Abstract

The METAFAST project aims to develop a novel class of synthetic nonlinear optical materials, or metamaterials, as a disruptive platform enabling unprecedented ultrafast dynamical control over polarization and wavefront of light. In particular, the project has the potential to develop novel ultracompact all-optical modulators capable of faster than ever structuring of the spin and orbital angular momentum (SOAM) of light beams. Such ultrafast optical modulation offers an exceptionally robust method for the encoding of digital information in free space optical links, being also resistant to eavesdropping thanks to topological protection.

Summary of the Project Results

The work performed during the four and a half years of the METAFAST project has concerned two main activities.

First of all, we developed a consistent theoretical framework for the modeling of the constitutive elements of optical metasurfaces, i.e. nonlinear metaatoms. In particular, we have reported a comprehensive electromagnetic description of photonic nanomaterials, from individual nano-objects to ultrathin planar periodic structures. The model comprises photogenerated carriers and their spatial inhomogeneities at the nanoscale, and the subsequent modification of the optical properties of noble metal (gold) and semiconductor (silicon-based and gallium arsenide-based) nanostructures. A particular emphasis is devoted to the modeling of light-induced modifications of the metasurfaces capable of changing light polarization (i.e. the direction of electric field oscillation). The theoretical results have been tested via nanofabrication technology and ultrafast spectroscopy means. In particular we have investigated a variety of metasurface configurations, seeking for the enhancement of nonlinear effects for the control of light polarization (one of the internal structural elements of light beams) and wavefront. To achieve this goal, three different strategies have been explored: (i) metasurfaces based on asymmetric metaatoms in 3D or even 2D configurations; (ii) metasurfaces with symmetric metaatoms to be made highly anisotropic by inducing inhomogeneous excitation of hot carriers or of electron-hole pairs at the nanoscale; (iii) polarization and wavefront control via third-harmonic or second-harmonic generated beams. Thanks to the synergy between the novel modeling approaches introduced in the field and the refined experimental techniques implemented, we have reported key results towards the final aim of the project, that is the development of a novel photonic platform based on nonlinear metasurfaces for the all-optical structuring of light beams. In particular we demonstrated: (i) full reconfiguration of light polarization at GHz speed; (ii) giant, record high, photoinduced transient dichroism; (iii) light-by-light reconfiguration of light beam wavefront on picosecond time scale; (iv) orbital angular momentum structuring of nonlinearly generated light; (v) novel multi-functional nonlinear metasurfaces also in stacking configurations; (vI) spatially-controlled all-optical tuning and switching of metasurfaces; (vii) efficient ultrafast all-optical metamirrors; and (vii) novel ultrafast lasers and modulation methods. Finally, the first demonstration of ultrafast light structuring with a multifunctional metasurface platform (exploiting a cascading design comprising linear and nonlinear metaoptics structures) has been reported, and this result was published in a high impact journal, Nature Communications Vol. 15, 2507 (2024).

The project has also disclosed novel research directions that had not been identified in the original submission of the proposal. In particular, we found that hot-carrier driven nonlinearities at the nanoscale can benefit from ultrafast spatial inhomogeneities as a novel degree of freedom to engineer all-optical reconfiguration of metasurfaces. Also, light polarization can be controlled via nonlinear light generation. Novel ultrafast laser sources and ultrafast modulation techniques have been also develped. Moreover, our research have explored alternative materials compared to those in the mainstream of the project, seeking for improved efficiency, mass-production capability and even faster optical response. These achievements represented unique skills of our Consortium that have been fruitfully exploited by reporting 7 innovations, establishing industrial partnerships and cooperations, patenting activities, and increasing the TRL in new projects. Several young researchers have been involved and started their training along the new research directions opened by METAFAST. Thanks to the expertise developed working on the METAFAST project, several young researchers from different Units of our Consortium have boosted their carrier with starting grants, including ERC Starting and MSCA Global fellowship.

The ground-breaking technology developed by METAFAST with ultrafast all-optically reconfigurable metasurfaces for light structuring at unprecedente speed can disclose a new route for next generation optical networks, based on free-space-optics (FSO) links. FSO links are much easier and cheaper to install compared to fiber-based-optics links and can thus support the bandwidth demand posed by the just now beginning second internet revolution (or the “internet of things”), with lower environmental impact and superior economic sustainability. This applicative scenario is timely, since the market of free-space optics communications is growing: more than 500 Light Fidelity (LiFi) startups are now operative worldwide. The researchers involved in the METAFAST project will definidely contribute to a breakthrough development in this new emerging market in the near future.