Hybrid Plasmonic/Photonic Nanoscale Strategy for Multilevel Anticounterfeit Labels.

Innovative goods authentication strategies are of fundamental importance considering the increasing counterfeiting levels. Such a task has been effectively addressed with the so-called physical unclonable functions (PUFs), being physical properties of a system that characterize it univocally. PUFs are commonly implemented by exploiting naturally occurring non-idealities in clean-room fabrication processes. The broad availability of classic paradigm PUFs, however, makes them vulnerable. Here, we propose a hybrid plasmonic/photonic multilayered structure working as a three-level strong PUF. Our approach leverages on the combination of a functional nanostructured surface, a resonant response, and a unique chromatic signature all together in one single device. The structure consists of a resonant cavity, where the top mirror is replaced with a layer of plasmonic Ag nanoislands. The naturally random spatial distribution of clusters and nanoparticles formed by this deposition technique constitutes the manufacturer-resistant nanoscale morphological fingerprint of the proposed PUF. The presence of Ag nanoislands allows us to tailor the interplay between the photonic and plasmonic modes to achieve two additional security levels. The first one is constituted by the chromatic response and broad iridescence of our structures, while the second by their rich spectral response, accessible even through a common smartphone light-emitting diode. We demonstrate that the proposed architectures could also be used as an irreversible and quantitative temperature exposure label. The proposed PUFs are inexpensive, chip-to-wafer-size scalable, and can be deposited over a variety of substrates. They also hold a great promise as an encryption framework envisioning morpho-cryptography applications.

Biomolecular Sensing at the Interface between Chiral Metasurfaces and Hyperbolic Metamaterials.

In recent times, biomolecular sensing to recognize genetic fragments and proteins is spurring unprecedented interest as a diagnostic protocol for cancer and infectious diseases. Significant efforts have been made to design nanomaterials able to control the light-matter interaction at the single nanometer scale, where genes and proteins bind specifically to receptors. Here, we numerically show how the interface between a chiral metasurface and hyperbolic metamaterials can enable both high sensitivity and specificity for low-molecular-weight nucleic acids and proteins. As we have recently reported, hyperbolic dispersion metamaterials allow molecular biorecognition with extreme sensitivity because of coupled and highly confined plasmon polaritons. Specificity is almost exclusively achieved by receptor-ligand interaction at the in-plane sensing surface. Interestingly, an adapted out-of-plane chiral metasurface enables three key functionalities of the hyperbolic metamaterial sensor. Computational effort reveals that helicoidal metasurfaces can act as (i) efficient diffractive elements to excite surface and bulk plasmon polaritons; (ii) out-of-plane sensing branches to reduce the diffusion limit and increase the sensing surface; and (iii) biorecognition assay also via circular dichroism and chiral selectivity.