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International audience; (K,Na)NbO 3 -based ferroelectric single crystals have recently undergone a substantial development, resulting in improved crystal quality and large piezoelectric coefficients, exceeding 700 pC/N, over a broad temperature range. However, further development necessitates a detailed understanding of the mechanisms defining the domain structure and its temperature evolution. This paper presents the investigation into the crystallographic structure and domain configurations of a (K,Na,Li)(Ta,Nb)O 3 single crystal over a broad temperature range. The crystal was grown by the submerged-seed solution growth technique and investigated using in situ transmission electron microscopy, X-ray diffraction, dielectric measurements, and polarized light microscopy. The lattice distortion, structural phase transitions, and domain configurations are reported. A transition from the lamellar orthorhombic to the rectangular tetragonal domain structure is observed upon heating. Moreover, the milky optical appearance of the crystal was investigated and found to result from the presence of regions with different domain configurations and domain sizes. The formation of these regions is related to the growth defects, which govern the domain formation when cooling below the Curie temperature.

International audience; Ferroelectric single crystals exhibit the largest known piezoelectric constants and therefore show high potential for application in electronic devices, including sensors, actuators, and transducers. Their large properties, however, can only be fully exploited if the behavior is not influenced by lattice defects, formed during high-temperature processing. Such defects can inhibit domain-wall movement, increase leakage currents, and are predominantly responsible for the poor performance of the emerging ferroelectric crystals. Here, an approach to considerably enhance the piezoelectric and ferroelectric properties of (K,Na,Li)(Nb,Ta,Sb)O3 crystals by oxygen annealing is investigated. As compared to the non-annealed crystals, polarization, strain, and piezoelectric constant of the annealed sample were enhanced by a factor of two, resulting in an outstanding room-temperature value of piezoelectric constant d33 = 732 pC/N and a peak value of d33 = 1431 pm/V at the orthorhombic-tetragonal transition. The behavior was analyzed using electromechanical measurements, Mössbauer spectroscopy, and impedance spectroscopy, revealing decreased concentrations of both oxygen vacancies and Sb3+ ions after annealing. The findings indicate that the control of the defect chemistry enables these ferroelectric (K,Na)NbO3-based single crystals to outperform their polycrystalline counterparts and reach the values of lead-containing compositions. Moreover, the present strategy enables further detailed studies of the inherent crystal anisotropy and the possibilities for domain engineering.

DOI: 10.1364/OME.6.002984 URL: https://www.osapublishing.org/ome/abstract.cfm?uri=ome-6-9-2984 Filiació URV: SI We measured a rotation of the dielectric frame as a function of wavelength of about 2° in the spectral range of 0.4-1.6 ¿m in the monoclinic double tungstate crystals, KRE(WO4)2 where RE = Gd, Y or Lu. We also refined the Sellmeier equations of their three principal refractive indices. By taking into account these data, we calculated the real orientation of the optical axes as a function of wavelength which is of key importance for the design of conical refraction lasers based on monoclinic double tungstate crystals.

International audience; In this work, we present the first crystal growth of cubic Tb2O3 single crystals by a controlled atmosphere flux method which uses a heavy metal free solvent working at less than half the melting temperature of this sesquioxide. Cubic millimeter-sized crystals extracted from as-grown boules are phase (powder XRD) and chemically (GDMS) pure and exhibit a Verdet constant in the visible and near-infrared spectral ranges, which is at least three times higher than that of a commercial Tb3Ga5O12 (TGG) crystal. The 1.36 mm thick crystals display a transmission coefficient higher than 77% over the 525 nm–1.38 μm spectral range. The absorption spectrum, magnetic susceptibility and specific heat measurements on the single crystals confirm the absence of detectable Tb4+ cations and other impurities.

Bismuth Ferrite (BFO) nanoparticles (BFO-NP) display interesting optical (nonlinear response) and magnetic properties which make them amenable for bio-oriented applications as intra- and extra membrane contrast agents. Due to the relatively recent availability of this material in well dispersed nanometric form, its biocompatibility was not known to date. In this study, we present a thorough assessment of the effects of in vitro exposure of human adenocarcinoma (A549), lung squamous carcinoma (NCI-H520), and acute monocytic leukemia (THP-1) cell lines to uncoated and poly(ethylene glycol)-coated BFO-NP in the form of cytotoxicity, haemolytic response and biocompatibility. Our results support the attractiveness of the functional-BFO towards biomedical applications focused on advanced diagnostic imaging., Word Count: 109 Complete Word Count (body text and figure legends) : 4886 Number of Figures: 6 Number of Tables: 0 Number of reference: 45

International audience; We report on the wavelength dependence of the orientation of the optic axes in monoclinic crystals using undoped and neodymium-doped KGd(WO4)2 (KGW) samples. Measurements have been performed over a wide wavelength range from 420 up to 1,580 nm with more than 70 measurement points. This provides the widest measurement range of the optic axis variation ever reported in literature for KGW crystals. With such a measurement, which can be performed in principle in all biaxial crystals, the refractive indices values can be checked with a precision down to 10−3.

Second Harmonic Generation (SHG) from BiFeO3 nanocrystals is investigated for the first time to determine their potential as biomarkers for multiphoton imaging. Nanocrystals are produced by an auto-combustion method with 2-amino-2-hydroxymethyl-propane-1,3-diol as a fuel. Stable colloidal suspensions with mean particle diameters in the range 100–120 nm are then obtained after wet-milling and sonication steps. SHG properties are determined using two complementary experimental techniques, Hyper Rayleigh Scattering and nonlinear polarization microscopy. BiFeO3 shows a very high second harmonic efficiency with an averaged hdi coefficient of 79612 pm/V. From the nonlinear polarization response of individual nanocrystals, relative values of the independent dij coefficients are also determined and compared with recent theoretical and experimental studies. Additionally, the particles show a moderate magnetic response, which is attributed to gamma-Fe2O3 impurities. A combination of high nonlinear optical efficiency and magnetic response within the same particle is of great interest for future bio-imaging and diagnostic applications.

A novel bio-photonics approach based on the nonlinear optical process of second harmonic generation by non-centrosymmetric nanoparticles is presented and demonstrated on malignant human cell lines. The proposed method allows to directly interact with DNA in absence of photosensitizing molecules, to enable independent imaging and therapeutic modalities switching between the two modes of operation by simply tuning the excitation laser wavelength, and to avoid any risk of spontaneous activation by any natural or artificial light source.

International audience; We present, to the best of our knowledge, the first laser demonstration of an Yb-doped Gd2O3 cubic crystal. This crystal was obtained by the flux method using an original borate-based solvent, which was particularly well suited to the growth of rare earth sesquioxide crystals at half the working temperature of classical growth techniques. This flux method is a very interesting alternative for the production of laser sesquioxide crystals, not only because it provides access to new matrices of the cubic polymorph, but also because it permits high Yb3+-doping levels for these crystals. The first laser results of two highly Yb3+-doped sesquioxides, namely Gd2O3 and Y2O3, grown by this flux method are presented here, including the Ti:sapphire and diode pumping configurations. Laser efficiencies and emission spectra for these two crystals were studied and compared.

International audience; A mode-locked thin-disk laser based on Yb:CALGO is demonstrated for the first time. At an average output power of 28 W we obtained pulses with a duration of 300 fs and a pulse energy of 1.3 μJ. 197 fs pulses with 0.9 μJ of energy were achieved at an average output power of 20 W. The shortest pulse duration measured in our experiments was 135 fs with a spectrum centered at 1043 nm. The experiments also revealed a very broad tunability from 1032 to 1046 nm with sub-200 fs pulses

International audience; Nonlinear optical nanocrystals have been recently introduced as a promising alternative to fluorescent probes for multiphoton microscopy. We present for the first time a complete survey of the properties of five nanomaterials (KNbO3, LiNbO3, BaTiO3, KTP, and ZnO), describing their preparation and stabilization and providing quantitative estimations of their nonlinear optical response. In the light of their prospective use as biological and clinical markers, we assess their biocompatibility on human healthy and cancerous cell lines. Finally, we demonstrate the great potential for cell imaging of these inherently nonlinear probes in terms of optical contrast, wavelength flexibility, and signal photostability.

International audience; We present the first demonstration of an Yb:CALGO thin-disk laser. In a slightly multimode configuration we obtained up to 30 W of average power at a slope efficiency of 40% and an optical-to-optical efficiency of 32%. With a single-mode cavity, an average power of 25 W was achieved. A tuning range from 1018 to 1052nm could be demonstrated by inserting a prism into the cavity. In Q-switched regime, we obtained 1 mJ of pulse energy at a repetition rate of 100 Hz.

International audience; Brightness improvement of high-power semiconductor lasers can be achieved by placing a broad-area laser diode, used as a pure optical amplifier, in an extended cavity. We report the modal behavior of an extended cavity which contains a photorefractive crystal. The dynamic holographic medium that plays the role of a spatial and spectral filter acts on the transverse and axial structure of the extended cavity modes and improves the single-mode operating range of the laser. An enhancement of the brightness in single-mode operation by a factor of seven has been experimentally observed thanks to the intracavity photorefractive crystal.

International audience; High contrast optical bistability is found experimentally in the pump-ratio dependences of the output intensity of a semilinear photorefractive coherent oscillator with two counterpropagating pump waves. The data are in qualitative agreement with the results of calculation.

International audience; Previous works on photorefractive self-organizing laser cavities were about lasers that oscillate, prior the self-organization process occurs, on a set of axial modes sharing the same transverse structure. In a welldesigned broad-area laser diode extended cavity, we theoretically and experimentally demonstrate that the insertion of a photorefractive crystal can also affect the transverse modal structure to force the laser, initially oscillating on several transverse modes, to oscillate on a single transverse and axial mode. This spatial self-organization process leads to an enhancement of the single mode operating range of the laser.

International audience; The self-development of a strong reflection grating in a BaTiO3 sample illuminated with two mutually incoherent counter-propagating light waves is reported. The two transmitted waves diffracted from this grating become partially mutually coherent.

International audience; A photorefractive crystal inserted inside an extended cavity laser diode is an adaptive frequency filter. Without any other selective element, the mutual adaptation between the modes and the dynamic photorefractive hologram leads to a single-longitudinal-mode oscillation. We demonstrated this adaptive process with a 810-nm diffraction limited laser diode and a BaTiO3 photorefractive crystal. We analyzed the transient spectra from the initially multimode oscillation until the single-mode steady state.

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International audience; We characterize Rh:BaTiO, by cw two-beam coupling experiments at 1.06 µm. We demonstrate that all our measurements are well described by a single-carrier, single-site model as soon as the incident intensity is large enough. The effective trap density is found to be equal to N,, = 6 X 10^16 cm-3 and the maximum measured gain is 23 cm-‘. At 20 W cm-2 with a measured absorption of 0.15 cm-‘, the time constant is 75 s.

International audience; We present self-pumped phase conjugation experiments in a ring cavity at 1.06 pm in a BaTiO,:Rh crystal both in the cw and in the nanosecond regime. The measured reflectivities are respectively 50% and 66%. At 4 W cme2 of cw illumination, it only takes 90 s to reach the steady state. A preliminary experiment of correction of the focusing effect of a lens is reported.

International audience; We implement a feedback loop oscillator, using a BaTiO3 crystal that provides stable high-quality phase conjugation for nanosecond pulses, with an efficiency close to the theoretical maximum.

The thesis aims to find possible approaches for improved electromechanical properties in KNN-based piezoelectric single crystals. Both submerged-seed and top-seeded solution growth techniques were employed to produce single crystals. Conclusions from the crystal growth aspect are: (i) For individual elements, segregation coefficients highly rely on the initial concentration in the liquid solution. (ii) A competition between elements occupied on the same lattice site was found. (iii) The very low Li segregation coefficient in the KNN matrix is responsible for the occurrence of a secondary phase with the tetragonal tungsten bronze structure. (iv) Observed optically-cloudy regions in as-grown crystals decrease the electrical response and can be reduced by thermal treatment with slow cooling. In the second part, we used three approaches to enhance the piezoelectric and ferroelectric behavior of KNN-based single crystals. Ta or Sb substitutions indicates that enhanced electromechanical response is achieved when the orthorhombic-tetragonal phase transition is near room temperature. Thermal treatment in pure O2 atmosphere resulted in a twofold increase of the piezoelectric coefficient and ferroelectric parameters of a (K,Na,Li)(Ta,Nb,Sb)O3 single crystal. The highest room-temperature piezoelectric coefficient in annealed KNN-based single crystals of 732 pC/N was obtained. The third approach, doping with Mn ions in (K,Na,Li)(Ta,Nb)O3 single crystals, is also presented.; Cette thèse vise à trouver des approches possibles pour l’amélioration des propriétés électromécaniques de monocristaux piézoélectriques à base de KNN. La TSSG et la SSSG sont entreprises afin de faire croître des monocristaux La conclusion de l'aspect de croissance cristalline est: (1) Pour chaque élément pris individuellement, leurs coefficients de ségrégation reposent fortement sur leurs concentrations initiales dans la solution liquide. (2) La compétition entre éléments occupant le même site du réseau est démontrée. (3) Le très faible coefficient de ségrégation de Li dans la matrice KNN est responsable de l'apparition d'une phase secondaire présentant la structure bronze de tungstène quadratique. (4) Les régions optiquement laiteuses observées dans les monocristaux diminuent la réponse électrique et peuvent être réduites par traitement thermique et refroidissement lent. Dans la deuxième partie, nous avons utilisé trois approches pour améliorer le comportement piézo/ferroélectrique des monocristaux à base de KNN. La Ta ou Sb substitution indique qu'une réponse électromécanique améliorée est obtenue lorsque la transition orthorhombique-quadratique est à proximité de la température ambiante. Le traitement thermique sous atmosphère d'O2 pur a conduit au doublement de la valeur du coefficient piézoélectrique et des paramètres ferroélectriques d'un monocristal de (K,Na,Li) (Ta,Nb,Sb)O3. Son coefficient piézoélectrique à la température ambiante, qui constitue un record mondial à l’heure actuelle vis-à-vis de ce qui est reporté dans la littérature internationale, vaut 732 pC/N. La troisième approche consiste au dopage des monocristaux de (K,Na,Li)(Ta,Nb)O3 avec Mn.