Browsing by Author "Della Valle, Giuseppe"

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    All-Optical Reconfiguration of Ultrafast Dichroism in Gold Metasurfaces
    (Wiley, 2022) Schirato, Andrea; Toma, Andrea; Proietti Zaccaria, Remo; Alabastri, Alessandro; Cerullo, Giulio; Della Valle, Giuseppe; Maiuri, Margherita
    Optical metasurfaces have come into the spotlight as a promising platform for light manipulation at the nanoscale, including ultrafast all-optical control via excitation with femtosecond laser pulses. Recently, dichroic metasurfaces have been exploited to modulate the polarization state of light with unprecedented speed. This work theoretically predicts and experimentally demonstrates by pump–probe spectroscopy the capability to reconfigure the ultrafast dichroic signal of a gold metasurface by simply acting on the polarization of the pump pulse, which is shown to reshape the spatio-temporal distribution of the optical perturbation. The photoinduced anisotropic response, driven by out-of-equilibrium carriers and extinguished in a sub-picosecond temporal window, is readily controlled in intensity by tuning the polarization direction of the excitation up to a full sign reversal. Hence, nonlinear metasurfaces are here demonstrated to offer the flexibility to tailor their ultrafast optical response in a fully all-optically reconfigurable platform.
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    All-Optically Reconfigurable Plasmonic Metagrating for Ultrafast Diffraction Management
    (American Chemical Society, 2021) Schirato, Andrea; Mazzanti, Andrea; Proietti Zaccaria, Remo; Nordlander, Peter; Alabastri, Alessandro; Della Valle, Giuseppe; Laboratory for Nanophotonics
    Hot-electron dynamics taking place in nanostructured materials upon irradiation with fs-laser pulses has been the subject of intensive research, leading to the emerging field of ultrafast nanophotonics. However, the most common description of nonlinear interaction with ultrashort laser pulses assumes a homogeneous spatial distribution for the photogenerated carriers. Here we theoretically show that the inhomogeneous evolution of the hot carriers at the nanoscale can disclose unprecedented opportunities for ultrafast diffraction management. In particular, we design a highly symmetric plasmonic metagrating capable of a transient symmetry breaking driven by hot electrons. The subsequent power imbalance between symmetrical diffraction orders is calculated to exceed 20% under moderate (∼2 mJ/cm2) laser fluence. Our theoretical investigation also indicates that the recovery time of the symmetric configuration can be controlled by tuning the geometry of the metaatom, and can be as fast as 2 ps for electrically connected configurations.
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    Nanoporous Titanium Oxynitride Nanotube Metamaterials with Deep Subwavelength Heat Dissipation for Perfect Solar Absorption
    (American Chemical Society, 2023) Afshar, Morteza; Schirato, Andrea; Mascaretti, Luca; Hejazi, S. M. Hossein; Shahrezaei, Mahdi; Della Valle, Giuseppe; Fornasiero, Paolo; Kment, Štěpán; Alabastri, Alessandro; Naldoni, Alberto
    We report a quasi-unitary broadband absorption over the ultraviolet–visible–near-infrared range in spaced high aspect ratio, nanoporous titanium oxynitride nanotubes, an ideal platform for several photothermal applications. We explain such an efficient light–heat conversion in terms of localized field distribution and heat dissipation within the nanopores, whose sparsity can be controlled during fabrication. The extremely large heat dissipation could not be explained in terms of effective medium theories, which are typically used to describe small geometrical features associated with relatively large optical structures. A fabrication-process-inspired numerical model was developed to describe a realistic space-dependent electric permittivity distribution within the nanotubes. The resulting abrupt optical discontinuities favor electromagnetic dissipation in the deep sub-wavelength domains generated and can explain the large broadband absorption measured in samples with different porosities. The potential application of porous titanium oxynitride nanotubes as solar absorbers was explored by photothermal experiments under moderately concentrated white light (1–12 Suns). These findings suggest potential interest in realizing solar-thermal devices based on such simple and scalable metamaterials.