Light on-demand with meta-surfaces
An international team of researchers, led by Giuseppe Della Valle of the Department of Physics at Politecnico di Milano, has demonstrated the possibility of using an active meta-surface for high speed and extremely efficient manipulation of light. The results of the study are of crucial interest for the development of free-space optical communications, in which digital information transfer is achieved robustly without the use of optical fibers.
In the study, recently published in Nature Light: Science & Applications, researchers propose a meta-surface, an ultra thin, fully synthetic material, capable of inducing highly efficient modulations of light, reaching strong variations in both intensity (>400%) and polarization (i.e., the direction of oscillation of the light wave), for low intensity photoexcitation.
The extraordinary efficiency of these processes was achieved by exploiting the peculiar properties of the photoexcited semiconductor material, in combination with the optimized geometrical parameters of the meta-surface obtained through numerical simulations at the nanoscale (the spatial scale of meta-atoms) and a refined experimental apparatus for studying ultrafast interactions between light and matter.
Giulia Crotti e Mert Akturk, researchers and lead authors of the paper
The multidisciplinary research team designed, produced and tested a nanostructured material consisting of semiconductor (gallium arsenide and aluminum) wires less than a hundredth of a hair's breadth thick (so-called meta-atoms) arranged parallel to each other to form a synthetic surface, or meta-surface.
The experiments conducted revealed record-breaking performance in terms of light modulation, and particularly light polarization, via laser pulse control. Indeed, laser photoexcitation allows to radically change meta-surface properties, varying transmission by 470% in a picosecond (a very short time, precisely 1 billionth of a second).
In addition, for some specific colors of light, this strategy makes it possible to change the direction of oscillation of the light wave, transforming it from linear to circular, an effect known as “quarter-wave foil” that, in this case, is activated on demand at an unprecedented speed.
These results on gallium aluminum arsenide meta-surfaces suggest a promising approach to achieve all-optical modulation of light polarization in a very efficient way. Moreover, the concept proposed and demonstrated in the paper can be easily extended to other semiconductor platforms and for other properties of light. Having huge variations of such properties via low power control pulses is of crucial importance for the development of real devices of industrial interest, for example for encoding and on-demand manipulation of optical signals for free-space telecommunications.
Giuseppe Della Valle, corresponding author of the study and coordinator of the METAFAST project
The work coordinated by Politecnico di Milano involved researchers from Université Paris-Cité in Paris who produced the meta-surface, while the design, numerical simulations and experiments were conducted in Milan. The study was carried out in collaboration with the Italian Institute of Technology, the University of Sofia (Bulgaria) and the Australian National University. It is part of the Horizon Europe project METAFAST.