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Metasurfaces for ultrafast light structuring

Periodic Reporting for period 1 - METAFAST (Metasurfaces for ultrafast light structuring)

Periodo di rendicontazione: 2020-10-01 al 2021-09-30

The METAFAST project aims to develop a novel technological paradigm for the control of light beams at unprecedented speed and in ultracompact footprints. The idea is to demonstrate light-by-light (all-optical) modulation from synthetic photonic materials, so-called metasurfaces.

The current digital era has been founded, from its origins, on optical communications technology, providing data transfer at a much faster rate compared to communications based on electrical lines. The current scenario is dominated by optical fiber transmissions, while free-space (i.e. in air) optical links have not seen a significant development. However, the interest in this type of transmission is huge, being without cables. This could be extremely useful in metropolitan areas, where there are strong logistical constraints due to the high density of buildings and where the demand for bandwidth is inexorably growing, due to the 5G-6G revolution, already underway.
High-speed optical links in free space may be the best solution to this problem.

At present, FSO technology suffers from two main limitations: (i) the disturbance and attenuation effects introduced by atmospheric perturbations; (ii) the speed limits of state-of-the-art FSO modulators.

The ultimate goal of the project is to develop ultracompact all-optical FSO modulators capable of faster than ever structuring of the spin and orbital angular momentum (SOAM) of light beams.
Such ultrafast optical modulation can offer an exceptionally robust method for the encoding of digital information in free space optical links, being also resistant to eavesdropping thanks to topological protection.
The work performed during the first year of the project have 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). 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 control via third-harmonic or second-harmonic generated beams.
The progress beyond state-of-the-art during the first year is ascertained by the number of articles published by our Consortium on international peer-referred scientific journals, 16 in total, including two articles on the most prestigious journal in the field, Nature Photonics.

The first-year intermediate objectives have been reached, but the project has also disclosed two 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.
These achievements represent unique skills of our Consortium that will be fruitfully exploited within the project development and beyond. Also, the project partners have already enrolled six post-docs and one PhD student, and several young researchers have been involved and started their training along the new research directions opened by the project.

The most important challenge now facing the project is to demonstrate ultrafast control over the wavefront of light. This is the main topic of the ongoing studies and the first results are expected by the end of the second year of activity.

In terms of impact, the long-term vision of the METAFAST project is represented by the development of a ground-breaking technology based on ultrafast all-optically reconfigurable metasurfaces. If successful, the project 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 very timely, since the market of free-space optics communications is growing: more than 500 Light Fidelity (LiFi) startups are now operative worldwide. If successful, the project can contribute to a breakthrough development in this new emerging market.
Ultrafast reconfigurable metasurface for transient optical dichroism