Fine structuring of optical materials is a powerful tool to shape their optical response, to control the way in which light impinging on them is reflected, scattered or transmitted. Scientists and engineers have been pursuing this route for a long time by now, creating sophisticated devices in which nanometric patterns mold the flow of light to give rise to the desired response, in a similar way to what nature does to create the stunning beauty and iridescent colors of peacock feathers or opal gem stones.

The approach has nevertheless one drawback: such structures are generally not tunable. Once embedded in the material, they can no longer be modified at will, which means that their optical response is fixed. Imagine instead we could make a peacock feather change color with the intensity of the light we shine on it, or in a similar way reconfigure the way in which an optical chip processes the incoming signals simply by adjusting their power.

The realization of such optical ‘metamorphic’ devices is indeed a holy grail in current nanophotonic research. To do the trick we need optical nonlinearities. Countless new recipes and materials are being explored towards optically tunable devices, yet how to achieve this goal while avoiding complex and costly material processing, potential material damage and device instabilities and prohibitively high operational powers, remains still an open issue.

Y. Zhou and collaborators present an appealing approach to engineer a nonlinear material, based on embedding silicon nanoparticles in a solid organic wax, which has the benefits of being cheap and simple, exploiting the almost ubiquitous silicon, and overcoming the limitations of unstable aqueous and organic solutions, in order to achieve a tunable nonlinear response. At low nanoparticle concentrations (20-30 mM), the material behaves as an optical limiter, with a response underpinned by an increased nonlinear scattering arising from the laser-induced formation of micro-bubbles around the silicon nanoparticles. At higher concentrations (60 mM) the material turns into a saturable absorber, due to the predominance of the silicon carrier dynamic response. Furthermore, the authors demonstrate memory effects associated with prolonged laser exposure, as well as self-healing capabilities of the nano-composite system, which can recover its original properties upon removal of the light irradiation over time scales of around 30s.

All this points out to new possibilities to tailor metamorphic optical nonlinearities in solid state form through a smart cocktail of functionalized nanoparticles and suitable organic hosts, which combines ease of fabrication and richness in the achievable scenario of optical functionalities.

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