Color Routing via Cross-Polarized Detuned Plasmonic Nanoantennas in Large-Area Metasurfaces

Bidirectional nanoantennas are of key relevance for advanced functionalities to be implemented at the nanoscale and, in particular, for color routing in an ultracompact flat-optics configuration. Here we demonstrate a novel approach avoiding complex collective geometries and/or restrictive morphological parameters based on cross-polarized detuned plasmonic nanoantennas in a uniaxial (quasi-1D) bimetallic configuration. The nanofabrication of such a flat-optics system is controlled over a large area (cm2) by a novel self-organized technique exploiting ion-induced nanoscale wrinkling instability on glass templates to engineer tilted bimetallic nanostrip dimers. These nanoantennas feature broadband color routing with superior light scattering directivity figures, which are well described by numerical simulations and turn out to be competitive with the response of lithographic nanoantennas. These results demonstrate that our large-area self-organized metasurfaces can be implemented in real-world applications of flat-optics color routing from telecom photonics to optical nanosensing.


TILTED OPTICAL NANOANTENNAS FABRICATION
A soda-lime glass substrate (20 x 20 x 2 mm) is repeatedly rinsed in ethanol and acetone. The sample is then placed in a custom-made vacuum chamber and irradiated with an 800 eV low energy defocused Ar + ion beam (gas purity N5.0). A biased tungsten filament avoids charge build-up through thermionic electron emission. The ion beam illuminates the glass surface at an incident angle of θ = 30° with respect to its normal. The ion fluence corresponds to 1.4 ×10 19 ions/cm 2 and the glass temperature is fixed at about 680 K during the Ion Beam Sputtering (IBS) process. After the rippled pattern is formed on the glass surface, thermal Au deposition is performed on the rippled facets at a glancing angle θ = 55° with respect to the flat sample normal. The Au beam directly illuminates the glass facets tilted at +35° while the opposite facets are completely shadowed. By means of a calibrated quartz microbalance, the thickness h of the Au stripes can be evaluated by basic geometrical arguments given the Au thickness (h0) deposited on a flat surface facing the crucible at normal incidence and the average slope of the illuminated facet measured with AFM as h = h0  cos(55°-35°). The sample is then put in a custom-made RF sputtering chamber where a layer of SiO2 is conformally grown all over the surface using a 2" fused silica target. The silica layer thickness was monitored by means of a calibrated quartz microbalance. The RF sputtering experiment is run in Argon atmosphere at a power P = 60 W, sample-target distance d = 8.5 cm and total pressure of about P = 710 -2 mbar. Finally, Ag stripes are confined on the rippled facets tilted at -50°, now coated with a conformal SiOx layer, by using the same strategy and arguments already described for the Au ones.

MORPHOLOGICAL CHARACTERIZATION
The rippled glass template morphology was characterized by means of an atomic force microscope  The template average periodicity λ is estimated from the real space distance between the maximum and the secondary neighboring peaks in the 2D self-correlation function (Figs. S1b and S1c). It's worth to note how the 2D self-correlation function of the rippled patterns rapidly decays to negligible values away from the central maxima, as the pattern loses its morphological coherence within 2-3 unit cells. This prevents the rippled glass template, and consequently the nanoantennas array confined on it, from showing grating optical effects which would lead to a more complex engineering of the color routing properties of our self-organized large area platform.

NUMERICAL OPTICAL MODEL
For the numerical analysis of the nanostrip antennas we employed a commercial software (Comsol Multiphysics 5.3), implementing the full-vectorial finite element method for scattered field formalism in two dimensions. We assumed a circular computational domain with 500 nm radius, surrounded by perfectly matched layers (PML) with scattering boundary conditions. The effective environment approximation was assumed (in accord with, e.g., Refs. 1,2) by embedding the nanostrips into a homogeneous dielectric medium with non-dispersive and lossless permittivity.
The sketch of the bimetallic nanoantenna is shown in Fig. S2a. To avoid numerical artifacts, the vertices of the Au and Ag nanostrips have been rounded with 5 nm and 15 nm radius of curvature, respectively. The FEM mesh was accordingly defined so to resolve these radii with at least 5 elements. For the dielectric domain we set a maximum mesh element size corresponding to  Starting from the scattered electric ES and magnetic HS vector fields (numerically solved for as a function of ), the total extinction cross-section is computed as E = A +S, with A and S the total absorption and scattering cross-section spectra, respectively given by: With the total extinction cross-section at hand, the transmittance of the sample at normal incidence is estimated as following: where L = 200 nm is the measured average periodicity of the sample and  a dimensionless fitting parameter of the order of 1 (in our simulations we set  Concerning the far-field scattering patterns, we employed the FAR-FIELD procedure in Comsol, implementing the Stratton-Chu formulas (see, e.g., Ref. 7) using Σ as the aperture enclosing our 2D antennas (either being the Ag or Au monomer, or the Ag-Au dimer).

OPTICAL CHARACTERIZATION
VIS-NIR extinction measurements were performed at normal incidence using a halogen-deuterium  In Fig. S4 we show the measured directivity data for the Au, Ag and Au/Ag NSA reported in the main manuscript in Fig. 4a-b-c respectively, but using the same dB scale for all the panels. Fig. S4 evidences how the signal-to-noise ratio is similar for all the three different considered configurations when the directivity is not steeply changing with wavelenght.