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Managing light-matter interactions on timescales faster than the loss of electronic coherence is key for achieving full quantum control of the final products in solid-solid transformations. In this work, we demonstrate coherent electronic control of the photoinduced insulator-to-metal transition in the prototypical Mott insulator V$_2$O$_3$. Selective excitation of a specific interband transition with two phase-locked light pulses manipulates the orbital occupation of the correlated bands in a way that depends on the coherent evolution of the photoinduced superposition of states. A comparison between experimental results and numerical solutions of the optical Bloch equations provides an electronic coherence time on the order of 5 fs. Temperature-dependent experiments suggest that the electronic coherence time is enhanced in the vicinity of the insulator-to-metal transition critical temperature, thus highlighting the role of fluctuations in determining the electronic coherence. These results open new routes to selectively switch the functionalities of quantum materials and coherently control solid-solid electronic transformations.

Mott transitions in real materials are first order and almost always associated with lattice distortions, both features promoting the emergence of nanotextured phases. This nanoscale self-organization creates spatially inhomogeneous regions, which can host and protect transient non-thermal electronic and lattice states triggered by light excitation. Here, we combine time-resolved X-ray microscopy with a Landau-Ginzburg functional approach for calculating the strain and electronic real-space configurations. We investigate V$_2$O$_3$, the archetypal Mott insulator in which nanoscale self-organization already exists in the low-temperature monoclinic phase and strongly affects the transition towards the high-temperature corundum metallic phase. Our joint experimental-theoretical approach uncovers a remarkable out-of-equilibrium phenomenon: the photo-induced stabilisation of the long sought monoclinic metal phase, which is absent at equilibrium and in homogeneous materials, but emerges as a metastable state solely when light excitation is combined with the underlying nanotexture of the monoclinic lattice.

The insulator-to-metal transition in Mott insulators is the key mechanism for a novel class of electronic devices, belonging to the Mottronics family. Intense research efforts are currently devoted to the development of specific control protocols, usually based on the application of voltage, strain, pressure and light excitation. The ultimate goal is to achieve the complete control of the electronic phase transformation, with dramatic impact on the performance, for example, of resistive switching devices. Here, we investigate the simultaneous effect of external voltage and excitation by ultrashort light pulses on a single Mottronic device based on a V$_2$O$_3$ epitaxial thin film. The experimental results, supported by finite-element simulations of the thermal problem, demonstrate that the combination of light excitation and external electrical bias drives a volatile resistivity drop which goes beyond the combined effect of laser and Joule heating. Our results impact on the development of protocols for the non-thermal control of the resistive switching transition in correlated materials.

The full control of the fundamental photophysics of nanosystems at frequencies as high as few THz is key for tunable and ultrafast nano-photonic devices and metamaterials. Here we combine geometrical and ultrafast control of the optical properties of halide perovskite nanoparticles, which constitute a prominent platform for nanophotonics. The pulsed photoinjection of free carriers across the semiconducting gap leads to a sub-picosecond modification of the far-field electromagnetic properties that is fully controlled by the geometry of the system. When the nanoparticle size is tuned so as to achieve the overlap between the narrowband excitons and the geometry-controlled Mie resonances, the ultrafast modulation of the transmittivity is completely reversed with respect to what is usually observed in nanoparticles with different sizes, in bulk systems and in thin films. The interplay between chemical, geometrical and ultrafast tuning offers an additional control parameter with impact on nano-antennas and ultrafast optical switches.

Unveiling the physics that governs the intertwining between the nanoscale self-organization and the dynamics of insulator-to-metal transitions (\textit{IMT}) is key for controlling on demand the ultrafast switching in strongly correlated materials and nano-devices. A paradigmatic case is the \textit{IMT} in V$_2$O$_3$, for which the mechanism that leads to the nucleation and growth of metallic nano-droplets out of the supposedly homogeneous Mott insulating phase is still a mystery. Here, we combine X-ray photoemission electron microscopy and ultrafast non-equilibrium optical spectroscopy to investigate the early stage dynamics of isolated metallic nano-droplets across the \textit{IMT} in V$_2$O$_3$ thin films. Our experiments show that the low-temperature monoclinic antiferromagnetic insulating phase is characterized by the spontaneous formation of striped polydomains, with different lattice distortions. The insulating domain boundaries accommodate the birth of metallic nano-droplets, whose non-equilibrium expansion can be triggered by the photo-induced change of the 3$d$-orbital occupation. We address the relation between the spontaneous nanotexture of the Mott insulating phase in V$_2$O$_3$ and the timescale of the metallic seeds growth. We speculate that the photoinduced metallic growth can proceed along a non-thermal pathway in which the monoclinic lattice symmetry of the insulating phase is partially retained., Comment: 17 pages, text+figures+methods+supplementary

Multiorbital correlated materials are often on the verge of multiple electronic phases (metallic, insulating, super- conducting, charge and orbitally ordered), which can be explored and controlled by small changes of the external parameters. The use of ultrashort light pulses as a mean to transiently modify the band population is leading to fundamentally new results. In this paper we will review recent advances in the field and we will discuss the pos- sibility of manipulating the orbital polarization in correlated multi-band solid state systems. This technique can provide new understanding of the ground state properties of many interesting classes of quantum materials and offers a new tool to induce transient emergent properties with no counterpart at equilibrium. We will address: the discovery of high-energy Mottness in superconducting copper oxides and its impact on our understanding of the cuprate phase diagram; the instability of the Mott insulating phase in photoexcited vanadium oxides; the manipulation of orbital-selective correlations in iron-based superconductors; the pumping of local electronic excitons and the consequent transient effective quasiparticle cooling in alkali-doped fullerides. Finally, we will discuss a novel route to manipulate the orbital polarization in a a k-resolved fashion.

Many puzzling properties of high-$T_c$ superconducting (HTSC) copper oxides have deep roots in the nature of the antinodal quasiparticles, the elementary excitations with wavevector parallel to the Cu-O bonds. These electronic states are most affected by the onset of antiferromagnetic correlations and charge instabilities and they host the maximum of the anisotropic superconducting gap and pseudogap. In this work, we use time-resolved extreme-ultra-violet (EUV) photoemission with proper photon energy (18 eV) and time-resolution (50 fs) to disclose the ultrafast dynamics of the antinodal states in a prototypical HTSC cuprate. After photoinducing a non-thermal charge redistribution within the Cu and O orbitals, we reveal a dramatic momentum-space differentiation of the transient electron dynamics. While the nodal quasi-particle distribution is heated up as in a conventional metal, new quasiparticle states transiently emerge at the antinodes, similarly to what is expected for a photoexcited Mott insulator, where the frozen charges can be released by an impulsive excitation. This transient antinodal metallicity is mapped into the dynamics of the O-2$p$ bands thus directly demonstrating the intertwining between the low- and high-energy scales that is typical of correlated materials. Our results suggest that the correlation-driven freezing of the electrons moving along the Cu-O bonds, analogous to the Mott localization mechanism, constitutes the starting point for any model of high-$T_c$ superconductivity and other exotic phases of HTSC cuprates.

The Thermal Boundary Resistance at the interface between a nanosized Al film and an Al_{2}O_{3} substrate is investigated at an atomistic level. A room temperature value of 1.4 m^{2}K/GW is found. The thermal dynamics occurring in time-resolved thermo-reflectance experiments is then modelled via macro-physics equations upon insertion of the materials parameters obtained from atomistic simulations. Electrons and phonons non-equilibrium and spatio-temporal temperatures inhomo- geneities are found to persist up to the nanosecond time scale. These results question the validity of the commonly adopted lumped thermal capacitance model in interpreting transient nanocalorimetry experiments. The strategy adopted in the literature to extract the Thermal Boundary Resistance from transient reflectivity traces is revised at the light of the present findings. The results are of relevance beyond the specific system, the physical picture being general and readily extendable to other heterojunctions., Comment: 12 pages, 8 figures

The intrinsic instability of underdoped copper oxides towards inhomogeneous states is one of the central puzzles of the physics of correlated materials. The influence of the Mott physics on the doping-temperature phase diagram of copper oxides represents a major issue that is subject of intense theoretical and experimental effort. Here, we investigate the ultrafast electron dynamics in prototypical single-layer Bi-based cuprates at the energy scale of the O-2p$\rightarrow$Cu-3d charge-transfer (CT) process. We demonstrate a clear evolution of the CT excitations from incoherent and localized, as in a Mott insulator, to coherent and delocalized, as in a conventional metal. This reorganization of the high-energy degrees of freedom occurs at the critical doping p$_{cr}\simeq$0.16 irrespective of the temperature, and it can be well described by dynamical mean field theory calculations. We argue that the onset of the low-temperature charge instabilities is the low-energy manifestation of the underlying Mottness that characterizes the p