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.

The investigation of optical phenomena at the nanoscale has witnessed an amazing development in the last decades, with particular interest to metallic nanostructures, supporting so-called plasmonic resonances and subsequent spatial localization of intense optical fields far below the diffraction limit.[1−4] A key feature of subwavelength metallic structures is their capability to provide amplified light scattering and strong near-field confinement at the resonant frequency, thus behaving as optical nanoantennas.[5−10] Plasmonic nanoantennas have been thoroughly developed to demonstrate challenging functionalities in flat-optics nanodevices,[11−13] including waveplates,[14,15] polarization splitters,[16,17] directional nanoemitters,[18−20] unidirectional antennas,[21−23] and multidirectional color routers.[24−27] The availability of broadband and highly directive optical antennas is crucial for the development of a wide range of applications from optical sensing[2,28,29] to photon harvesting[12,30,31] and biosensing.[32−35] When dealing with color routing, i.e., wavelength-selective multidirectional scattering, different strategies, comprising single or multielement antenna systems, and different materials, have so far been explored: as summarized below, such approaches to color routing predominantly exploit the complex design of the nanoantenna geometry, interantenna gap and chemical composition which require cumbersome top-down nanofabrication methods.

A first approach relies on multielement nanoantennas. The simplest configuration exploits the scattering interference between two asymmetric nanoantenna elements whose resonant dipoles oscillate with a large phase shift with respect to the illuminating field, in the spectral range in-between their resonances.[24,25,36,37] The wavelength selectivity, enabling color routing operation, is obtained via a symmetry breaking induced, e.g., by using two different materials for the two nanoantenna elements.[24,37] The unidirectionality can also be enhanced with several elements in order to realize a downscaling of the classic RF Yagi-Uda antenna[21,38,39] or other array configurations.[38,40,41] Another approach exploits the near-field interference of multiple resonances (e.g., electric and magnetic) simultaneously excited in the same antenna element.[26,42−44] Plasmonic nanoantennas can be engineered to achieve unidirectionality by resorting to specific antenna geometries,[45,46] e.g., gold split-ring resonators[47] or even nanodisk antennas,[48] but generally only dielectric and hybrid metal/dielectric antennas are suited for color routing due to the strength and phase shifts of magnetic modes in dielectric materials.[26,49] The third approach is based upon wavefront manipulation by highly ordered metasurfaces, with meta-atoms made of either single or multielement nanoantennas, providing either wavelength-independent unidirectional scattering,[50] or wavelength selective narrow-band bidirectional color routing operation.[27,51] Very recently, active metasurfaces for bias controlled directional scattering have also started to gain attention.[52] All the outlined strategies require heavy computational effort and time-consuming top-down nanofabrication processes to control interantennas spacing, shape, and material composition, which are costly and inherently limited to small areas.

In this work, we achieve broadband and highly effective color routing functionalities over a large area (cm2) with much more relaxed fabrication constraints and requirements compared to the state-of-the-art antenna design. Our idea is based on a novel flat-optics approach to bidirectional passive color routing at optical frequencies with cross-polarized detuned plasmonic nanoantennas in a uniaxial (quasi-1D) bimetallic configuration. The directional routing mechanism is based purely on the relative tilt and material composition of the nanoantenna elements, rather than on the interference of their resonant optical modes, which is actually inhibited by the cross-polarization configuration. We demonstrate that our strategy can be effectively implemented via self-organized nanofabrication based on controlled anisotropic nanoscale wrinkling in transparent templates and maskless confinement of plasmonic bidirectional antennas.

Results and Discussion

The concept behind our approach is summarized in Figure . Let us consider the two-dimensional (2D) scattering problem of two lines of cross-polarized electric dipolar scatterers (having out-of-plane continuous translational invariance), excited by a uniform plane wave with an in-plane electric field (i.e., linear TM polarization) (Figure a). If the two scatterers exhibit a resonant behavior with well-separated scattering peaks (Figure b), e.g., one in the blue (λ1) and one in the red (λ2), the blue light is mainly scattered at 90° with respect to the red light, with the negligible superposition of the two spectral components in the far-field along the two orthogonal directions.

Figure 1

(a) Sketch of the 2D cross-polarized electrical dipoles for color routing with (b) detuned scattering spectra; (c) implementation with bimetallic nanostrips (having continuous translational invariance along the out-of-plane y-axis) and (d) the actual scattering spectra of the gold monomer (red), of the silver monomer (blue) and of the dimer (black), retrieved by full-wave 2D numerical simulations; (e) scattering diagram of the nanostrip monomers at their peak resonance wavelengths; (f) same as panel e for the nanostrip dimer, also showing the scattering diagram at an intermediate wavelength (in between the two resonances).

Such an ideal system can be implemented with a couple of tilted metallic nanostrips, which are well-known to behave as dipolar scatterers, having a pronounced resonance under TM light, enabled by a plasmonic response.[53] The latter can be controlled and tuned by acting on the width (w) and height (h) of the nanostrips, as well as on the metal permittivity. Basically, the fundamental dipolar resonance wavelength of the nanostrip linearly scales with w and is inversely proportional to h. Also, note that for a given geometrical configuration, silver nanostrips resonate at shorter wavelengths compared to gold nanostrips.[53] The latter circumstance indicates that a hybrid Au/Ag configuration for the dimer should be more flexible to achieve detuned plasmonic resonances from the two nanostrips, which is a key feature for efficient color routing. Having this in mind, we designed a two-dimensional plasmonic dimer made of a gold nanostrip with (w1,h1) = (100,13) nm tilted at about 40° with respect to the x-axis and a silver nanostrip with (w2,h2) = (90,34) nm tilted at about −50° (Figure c). Finite element method (FEM) numerical analysis of the nanostrips scattering spectrum (Figure d) retrieves a peak resonance wavelength at λ1 = 500 nm for silver and at λ2 = 740 nm for gold isolated nanostrips (or monomers). The scattering diagram as a function of the polar angle Θ for the isolated monomers (Figure e) exhibits the typical pattern of a dipolar scatterer, with two main lobes orthogonal to the major axis of the nanostrip. In the dimer configuration, the scattering diagram, even though being more complex because of the near-field coupling between the two monomers, preserves the key features of the individual scattering patterns at the two resonance wavelengths (Figure f). In particular, note the wavelength-dependent bidirectional behavior, characterized by a strong rejection of the blue wing of the spectrum at Θ ∼ 140° and of the red wing at Θ ∼ 40°, implementing the desired color routing functionality.

Even though conceptually simple, the design above detailed poses a fundamental challenge in terms of fabrication, because of the tilted configuration of the nanostrips and the requirement of a large-area nanopatterning to allow flat-optics operation. Here, a self-organized nanofabrication method, based on anisotropic nanoscale wrinkling in glasses, enables the effective engineering of tilted plasmonic nanostrip antennas in the form of cross-polarized metallic dimers lying on opposite ridges of a faceted dielectric template.

A low-cost glass substrate is irradiated with a defocused Ar+ ion beam set at an incident angle of θ = 30° and at a low energy of 800 eV. The glass temperature is fixed at about 680 K during the ion beam sputtering (IBS) process. A quasi-1D rippled pattern is obtained all over the macroscopic sample surface, with a wavevector parallel to the ion beam direction (Figure a). The glass ripples are elongated for a length of several micrometers and show a remarkable degree of long-range order. The nanostructures are characterized by a steep asymmetric sawtooth profile with a pronounced vertical dynamic of approximately 90 nm (Figure b) and a periodicity of about 200 nm (see the 2D self-correlation function of the AFM topography in Figure S1). The ridges directly exposed to the ion beam develop a broad facet slope distribution (negative values in the slope frequency plot of Figure c) with a characteristic slope peaked at about −50°. The opposite ridges (positive values in Figure c) develop wider and very defined facets with slopes peaked at +35°.

Figure 2

(a) Atomic force microscopy (AFM) topography of the fabricated rippled glass template. Scale bar = 1 μm. (b) AFM cross section of nanoripples corresponding to the green line in panel a. (c) Histogram of the slope distribution extracted by the AFM image of panel a. (d) Scanning electron microscope (SEM) images of the bimetallic metasurface detected in the top view and (e) cross section configuration. Scale bars 1 μm and 600 nm, respectively. The inset represents a sketch of the unit cell cross section of the bimetallic surface.

As reported by some of the authors in a recent study,[54] raising the substrate temperature near the glass transition is crucial to activating a peculiar solid-state wrinkling instability, which strongly enhances the nanopattern growth, acting in parallel with the ion beam erosion process. Under this condition, high-aspect ratio ordered 1D templates are obtained, with a strong improvement with respect to state-of-the-art large-area IBS nanopatterning of semiconductor and insulating substrates at room temperature, which commonly yields low-aspect-ratio, disordered ripples.[55,56]

Such high-aspect-ratio 1D rippled templates, with well-defined tilted facets, are the ideal platform for the maskless confinement of plasmonic uniaxial nanostrip antennas (NSA) with controlled tilt and morphology. The controlled growth is achieved by grazing angle physical vapor deposition of metal atoms exploiting shadowing effects on the tilted ripple facets.[57] In order to fabricate the bimetallic dipolar antennas depicted in the theoretical model of Figure c, we first confined Au NSA by glancing evaporation on the wider rippled facets, which are tilted at +35° (see inset sketch in Figure e). By the statistical analysis of SEM cross-sectional images (Figure e), we measured the average Au NSA width w ≈ 105 nm, controlled by the periodicity of the underlying template and by the local slope of the illuminated ripple facets. The thickness of the Au NSA reads h ≈ 12 nm and is determined by the sublimated metal dose and deposition angle (see Supporting Information for details). As a second step, an insulating layer of substoichiometric silica was grown on the sample at normal incidence by radio frequency magnetron sputtering. Finally, Ag NSA were confined on the narrower and steeper rippled facets with an average slope of −50°, on top of the conformally grown silica layer. Ag NSA average w and h are about 83 and 36 nm, respectively. The average dielectric gap separating the Au and Ag NSA corresponds to 40 nm, again evaluated from the SEM cross-sectional analysis. This method enables the controlled growth of subwavelength bimetallic antennas whose length exceeds several micrometers and form a large-area quasi-1D metasurface, as demonstrated by the top-view and cross-sectional SEM images (Figure d,e). The image of the sample cross section (Figure e) evidence the selective lateral confinement of nanoantennas on top of the tilted ridges of the ripple, thus well mimicking the cross section of the ideal structure sketched in the inset of Figure e.

Optical transmittance measurements at normal incidence were performed on the sample using a halogen/deuterium lamp source. Scattering measurements were instead acquired with a custom-made scatterometer setup, following the scheme of Figure l,m (see Supporting Information for details). The sample was illuminated from the glass side at normal incidence, and the scattered light was collected at a fixed polar angle Θ = 50°, as a function of the azimuthal angle Φ. For both transmittance and scattering measurements, the light source was linearly polarized orthogonally to the NSA long axis (i.e., TM polarization). The same optical measurements were also performed on two other samples: Au NSA, and Ag NSA, fabricated with the same morphological and geometrical configuration as in the complete bimetallic Au–Ag NSA. This allows us to investigate the properties of the final system as well as the optical behavior of its two building blocks. The optical transmittance and the scattering intensities measured for the Au, Ag, and Au–Ag NSA samples are shown in Figure d–f and Figure g–i, respectively. Remarkably, for all three configurations, a directional scattering maximum is detected, which is resonant to the localized surface plasmon mode (i.e., minimum in transmittance in Figure d–f).

Figure 3

(a, b, c) Sketch of the Au, Ag, and Au/Ag NSA meta-atom cross section, respectively. (d, e, f) Measured optical transmittance at normal incidence for Au, Au, and Au/Ag NSA arrays, respectively. All spectra are normalized to the transmittance of the bare glass substrate. (g, h, i) Scattered light intensity detected at Θ = 50° and Φ = 0° (solid curve) and at Θ = 50° and Φ = 180° (dashed curve) for Au, Ag, and Au/Ag NSA arrays, respectively. (l, m) Sketch of the optical setup exploited for scattering measurements, respectively, shown in top- and side-view.

As anticipated above, the scattering pattern of a plasmonic NSA is expected to resemble the one of a dipolar antenna. The transmitted scattering maximum is then expected to be centered along the direction normal to the NSA major axis. When Φ = 0° and Θ = 50°, the scattered light collection is essentially facing the +35° tilted Au NSA (as it can be clearly appreciated in the inset of Figures l,m), i.e., the direction where the scattering intensity is expected to be stronger. Indeed, for this collection configuration (solid yellow curve in Figure g), the measured scattering intensity shows an intense and broad maximum centered around 750 nm. Note that the scattering maximum and the plasmonic transmittance dip are resonant (the slight frequency shift being ascribable to the well-known mismatch between absorption and scattering resonances in plasmonic nanostructures), thus clearly demonstrating the plasmonic nature of the enhanced scattered light. When the collection of the scatterometer is set to the azimuthal angle Φ = 180°, the scattered light is measured along the direction of the Au NSA major axis. As a consequence, the scattering intensity drops by about an order of magnitude, in accord with the radiation pattern simulations of Figure e. Figure e shows the transmittance measurement for the Ag NSA, with the Ag nanostrips confined on a silica layer grown upon the rippled glass template, to preserve the effective refractive index of the medium surrounding the NSA in the complete bimetallic NSA. The plasmonic transmittance dip is blue-shifted to 500 nm compared to the Au NSA, due to the different Ag NSA aspect ratio (w/h), and higher permittivity (in modulus) of silver compared to gold.[58] This time, when Φ = 180° (keeping Θ = 50°), the scattered light collection is perfectly aligned to the normal of the Ag nanostrips, which are tilted at −50°, and an intense scattering peak is recorded at resonance, i.e., around 500 nm, near the edge of our laser source spectrum (dashed gray curve in Figure h). Note that the resonant scattering intensity of the Ag NSA is about 3 times higher than in the Au NSA, mainly because of the better optimized collection angle with respect to the NSA axis and also thanks to the higher optical density of Ag, which implies a larger Ag scattering cross section.[53] When the azimuthal angle is set to Φ = 0°, the scattered light is collected in a direction parallel to the Ag NSA axis, which corresponds to the direction of minimum scattered intensity emission pattern (cf. Figure ). As a consequence, the collected scattering intensity considerably drops down (solid gray curve in Figure h). Finally, the transmittance spectrum for the complete bimetallic Ag–Au NSA is shown in Figure f. Two separate transmittance dips are distinguishable at 500 and 740 nm, which are associated with the plasmonic resonances of the Ag NSA and Au NSA monomers, respectively. When the azimuthal angle is fixed to Φ = 0°, the collection of the scattering intensity faces the Au NSA, and a scattering maximum is recorded at the Au NSA resonance of 740 nm (solid yellow curve in Figure g), with negligible contributions from the Ag NSA (cf. Figure h, solid gray curve). When the azimuthal angle is set to Φ = 180°, the collection direction faces the Ag NSA, and a scattering intensity peak is recorded at the Ag NSA resonance of 500 nm, with negligible contribution from the Au NSA (cf. Figure g, dashed yellow curve).

Note also that the scattering from the Au–Ag dimeric NSA is partially attenuated with respect to the case of individual Au or Ag NSA building blocks, mostly because of nonresonant shadowing effects. Actually, even though hybridization is negligible in our detuned cross-polarized configuration (in agreement with the numerical simulations of Figure ), each monomer is placed within the cross-section region of the other, thus causing an effective partial attenuation of the beam. Remarkably, the detected directivity figures highlight the high efficiency of self-organized bimetallic antennas in wavelength selective bidirectional scattering. This response is well explained in terms of the cross-polarized detuned plasmonic nanoantenna model. Provided the tilted nanoantennas configuration, the Ag and Au NSA monomer resonantly scatters light to the “right side” at about 500 nm wavelength, and to the “left side” at about a 740 nm wavelength, respectively. This color routing action taking place over the whole sample macroscopic area can be clearly appreciated in the digital camera images of Figure S3.

To quantify the color routing performance in our samples, we plot in Figure the directivity spectrum (D), defined as follows:

Figure 4

Measured (a, b, c) and simulated (d, e, f) directivities at two different polar angles (Θ = 30° in blue, Θ = 50° in red) for (a, d) Au NSA monomer, (b, e) Ag NSA monomer, and (c, f) Au–Ag NSA dimer configurations, respectively.

The directivity spectra at polar angles Θ = 30° and Θ = 50° are shown in Figure (top panels), together with the corresponding numerical simulations (bottom panels). Note that the vertical axis dB scale is different for every panel of Figure for better reading of the traces. In particular, as can be appreciated in Figure S4, the signal-to-noise ratio of the measured directivity data is indeed similar for all three different considered NSA configurations (Au monomer, Ag monomer, and Au–Ag dimer). Note that the monomeric configurations based on Au (Figure a) or Ag (Figure b) NSA, even though giving rise to a wavelength-dependent directional scattering, are not capable of bidirectional functionality, having no sign changes in their directivity. Conversely, the bimetallic Au–Ag NSA uniquely provides an inversion of D as a function of wavelength. The sign flips from negative to positive by increasing λ from the blue wing of the spectrum (dominated by the Ag NSA scattering) to the red one (dominated by Au NSA scattering), the zero being at around 620 nm (i.e., in correspondence to a local maximum of the transmittance spectrum of Figure f, placed in-between the two orthogonal plasmonic resonances of the dimer configuration). Notably, the maximum directivity values for both the monomer and dimer configurations are well within the same order of magnitude reported for lithographic antennas and, at the same time, show a uniform broadband response, which is particularly evident in the NIR range.[24,26,45] The quasi-1D nature of the arrays and the polydispersion of the NSA morphology lead to the absence of collective grating effects or surface lattice resonances and also ensure the spectrally broad response of the LSP resonance (cf. ref (59) for details and Figure S1). Therefore, the optical properties of the NSAs are dominated by the resonant features of the unit cell, which is the typical regime of metasurface operation. Thus, the color routing properties of the system are not related to complex and/or restrictive collective geometries and morphological parameters, greatly relaxing the fabrication demands and issues.

Finally, it is worth noting that even though our implementation of the NSA employs finite length (i.e., few μm long) structures, thus breaking the continuous translational invariance of the ideal 2D configuration along the y-axis (Figure a), the measured azimuthal scattering pattern turns out to be well concentrated along the Φ = 0° and Φ = 180° directions of the color routing (Figure a). This is especially true at around the plasmonic resonance wavelengths, as elucidated by the 90° scattering rejection ratio of Figure b, defined as the ratio (in dB scale) between the light scattered at Φ = 0° (or Φ = 180°) and that scattered at Φ = 90° for a given polar angle (Θ = 50° in the present case). The finite length of the nanostrips and their slight deviations to parallelism along the y-axis (Figure a), does not prevent to consider the system as substantially invariant in the direction of the ridges, which is consistent with our design and modeling strategy. A related issue pertains to the polarization sensitivity of our structures, which belongs to the quasi-1D nature of the array configuration.[57,60] To address this point, we have repeated all the optical experiments with TE polarization (electric field of the source parallel to the nanostrips, i.e., orthogonal to their cross-sectional plane, see the top-view sketch in Figure l). The results indicate a very shallow directional scattering in terms of the azimuthal angle and the absence of bidirectional wavelength sensitivity, which is consistent with the basic theoretical argument according to which no plasmonic resonances can be excited under TE illumination in a two-dimensional configuration.

Figure 5

(a) Azimuthal plot of the scattered light at polar angle Θ = 50° for 5 different wavelengths; (b) 90° scattering rejection ratio as a function of wavelength for the left (red) and right (purple) routing operation.

In conclusion, we demonstrated broadband color routing from cross-polarized detuned plasmonic nanoantennas, fabricated over a large (cm2) area via self-organized metal confinement on a faceted glass template. The latter represents a natural platform for guiding the growth of vertically tilted nanoantenna arrays in a single maskless step. The resulting plasmonic metasurfaces exhibit highly directional and wavelength selective properties that are widely tunable and not related to complex and/or restrictive collective geometries and morphological parameters, greatly relaxing the fabrication demands and related critical issues. All the main optical and scattering features of these structures can be effectively predicted by numerical simulations in which the key parameters (such as antennas tilt and materials dielectric function) can be readily implemented by our self-organization methods. The scattering directivities of the proposed large-area beam-splitting metasurfaces are competitive with the figures of merit of lithographically patterned nanoantennas. We thus believe that our results can open the way to the use of flat-optics broadband color routers in large-area applications, with potential impact on a wide range of real-world devices involving, e.g., dichroic beam splitters, broadband polarizers, and multiplexed plasmonic biosensors.