Hoshang Sahib

Des guides d’ondes chiral canaux pour capteurs énantiosélectifs: La symétrie planaire rend les guides d'ondes de la photonique intégrée (IP) biréfringents linéaires ; ils ne supportent que deux modes propres polarisés linéairement, notamment les modes TE et TM. En raison de la biréfringence linéaire (LB), sauf pour TE et TM, l'état de polarisation de la lumière guidée dans IP n'est pas conservé. Par conséquent, toute application qui nécessite d'autres polarisations, comme la polarisation circulaire (la polarisation clé dans la détection chirale), ne peut pas bénéficier de la technologie IP. Une façon de dépasser la limitation de la LB dans les guides d'ondes optiques est d'utiliser des guides d'ondes composés de matériaux chiraux, connus sous le nom de chirowaveguides. Dans cette thèse, nous présentons la réalisation de chirowaveguides optiques à canaux qui peuvent être utilisés pour des applications de polarisation autres que TE/TM, en particulier pour des applications de polarisation circulaire. Les guides d'ondes à canaux sont développés sur la base de notre étude théorique sur les propriétés de polarisation des chirowaveguides et des simulations numériques du comportement de biréfringence des guides d'ondes à canaux. La méthode sol-gel est utilisée pour fabriquer des couches transparentes de silice chirale organiquement modifiée (ORMOSIL) avec une biréfringence circulaire élevée et une qualité optique, un indice de réfraction et une épaisseur contrôlés. Ensuite, en fabriquant des moules en PDMS et en les imprimant sur la forme de gel des couches ORMOSIL, des guides d'ondes à canaux de différentes tailles sont fabriqués. En combinant la biréfringence circulaire élevée de notre matériau avec le LB modulé par la taille des guides d'ondes à canaux fabriqués, nous avons obtenu des guides d'ondes ayant des modes propres linéaires à quasi-circulaires. En effet, les guides d'ondes avec des modes propres quasi-circulaires peuvent être utilisés pour des applications de polarisation quasi-circulaire et circulaire dans la technologie IP, comme capteurs de chiralité miniaturisés pour la pharmacologie, la médecine, l'industrie alimentaire ou la mesure de la pollution environnementale.

Two-dimensional electron gases (2DEGs) at oxide interfaces are promising for electronics because of desirable ingredients such as spin–orbit coupling and strong correlations that can be leveraged to bridge into spintronics or photonics. In this context, the ability to manipulate oxide 2DEGs via external knobs is particularly important. Here we show that a volatile high-mobility 2DEG can be photogenerated at the interface between SrTiO₃ (001) and infinite-layer NdNiO₂, where such an electronic state is otherwise absent. This allows us to optically switch the 2DEG between ON and OFF, leading to a giant, instantaneous conductivity variation. The key ingredients for this effect are the structural and electronic reconstructions at the NdNiO₂//SrTiO₃ interface, together with a built-in interfacial electric field that promotes the occupation of the Ti 3d_xy band by the photogenerated carriers. By contributing to understanding photoconductance at complex-oxide interfaces, our results pave the way to engineer the photoresponse of strongly correlated electrons.

In transition metal oxides, electron–electron interaction and lattice degree of freedom are basic ingredients of emergent phenomena, such as metal-to-insulator transition (MIT) and superconductivity. Perovskite rare-earth nickelates are largely studied for their temperature-driven MIT, which is accompanied by a breathing-mode distortion and associated with a bond-disproportionation of the expanded (3d8L0) and compressed (3d8L2) NiO6 octahedra. Steric effects control the onset temperature of the MIT, the latter being concomitant or not with a complex antiferromagnetic spin arrangement depending upon the choice of the rare-earth ion (TMIT ≥ TNéel). Interface engineering of oxygen octahedra tilting, as imposed by the symmetry and orientation of the substrate, has resulted in an efficient pathway to modify both TMIT and TNéel, hence suggesting a key role of the electron–phonon coupling for both transport and magnetic properties in nickelate thin films. Here, via a combination of resonant elastic x-ray scattering and transport experiments, we show control over both TMIT and TNéel in heteroepitaxial PbZr0.2Ti0.8O3(d)/NdNiO3(7 nm)//SrTiO3 heterostructures, which are characterized by different strains and polarization states of the PbZr0.2Ti0.8O3 layer grown at different thicknesses d. We found the expected NdNiO3 bulk behavior (TMIT = TNéel), for a fully relaxed PbZr0.2Ti0.8O3 layer showing a monodomain polarization state. On the other side, an almost 30 K difference (TMIT > TNéel), is found for a fully strained PbZr0.2Ti0.8O3 characterized by a multidomain texture of the polarization state. We discuss our results in terms of an altered breathing distortion pattern of the underlying nickelate layer as supported by x-ray absorption spectroscopy measurements. We infer that locally different polar distortions controlled by a combination of polarization direction and strength of the strain state play the main role in the observed TMIT and TNéel variations.

Several reports about infinite‐layer nickelate thin films suggest that the superconducting critical temperature versus chemical doping phase diagram has a dome‐like shape, similar to cuprates. Here, a highly reproducible superconducting state in undoped PrNiO₂ thin films grown on SrTiO₃ are demonstrated. Scanning transmission electron microscopy measurements show coherent infinite‐layer phase with no visible stacking‐fault defects, an overall high structural quality where possible unintentional chemical doping or interstitial oxygen, if present, sum well below the measurable threshold of the technique. X‐ray absorption measurements show very sharp features at the Ni L_3,2‐edges with a large linear dichroism, indicating the preferential hole occupation of Ni¹⁺‐3 orbitals in a square planar geometry. Resonant inelastic X‐ray scattering measurements reveal sharp magnon excitations of 200 meV energy at the magnetic Brillouin zone boundary, highly resonant at the Ni^{1 +} absorption peak. The results indicate that, when properly stabilized, infinite‐layer nickelate thin films are superconducting without chemical doping.

Several reports about infinite-layer nickelate thin films suggest that the superconducting critical temperature versus chemical doping phase diagram has a dome-like shape, similar to cuprates. Here, we demonstrate a highly reproducible superconducting state in undoped PrNiO$_2$ thin films grown onto SrTiO$_3$. Scanning transmission electron microscopy measurements demonstrate coherent and defect-free infinite-layer phase, a high structural quality with no unintentional chemical doping and a total absence of interstitial oxygen. X-ray absorption measurements show very sharp features at the Ni L$_{3,2}$-edges with a large linear dichroism, indicating the preferential hole-occupation of Ni$^{1+}$-3d$_{x^2-y^2}$ orbitals in a square planar geometry. Resonant inelastic X-ray scattering measurements reveal sharp magnon excitations of 200\,meV energy at magnetic Brillouin zone boundary, highly resonant at the Ni$^{1+}$ absorption peak. The results indicate that, when properly stabilized, infinite-layer nickelate thin films are superconducting without chemical doping.

Following the discovery of superconductivity in hole-doped NdNiO$_2$ infinite-layer thin films, extensive research has been conducted particularly to compare these materials with cuprates. Superconductivity has also been observed in nickelate thin films with other rare-earth elements like Pr and La, but not in their bulk forms, suggesting a critical role for substrate-induced strain/interface or dimensionality effects. In this study, we use water-soluble (Ca,Sr)$_3$Al$_2$O$_6$ sacrificial layers to fabricate freestanding perovskite nickelate membranes and explore topotactic reduction without the SrTiO$_3$ substrate as a template. NdNiO$_3$-based heterostructure membranes transferred from a LaAlO$_3$ substrate exhibit better metallic behavior and higher hysteresis than those transferred from SrTiO$_3$, owing to the different strain states that the NdNiO$_3$ layer experience when grown on these two substrates. Despite the expected X-ray diffraction shifts, membranes reduced with CaH$_2$ display insulating characteristics, similar to bulk infinite-layer nickelates. Our findings strongly indicate that a template is necessary to stabilize a coherent and robust infinite-layer phase with optimal transport properties.