Edoardo Baldini

Femtosecond X-ray absorption study of electron localization in photoexcited anatase TiO2

Transition metal oxides are among the most promising solar materials, whose properties rely on the generation, transport and trapping of charge carriers (electrons and holes). Identifying the latter’s dynamics at room temperature requires tools that combine elemental and structural sensitivity, with the atomic scale resolution of time (femtoseconds, fs). Here, we use fs Ti K-edge X-ray absorption spectroscopy (XAS) upon 3.49 eV (355 nm) excitation of aqueous colloidal anatase titanium dioxide nanoparticles to probe the trapping dynamics of photogenerated electrons. We find that their localization at Titanium atoms occurs in <300 fs, forming Ti3+ centres, in or near the unit cell where the electron is created. We conclude that electron localization is due to its trapping at pentacoordinated sites, mostly present in the surface shell region. The present demonstration of fs hard X-ray absorption capabilities opens the way to a detailed description of the charge carrier dynamics in transition metal oxides.

Filming the formation and fluctuation of skyrmion domains by cryo-Lorentz transmission electron microscopy

Significance The need for denser storage devices calls for new materials and nanostructures capable of confining single bits of information in a few nanometers. A new topological distribution of spins termed skyrmions is emerging, which promises to robustly confine a small magnetization in a few-nanometers-wide circular domain. A great deal of attention is being devoted to the understanding of these magnetic patterns and their manipulation. We manufactured a large nanoslice supporting over 70,000 skyrmions, and film their evolution in direct-space via cryo-Lorentz transmission electron microscopy. We reveal the octagonal distortion of the skyrmion lattice and show how these distortions and other defects impact its long-range order. These results pave the way to the control of a large two-dimensional array of skyrmions. Magnetic skyrmions are promising candidates as information carriers in logic or storage devices thanks to their robustness, guaranteed by the topological protection, and their nanometric size. Currently, little is known about the influence of parameters such as disorder, defects, or external stimuli on the long-range spatial distribution and temporal evolution of the skyrmion lattice. Here, using a large (7.3×7.3 μm2) single-crystal nanoslice (150 nm thick) of Cu2OSeO3, we image up to 70,000 skyrmions by means of cryo-Lorentz transmission electron microscopy as a function of the applied magnetic field. The emergence of the skyrmion lattice from the helimagnetic phase is monitored, revealing the existence of a glassy skyrmion phase at the phase transition field, where patches of an octagonally distorted skyrmion lattice are also discovered. In the skyrmion phase, dislocations are shown to cause the emergence and switching between domains with different lattice orientations, and the temporal fluctuation of these domains is filmed. These results demonstrate the importance of direct-space and real-time imaging of skyrmion domains for addressing both their long-range topology and stability.

Nonlinear characterization of hydrogenated amorphous silicon waveguides and analysis of carrier dynamics

In this paper, we determine the optical nonlinear coefficient of hydrogenated amorphous silicon (a-Si:H) waveguides. Up to date, the data reported in the scientific literature for similar structures show a very large variability and the final assessment of their nonlinear performance is still an open issue. We performed a complete and careful characterization of more than 50 waveguides. A nonlinear coefficient of 790 + j20 W−1 m−1 was found, confirming that a-Si:H is a good candidate for nonlinear silicon photonic devices. Nevertheless, free-carrier-dynamics exhibits a recombination time in the nanosecond range, which can hinder their exploitation in ultrafast applications requiring high-power optical beams.

Experimental study of the optical forces exerted by a Gaussian beam within the Rayleigh range

Numerical tools for the evaluation of optical forces exerted by non-focused Gaussian beams are becoming increasingly important for the design of integrated devices dedicated to cell manipulation without physical contact. Here we consider two methods for the evaluation of optical forces based on a ray-optics approach and we compare them with experimental results. We show that optical forces can be calculated with good accuracy also within the Rayleigh range of the Gaussian beam and for a wide range of particle sizes.

Probing the electron-phonon interaction in correlated systems with coherent lattice fluctuation spectroscopy

Tailoring the properties of correlated oxides is accomplished by chemical doping, pressure, temperature, or magnetic field. Photoexcitation is a valid alternative to reach out-of-equilibrium states otherwise inaccessible. Here, we quantitatively estimate the coupling between a lattice distortion and the charge-transfer excitation in La2CuO4+delta. We photoinduce a coherent La ion vibration and monitor the response of the optical constants in a broad energy range, providing quantitative information on the electron-phonon matrix element that can be compared to theoretical models. We propose the same methodology to probe electron-phonon and electron-electron interactions in other materials.

Interfacial Electron Injection Probed by a Substrate-Specific Excitonic Signature

Ultrafast interfacial electron transfer in sensitized solar cells has mostly been probed by visible-to-terahertz radiation, which is sensitive to the free carriers in the conduction band of the semiconductor substrate. Here, we demonstrate the use of deep-ultraviolet continuum pulses to probe the interfacial electron transfer, by detecting a specific excitonic transition in both N719-sensitized anatase TiO2 and wurtzite ZnO nanoparticles. Our results are compared to those obtained on bare nanoparticles upon above-gap excitation. We show that the signal upon electron injection from the N719 dye into TiO2 is dominated by long-range Coulomb screening of the final states of the excitonic transitions, whereas in sensitized ZnO it is dominated by phase-space filling. The present approach offers a possible route to detecting interfacial electron transfer in a broad class of systems, including other transition metal oxides or sensitizers.

Clocking the onset of bilayer coherence in a high- Tc cuprate

In cuprates, a precursor state of superconductivity is speculated to exist above the critical temperature

Real-Time Observation of Phonon-Mediated σ-π Interband Scattering in MgB2

{\mathrm{T}}_{\mathrm{C}}

A versatile setup for ultrafast broadband optical spectroscopy of coherent collective modes in strongly correlated quantum systems

. Here we show via a combination of far-infrared ellipsometry and ultrafast broadband optical spectroscopy that signatures of such a state can be obtained via three independent observables in an underdoped sample of

Anomalous anisotropic exciton temperature dependence in rutile TiO2

{\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+\ensuremath{\delta}}