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Hyperbolic Meta-Antennas Enable Full Control of Scattering and Absorption of Light Nicolò Maccaferri, Yingqi Zhao, Tommi Isoniemi, Marzia Iarossi, Antonietta Parracino, Giuseppe Strangi, and Francesco De Angelis Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04841 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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Hyperbolic Meta-Antennas Enable Full Control of Scattering and Absorption of Light Nicolò Maccaferria,b¶*, Yingqi Zhaoa¶,Tommi Isoniemia, Marzia Iarossia,c, Antonietta Parracinoa, Giuseppe Strangia,d,e, and Francesco De Angelisa* aIstituto

Italiano di Tecnologia, Via Morego 30, 16163, Genova, Italy

bPresent

address: Physics and Materials Science Research Unit, Université du Luxembourg, L-1511

Luxembourg, Luxembourg cDIBRIS,

Università degli Studi di Genova, Via Balbi 5, 16126 Genova, Italy

cDepartment

of Physics, Case Western Reserve University, 10600 Euclid Avenue,

Cleveland, Ohio 44106, USA eCNR-NANOTEC

Istituto di Nanotecnologia and Department of Physics, University of

Calabria, 87036, Italy

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ABSTRACT. We introduce a novel concept of hybrid metal-dielectric meta-antenna supporting type II hyperbolic dispersion, which enables a full control of absorption and scattering of light in the visible/near-infrared spectral range. This ability lies on the different nature of the hyperbolic Bloch-like modes excited within the meta-antenna. The experimental evidence is corroborated by a comprehensive theoretical study. In particular, we demonstrate that two main modes, one radiative and one non-radiative, can be excited by direct coupling with the free-space radiation. We show that the scattering is the dominating electromagnetic decay channel, when an electric dipolar mode is induced in the system, whereas a strong absorption process occurs when a magnetic dipole is excited. Also, by varying the geometry of the system, the relative ratio of scattering and absorption as well their relative enhancement and/or quenching can be tuned at will over a broad spectral range, thus enabling a full control of the two channels. Importantly, both radiative and non-radiative modes supported by our architecture can be excited directly by the farfield radiation. This was observed to occur even when the radiative channels (scattering) are almost totally suppressed, thereby making the proposed architecture suitable for practical applications. Finally, the hyperbolic meta-antennas possess both angular and polarization independent structural integrity, unlocking promising applications as hybrid meta-surfaces or as solvable nanostructures.

KEYWORDS: Hyperbolic Metamaterials, Scattering, Absorption, Nanoantennas, Plasmonic Nanostructures, Hybrid Nanostructures


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Unlike conventional optics, plasmonics enables unrivalled concentration of light well beyond the diffraction limit, leading to extremely confined and enhanced electromagnetic fields at the nanoscale1-4. Besides its fundamental importance, manipulation of light at the subwavelength level is of great interest for the prospect of real-life applications5, such as energy harvesting and photovoltaics6-8, wave-guiding and lasing9, optoelectronics10, and biomedicine11,12. Along with the ongoing efforts to synthesize novel plasmonic materials to improve the performances of the aforementioned uses13, novel optical designs and architectures that modify the optical power flowing through plasmonic metasurfaces represent another crucial step toward nanoscale manipulation of light-matter interactions14. Plasmonic nanostructures are known to exhibit, when coupled to light, collective electronic oscillations, so-called localized surface plasmon resonances (LSPRs), which determine their optical response in the visible and near-infrared spectral range and induce either absorption or scattering of light. One of the drawbacks of plasmonic nanostructures exhibiting LSPRs is the spectral overlapping of such scattering and absorption processes due to both the intrinsic nature of the excited plasmonic mode and the optical properties of the constituent material. For guiding light, for instance, it is essential that the photonic or plasmonic circuit does not have a high absorption, while for other kind of applications, such as photo-acoustic imaging, it is crucial that the light is absorbed rather than scattered. To overcome these issues, one can shift the LSPR of interest, just modifying the geometry of the nanostructure, to reduce or increase the weight of the absorption compared to the scattering, for instance using plasmonic nanorods15, although these two channels are at the same wavelength and one can choose to have only either absorption or scattering at a same time. An ideal solution would be an architecture and/or material which allows in the same platform a full control of the spectral distribution of scattering and absorption processes. A manipulation of plasmon-induced absorption and scattering processes has been proposed by using metal-insulatormetal (MIM) antennas16-18 or by inducing optical interference between MIMs19, although also these


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approaches do not still enable a full-control of the absolute intensity and relative ratio of the absorption and scattering channels at the same time. Hyperbolic metamaterials (HMM)20-22 have received great attention from the scientific community since they show unusual and unexpected properties never observed in nature such as negative refraction23, multiband perfect absorption24 and resonant gain singularities25. Moreover, these materials have a huge range of applications such as single-antenna biosensing26,27, nonlinear optics28, super resolution imaging29, ultra-compact optical quantum circuits30, plasmonic-based lasing31 and graphene-based technologies32. One of the most interesting properties of these materials is that they show also a strong optical anisotropy, for instance a periodic stack of metallic and dielectric layers33,34 appears as a metal in one plane and as a dielectric in the perpendicular axis35 (type II HMM). In this work, we show that type II hyperbolic meta-antennas enable in an easy and straightforward way the formation and a full control of almost pure and spectrally separated highly-radiative or highly-non-radiative regions and channels in the visible/near-infrared spectral range (Figure 1). Our approach can enable a full-control of (i) intensity, (ii) spectral position and (iii) ratio of the absorption and scattering processes at the same time. This property, which lies on the intrinsic hyperbolic nature of the multilayered structure, can be tuned at will by playing with the geometry (diameter, thickness and number of layers) and the composition (metal and dielectric materials) of the meta-antenna. We provide a detailed study, supported by experimental evidence, where we explain the physical mechanism underlying the aforementioned effects. In particular we focus on two main modes that can be excited within the meta-antenna, one purely absorptive and the other one of almost pure scattering. We show that the activation of a strong scattering channel depends on the excitation of a bonding super-radiant electric dipolar mode, whereas a high-absorption channel is enabled by the excitation of an anti-bonding sub-radiant magnetic dipolar mode. Interestingly, all the modes supported by our architecture, either radiative or non-radiative, can be excited by direct coupling with the external radiation, even when the scattering channels are practically totally suppressed, thus making the proposed system suitable for practical applications. Furthermore, the hyperbolic metaACS Paragon Plus Environment

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antennas are directional and show both polarization and angular independence, which is an important property if they are dispersed in solvents or grown on different kind of surfaces. The first and most important optical property of our system, namely the ability to display in the same platform almost pure radiative (scattering) or non-radiative (absorption) regions and or specific channels at different wavelengths and with the same intensity, is presented in Figure 1. A plasmonic antenna (top-left panel of Figure 1) made of a metal like gold displays a dominant scattering (red curve) efficiency (that is, scattering cross section divided by the geometrical cross section, in this case r2, where r is the radius of the antenna) over a low absorption (blue curve) behavior, and so a strong radiative spectral response (bottom-left panel of Figure 1). Moreover, both the scattering and absorption peaks are at the same wavelength. This is indeed the expected optical response for metallic nanoantennas with these specific sizes and shape in this spectral range1. If we cut the plasmonic antenna in, for instance, five slices (central panel of Figure 1) and then we connect them using dielectric spacers, we obtain a hyperbolic meta-antenna (top-right panel of Figure 1). We call it hyperbolic since the constituent multilayered structure displays hyperbolic dispersion of type II. In this case, we can see two well separated radiative and non-radiative regions, as well as two strong scattering and absorption bands with the same intensity (bottom-right panel of Figure 1). Although the structure considered in the sketch is made of five bilayers of metal and dielectric, up to four bilayers we can state that our system is hyperbolic because there are enough bilayers to display the hyperbolic features shown by an infinite multilayer35,36. Furthermore, to better clarify how important it is to be in the hyperbolic regime, it is seen that when the dimensions of the structure are changed so that its supported hyperbolic modes are shifted out of the hyperbolic spectral region, their non-radiative nature shown in the bottom-right panel of Figure 1 vanishes (see Supplementary Figure S1), as was also demonstrated experimentally by Yang et al. for similar multilayered nanostructured cavities37. Importantly, the multilayered structure enables a fine tuning of the effective in-plane permittivity of the material, that is a change in the effective free electron density (see Supplementary note 1 and the top-panel of Supplementary Figure S1). ACS Paragon Plus Environment

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Figure 1. Hyperbolic meta-antenna concept. A plasmonic antenna (top-left panel) display a highly radiative spectral response (bottom-left panel). If this antenna is cut in slices (central panel), which are then re-organized in a hyperbolic meta-antenna by using a dielectric spacer between them (top-right panel), the spectral response displays a radical change. Now, an almost pure radiative region and an almost pure non-radiative region can be enabled by the coupling of free space radiation with the hyperbolic metaantenna (bottom-right panel).

We want now to go more deeply inside the physical properties of our system and disclose the physical mechanisms responsible of its unconventional properties. We considered a single hyperbolic meta-antenna with a diameter D = 200 nm, made of five alternating layers of gold (10 nm each) and of a dielectric material (20 nm each) on a transparent substrate, such as glass (n = 1.5). From now on and where not specified, the environment is air, and the substrate is glass. We first calculated the properties of this meta-antenna by using the finite element method (for more details see Methods). We decided to use the aforementioned dimensions after an optimization study (see Supplementary note 2 and Supplementary Figure S2 and S3), to bring the functionality presented in the bottom-right panel of Figure 1 in a specific spectral range, namely the red/near-infrared spectral range (650-1800 nm), which useful for a plenty of emerging light-based technologies. In addition, the cylindrical shape ACS Paragon Plus Environment

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we chose is not crucial to achieve the effects introduced in this work. In fact, if we consider different shapes, such as conical or squared, we keep the full-control of the properties shown in the bottomright panel of Figure 1 (see Supplementary Note 3 and Supplementary Figure S4). First, we wanted to achieve a controlled tuning of the spectral separation between the scattering and absorption bands. It is actually possible to do so by changing, for instance, the thickness of the metallic or dielectric layers (see Supplementary Figure S2), the shape and size (diameter) of the nanostructure (see Supplementary Figure S3) or, in a more convenient way, the refractive index (RI) n of the dielectric spacers. To demonstrate that the latter possibility can produce a desired and significant variation of the spectral separation between the main absorption and scattering channels (the first two peaks in the bottom-right panel of Figure 1), we chose three different and well-known materials: SiO2 ( = 1.45), Al2O3 ( = 1.75) and TiO2 ( = 2.25). As can be inferred by looking at Figure 2a, the spectral separation between the scattering and the larger absorption process becomes larger by increasing the RI of the dielectric material. A spectral separation of 250 nm with SiO2 can be increased up to 580 nm by using TiO2. It is worth noticing that the dependence of the spectral separation between absorption and scattering channels on the value of the RI of the dielectric material chosen is linear, as shown in the inset in the top-panel of Figure 2a. Moreover, while the scattering peak redshifts less than 100 nm passing from n = 1.45 to n = 2.25, the absorption band displays a redshift of more than 400 nm. This higher sensitivity of the absorption channels compared to the scattering one is related to the nature of the absorption mode itself. This mode is due mainly to the excitation of a magnetic dipole, as we will show later, and displays also a higher quality factor. These two characteristics make this mode more sensitive compared to the classical dipolar electric mode supported by plasmonic nanoantennas38.


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Figure 2. Physical properties and their underlying mechanisms. (a) Calculated scattering (dots) and absorption (triangles) of a hyperbolic meta-antenna with D = 200 nm and made of five bilayers of Au (10 nm each) and three different dielectric materials (SiO2 – blue curves, top-panel; Al2O3 – orange curves, middle-panel; TiO2 – red curves, bottom-panel – 20 nm each layer). In all three the cases the structures are assumed to be on a glass substrate (n = 1.5) and exposed to air (n = 1). The inset in the top-panel shows the linear dependence of the spectral separation as a function of the RI of the dielectric layers; the dotted line is a guide for eyes. (b) Cross-section ratios abs/scat (blue triangles) and scat/abs (pink squares) for the case of the hyperbolic meta-antenna made of five bilayers of Au (10 nm each) and SiO2 (20 nm each); log10-scale is used in the vertical axes. (c) Top-panel: near-field distribution of the z-component of the electric field and related current density distributions of the first two modes at the scattering peak (left) and at the first absorption peak (right); the symbols “+” and “-” with different dimensions are intended to give an idea of the strength of the charge at that particular position. Bottom-panel: far-field distribution for the cases presented in the top-panel. (d) Top-panel: near-field distribution of the x-component of the magnetic field for the three absorption modes (see colored arrows indicating them in (b)). Bottom-panel: calculated Bloch eigenmodes of a multilayered structure which is not confined along the x- and ydirections, namely a hyperbolic metamaterial continuous film with a type II dispersion.


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If we now focus our attention on the specific case of the Au/SiO2 hyperbolic meta-antenna, it is clear that on the scale of the ordinate of Figure 2a only one absorption peak is clearly visible at around  = 1120 nm, while other two absorption peaks are present at higher wavelength, even if very much smaller in intensity. If we plot the ratio between scattering and absorption, namely scat/abs, and the inverse of this quantity, we can see the signature of these other two absorption processes (see Figure 2b and the green and red arrows). If we now look at the electric near-field and at the current density J distributions at the two resonances wavelengths (see top-panel of Figure 2c), we can clearly see a huge difference between the two cases. While at 860 nm (scattering band resonant peak) we have a clear signature of an electric dipolar mode and the current densities at the boundary of the structure have the same direction (typical behavior of an anti-bonding mode), at 1120 nm (absorption band resonant peak) currents densities at the bottom and at the top of the meta-antenna go in opposite directions (typical configuration of a bonding mode). In the first case we can observe that the electric near-field is localized almost outside the meta-antenna but it is not enhanced to much (see also the color scale-bar), giving rise to the usual far-field pattern of a plasmonic nanoantenna due to the excitation of an electric dipolar super-radiant mode (see the corresponding far-field distribution in the bottom-panel of Figure 2c). In the second case the electric near-field is strongly concentrated at the center of the structure (see also the enhancement factor by one of order of magnitude compared to the other case), and this explains the almost total suppression of scattering in favor of a strong absorption behavior. Indeed, at  = 1120 nm we do not observe any scattering peak, but a resonance due to a huge absorption (see top-panel Figure 2a). This non-radiative coupling between the far-field radiation and the meta-antenna can be reconducted to the excitation of a magnetic dipole, in analogy with previously reported similar effects in MIM structures39-41. Furthermore, the dipolar nature of this mode is clearly appreciable if looking at the far-field pattern reported in the right-bottom panel of Figure 2c. It is also important to mention that, to display two well-separated bands of scattering and absorption, it is crucial to have also an index mismatch between the dielectric material in the hyperbolic meta-antenna and the external environment (see Supplementary note 4 and Supplementary ACS Paragon Plus Environment

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Figure S5). Moreover, and even more important, we can state here that our meta-antenna is truly hyperbolic, as shown in Figure 2d. In fact, if we focus our attention on the three absorption bands highlighted in Figure 2b by using three different colored arrows, are related to the excitation of three different localized modes within the meta-antenna, as shown in the top-panels of Figure 2d, in strong analogy with previously reported works, where similar confined Bloch modes can be excited in a continuous multilayered film, although the latter are guided and not localized35,36. To prove this hypothesis, in Figure 2d we plot the magnetic near-field distributions of both our meta-antenna (top panels) at the resonant absorption clearly visible in Figure 2b (see also the orange, green and red arrows indicating these modes) and of a multilayered structure which is not confined along the x- and y-directions, namely a hyperbolic metamaterial continuous film with a type II dispersion (bottom panels) made of five bilayers of Au (10 nm each) and SiO2 (20 nm each). In the latter case, we plot the first three Bloch eigenmodes supported by this system. As can be inferred by looking at the nearfield profiles of both cases (meta-antenna and continuous film), the magnetic field have the same distribution, thus confirming that the effect of having almost pure absorption channels is due to the hyperbolic nature of our meta-antenna. This is indeed a novel property introduced by our system, as here we are clearly exciting low-energy modes of different nature if compared to the high-energy higher-order modes excited for instance in MIM17 or engineered metal-dielectric nanostructures49. Based on these results and to prove the robustness of our model, we fabricated two different samples by keeping as reference the two extreme cases reported in Figure 2a, namely hyperbolic metaantennas with an average diameter of about 200 nm on glass substrates made of five bilayers of Au and either SiO2 or TiO2. We used a a top-down etching approach combined with the well-known bottom-up hole mask colloidal lithography technique42, which is an affordable, highly parallel and cm2-scale nanofabrication method (see also Materials and Methods). For large-scale applications it is potentially feasible to use sequential wet chemical deposition of gold and dielectric to fabricate similar hyperbolic particles by using gold nanoplates as initial seeds43. In Figure 3a we show representative SEM images of the randomly distributed Au/SiO2 structures. As shown in the right ACS Paragon Plus Environment

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image, the stacking layers can be well distinguished indicating that we were able to fabricate multilayered nanostructures without damaging the multilayers while maintaining the structural integrity. We then characterized the optical properties of the fabricated samples by measuring their absorption and scattering coefficients, indicated as S and A in Figure 3b. The total extinction coefficient E can be calculated then as E = A + S. By looking at Figure 3b, an almost pure scattering (absorption) band is visible at around 900 (1250) nm for the SiO2-based meta-antennas and another around 1100 (1800) for the TiO2-based structures. We were able to induce experimentally two separated decay channels by direct coupling with the far field radiation, which is very important in view of practical applications. Furthermore, it is clear from the experimental and the calculated curves that by increasing the RI of the dielectric we can increase the separation between the absorption and scattering bands. It is notable that the spectral separation is higher in the experimental case (350 nm and 700 nm for the Au/SiO2-based and Au/TiO2-based meta-antennas, respectively) if compared to the theoretical one predicted by Figure 2a. Indeed, the experimental effective RIs of the SiO2 and the TiO2 layers are a bit higher than the ones used in the calculations, and this is mostly due to the presence of almost 2-3 nm of Ti as adhesion layer between each Au and dielectric layer. This can explain the difference. Moreover, the presence of Ti might explain also the smaller intensity, compared to that shown in Figure 2a and in Figure 3c, of the absorption peak at 1250 nm and 1800 nm, as well as the presence of a strong absorption and broad contribution in addition to the scattering. The latter can be explained also by other reasons, such as morphological defects of the multilayers clearly visible in the SEM images, as well as by roughness, round edges, and dispersion in size and shape. Nevertheless, the most important thing here is that these experiments prove in an incontrovertible manner that a path to achieve a separated engineering of scattering and absorption with hyperbolic meta-antenna is indeed possible. Finally, in Figure 3c we show the experimental setup we used to measure the quantities necessary to retrieve the information about scattering and absorption contributions. This approach is in part similar to that used by Langhammer et al.44. When the incident light illuminates the sample, the scattering and absorption process will result in the loss ACS Paragon Plus Environment

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of light intensity while the transmitted light go through the substrate without changing direction, as indicated in figure3 (c). Forward and backward scattering can be collected by the integrating sphere.

Figure 3. Experimental realization of hyperbolic meta-antennas. (a) SEM images (scale bar 4 m) of hyperbolic metaantennas fabricated using hole mask colloidal lithography. (b) Experimental A, S and E coefficients of hyperbolic metaantennas on glass with an average diameter D = 200 nm, made of five bilayers of Au (10 nm each) and two different dielectric material (SiO2 – blue curve, top-panel; TiO2 – brown curve, bottom-panel). (c) Sketch of the experimental setup used to measure directly IFS,

IBS and IFS + IT.

In the “Configuration 1”, incident unpolarized light with intensity I0 was sent inside an integrating sphere where the sample was placed as depicted in the sketch. In this case we were able to measure the forward scattering FS=IFS/I0. In “Configuration 2” (central panel of Figure 3c) the sample was placed inside the sphere as shown in the figure, and in this case we measured the backward scattering BS= IBS/I0. It is worth noticing that every time we lose some light going out from the


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sphere, and the intensity IT of this light give us the transmission coefficient T = IT/I0 as in Ref. [44], but in this case we did not have a detector outside the sphere. In the last configuration, that is “Configuration 3”, we closed the integrating sphere and collected at the same time IT + IFS. With this information, and knowing that 1 = T + A + FS + BS, we were able to calculate E as 1 – T and S = FS + BS. In this way, A = E – S. (more details about the experimental set-up and the optical measurements can be found in the Methods Section). We want also to highlight that the optical response of our structure is strongly independent on both the polarization and the direction of the incident light. In Figure 4 we plot the calculated absorption and scattering efficiencies (top and bottom panels, respectively) of a hyperbolic meta-antenna with D = 200 nm and five bilayers of gold and a dielectric made of SiO2 (each bilayer is composed by 10 nm and 20 nm of material, respectively) as a function of both the wavelength of the incident light and the angle of incidence. The incident light is unpolarized, that is I0 = ITE + ITM, where TE and TM stay for transverse electric (s-polarized) and transverse magnetic (p-polarized), respectively. Up to 70° both the absorption and scattering processes show neither any angular dependence nor any polarization dependence a part for a decreasing of the intensity. Above 70° of incidence we start to see a drop of the scattering and absorption intensities, since the in-plane dipolar LSPR of the plasmonic nanoantennas composing the hyperbolic meta-antenna are excited with lower efficiency, as the inplane component of the incident electric field goes to zero. On top of the calculated color maps, we plotted also the experimental points indicating the spectral position of both the absorption and scattering peaks (white dots). The outstanding agreement between experiments and simulations proves again that our meta-antennas are really stable and provide a perfect polarization and angle of incidence independence crucial in view of possible applications using these structures dispersed in solvents. The polarization/angular independence of our architecture is of great significance since it means that the special optical property of the system reported here does not depend on the orientation of the meta-antenna (see also Supplementary Figure S8). This fact implies that our system can be randomly deposited on different surfaces and implemented in a large range of practical applications, ACS Paragon Plus Environment

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for instance in plasmon-based photovoltaic devices45 or solar transparent radiators46, as well as they can be dissolved in solution for biomedical applications47.

Figure 4. Angular and polarization independence of absorption and scattering. Calculated absorption (top) and scattering (bottom) efficiency (color-map) and peak experimental absorption (top) and scattering (bottom) wavelengths (white dots) of hyperbolic meta-antennas on glass with D = 200 nm, made of five bilayers of Au (10 nm each) and SiO2 (20 nm each), for unpolarized incident light as function of the wavelength and of the angle of incidence.

We finally want to show that a full control of scattering and absorption can be achieved by a simple modification of the structure of our meta-antenna. From Figure 2b is clear that when the absorption is maximum the scattering is almost totally suppressed, and this is indeed a crucial property if one wants to exploit one or the other effect in the same platform. In Figure 5 we show that by adding more layers to the original structure one can actually tailor at will the cross-section ratio. Figure 5a shows the calculated cross-section ratios (in log10-scale) abs/scat (blue curves) and scat/abs (red ACS Paragon Plus Environment

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curves) for a hyperbolic meta-antenna made of six bilayers Au[10nm]/SiO2[20nm] on a glass substrate. This configuration is quite close to the one studied above, as it displays dominant absorption peaks as shown in Figure 2. It should be noted here that at around  = 1350 nm the scattering and absorption efficiencies are equal, namely scat/abs = 1 (see the red arrow in Figure 5a). This particular case is really interesting since plasmon-coupled resonance energy transfer processes can be maximized when these two decay channels show the same efficiency48. One can tune even more these cross-section ratios by adding more bi-layers composing the meta-antenna (see also Supplementary note 5 and Supplementary Figure S6 and S7).

Figure 5. Tailoring of cross-section ratio through layers engineering. Hyperbolic meta-antenna made of six (a), seven (b) and eight (c) bilayers Au[10nm]/SiO2[20nm] and relative calculated cross-section ratios (in log10-scale) abs/scat (blue curves) and scat/abs (red curves).

If we add another bilayer Au[10nm]/SiO2[20nm], at the mode where before S = A (red arrow in Figure 5a), now S > A (red arrow in Figure 5b). Note also that at  = 1500 nm we are able to induce


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an additional channel where S > A. We can shift this channel toward higher energies by adding an additional bilayer to the structure and create a spectral region dominated by radiative processes (Figure 5c), in straight contrast with the case of six bilayers Au[10nm]/SiO2[20nm] reported in Figure 5a. It is important to highlight here that a full control of absorption and scattering channels can be achieved through a straightforward and clear physical concept and the use of just one single metaantenna. The tuning of the cross-section ratios can be additionally tuned by engineering the vertical resonances in multilayered metal/dielectric nanostructures49. In summary, we have introduced a novel functionality of hyperbolic nanostructured metamaterials. Our proposed architecture, a hyperbolic meta-antenna, displays separated scattering and absorption channels in the visible/near-infrared spectral range. This behavior is related to the excitation of either electric and magnetic modes within the nanostructure, as well as to hyperbolic Bloch-like plasmons, which can be effectively excited by direct coupling with the far field radiation, even when the radiative channels (scattering) are almost totally suppressed, hence making the proposed architecture suitable for practical applications. Furthermore, we have shown that hyperbolic meta-antennas enable a full control of scattering and absorption channels over a broad spectral range by changing the dielectric material within the meta-antennas, its geometry and layers composition. Finally, the proposed architecture displays both angular and polarization independent structural integrity, thus opening up new perspectives for applications on a broad range of surfaces or dissolved in solvents. We foresee that the concept presented here can be generalized by exploring more complex shapes and/or configurations (such as lattice-like configurations) to induce additional hybrid modes beyond the ones responsible for the effects shown in this work. The presented findings open novel routes to control the decay channels in light-matter coupling processes beyond what is offered by current plasmon-based systems, possibly enabling applications spanning from fluorescence emission manipulation and enhancement, as already demonstrated by other groups50,51, to theragnostic nanodevices, optical trapping and nano-manipulation, non-linear optical properties, plasmon-enhanced molecular spectroscopy, photovoltaics and future energy nanomaterials. ACS Paragon Plus Environment

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METHODS Numerical Simulations Numerical simulations have been performed using the finite element method implemented in Comsol Multiphysics. The RI values of gold and dielectrics have been taken from literature52-55. To simulate the optical properties of hyperbolic meta-antennas we have considered a simulation region where we specified the background electric field (a linearly polarized plane wave), and then we calculated the scattered field by a single meta-antenna to extract optical parameters such as absorption and scattering cross sections. The model computes the scattering, absorption and extinction cross-sections of the 1

particle on the substrate. The scattering cross-section is defined as 𝜎𝑠𝑐𝑎𝑡 = 𝐼0∬(𝒏 ∙ 𝑺)𝑑𝑆, where 𝐼0 is the intensity of the incident light, 𝒏 is the normal vector pointing outwards from the nanodot and 𝑺is the Poynting vector. The integral is taken over the closed surface of the meta-antenna. The absorption 1

cross section equals𝜎𝑎𝑏𝑠 = 𝐼0∭𝑄𝑑𝑉, where 𝑄 is the power loss density of the system and the integral is taken over the volume of the meta-antenna.

Sample fabrication Hyperbolic meta-antennas were prepared by inductively coupled plasma (ICP) etching of the gold/dielectric multilayers with the Cr disk as mask, which was fabricated by hole mask colloidal lithography42. With this approach it is possible to fabricate large areas of hyperbolic meta-antennas with the predicted properties, which can be easily transferred on other substrates or disperse in solution, as demonstrated recently by some groups who already proposed detailed and efficient protocols46,56,57. 1.

Stacking bi-layers fabrication. Microscope glass slides were cleaned with acetone and 2-

propanol with 2 min sonication respectively. After deionized water (DI) washing and blow drying under N2 flow, the glass wafers were ready for the multilayer deposition. For the Au/SiO2 stacking


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layer deposition, the glass wafers were loaded into an electron beam deposition (E-beam, PVD75 Kurt J. Lesker company) chamber. One unit of the metal-dielectric bi-layer consisted of 0.5 nm Ti +10 nm Au + 0.5 nm Ti + 20 nm SiO2, in which Ti served as the adhesion layer. The deposition of the bi-layer unit was repeated five times. For the Au/TiO2 stacking layers, the glass wafers were loaded into an electron beam deposition chamber (Kenosistec KE 500 ET), and 0.5 nm Ti + 10 nm Au + 0.5 nm Ti layers were deposited at a rate of 0.3 Å/s. The wafer was then transferred to an atomic layer deposition chamber (ALD, FlexAL, Oxford Instruments) and TiO2 was deposited using a process with titanium isopropoxide as the titanium precursor and oxygen plasma as the oxidizer. The process was repeated at 80 °C temperature for 383 cycles to produce a film with a thickness of 20nm, which was verified with ellipsometry. One unit of the Au/TiO2 metal dielectric bi-layer consisted of 0.5 nm Ti + 10 nm Au + 0.5 nm Ti + 20 nm TiO2. The deposition of the bi-layer unit was repeated five times. 2.

Cr disk etching mask fabrication. On the top of stacking bilayers, photoresist (950 PMMA

A8, Micro Chem) was spin coated at 6000 rpm and soft baked at 180℃ for 1min. After O2 plasma treatment (2 min, 100 W, Plasma cleaner, Gambetti), Poly(diallyldimethylammonium chloride) solution (PDDA, Mw 200,000-350,000, 20 wt. % in H2O, Sigma, three times diluted) was drop coated on the top of the PR surface and incubated for 5min to create a positively charged surface. The extra PDDA solution was washed away under flowing DI water after 5min incubation. Then negatively charged polystyrene (PS) beads (diameter 552 nm, 5 wt% water suspension, Micro Particle GmbH) were drop coated on the as prepared stacking bi-layers, cleaned after 30 s under flowing DI water and dried with N2 flow. Thereby, random distributed PS beads were attached on top of the photoresist. The samples were treated with O2 plasma etching in the inductively coupled plasmareactive ion etching system (ICP-RIE, SENTECH SI500) to reduce the size of PS beads. Gold film (40nm) was sputter coated (Sputter coater, Quorum, Q150T ES) on top of the sample to serve as an etching mask to protect the PR underneath. After removal of the PS beads by Polydimethylsiloxane (PDMS) film, the samples were treated again by O2 plasma in the ICP-RIE system to etch away the ACS Paragon Plus Environment

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PR and create randomly distributed holes as mask on top of the stacking bi-layers. The diameter of the holes was controlled by varying the PS bead O2 plasma treatment time. E-beam deposition of 100nm Cr was then performed with a vertical incident angle. Followed by liftoff of the PR in acetone, randomly distributed Cr disks on the stacking multilayer were fabricated. 3.

Meta-antennas fabrication. With the Cr disk mask, ICP-RIE etching was carried out with CF4

gas flow 15 sccm, radio frequency (RF) power 200 W, ICP power 400 W, temperature 5 ℃, pressure 1Pa. The etching time was adjusted according to the stacking film thickness to ensure all the extra stacking bi-layer material, except for the area under Cr mask, was removed. Then the sample was soaked in Cr etchant (Etch 18, OrganoSpezialChemie GmbH) for 2min to remove the Cr mask. Followed by DI water cleaning and drying under N2 flow, the sample morphology was characterized with a scanning electron microscope (SEM).

Optical characterization Cary 5000 UV-VIS-near-IR spectrophotometer with integrating sphere was used for the measurement of scattering and absorption. Before each measurement after changing configuration, the baseline correction was performed using a glass slide as the blanc sample. In configuration 1, the forward scattering signal (FS) was collected by placing the sample on the front port with structure facing the integrating sphere, and the back port was left open. Transmission mode in the software was used to collect the signal. In configuration 2, the backward scattering (BS) was collected by placing the sample at the back port with structure facing the integrating sphere, without any white board to block the light at both front and back port. Reflection mode in the software was used to collect the signal. In configuration 3, the sum of transmitted and forward scattered light (T+FS) was measured by placing the sample at front port with structure facing the integrating sphere, but with the back port of the integrating sphere closed using the standard white board blocking. Transmission mode was used to collect the signal.


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ASSOCIATED CONTENT Supporting Information. Calculation of the effective dielectric constant of a multilayered metal/dielectric structures. Optimization of the hyperbolic meta-antennas dimensions and composition. Dependence of the optical properties of the hyperbolic meta-antennas on the shape – the conical and square shape cases. Effect of the index contrast between the environment and the dielectric material composing the meta-antenna. Dependence of the cross-section ratios on the direction and polarization of the incident radiation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author Nicolò Maccaferri: [email protected] Francesco De Angelis: [email protected]

Author Contributions NM and FDA conceived the concept. NM performed numerical simulations, data analysis and wrote the manuscript. YZ fabricated the samples and did the optical measurements with


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the assistance of TI and MI. NM, YZ, MI analyzed the experimental data with the help of AP and GS. FDA supervised the entire work. All the authors contributed to the general discussion. ¶ NM and YZ contributed equally to this work and share first authorship. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) – ERC Grant Agreement No. [616213], CoG: Neuro-Plasmonics,and under the Horizon 2020 Program, FET-Open: PROSEQO, Grant Agreement No. [687089]. NM acknowledges Matteo Barelli, Dr. Andrea Toma and Dr. Cristian Ciracì for fruitful discussions. The Authors acknowledge all the Reviewers for their helpful and insightful comments.

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TOC FIGURE


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Hyperbolic Meta-Antennas Enable Full Control of ... - ACS Publications

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