optical switches and optical memories. They are also a source of cooperative emission doubling the infrared emission of the isolated Yb ion in green-blue light. In our studied materials, rare earth ions (Pr, Nd, Sm...) are incorporated. We get samples from four internationally recognized groups for the quality of their crystal growing expertise. Light scattering and infrared spectroscopy constitute our tools to detect the ionic vibrations (phonons), the magnetic excitations (magnons) and the energy levels of the 4f rare-earth electrons. These excitations and levels constitute local probe to detect the various electrical and magnetic interactions (electron-electron, electron-ion and ion-ion). Using quantum mechanics formalism, we then model the interactions that we tune and modify as a function of temperature, doping and applied magnetic and electric fields. We aim to develop a basic comprehension of these strategic materials properties in order to orient the crystal growers in generating new strategic compounds that would enhance the essential particularities of the rare-earth based multiferroics and laser materials for new high tech developments.