Composite Multilayer Shared-Aperture Nanostructures: A Functional

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Composite Multilayer Shared Aperture Nanostructures: A Functional Multispectral Control Ali Forouzmand, and Hossein Mosallaei ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01441 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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Composite Multilayer Shared Aperture Nanostructures: A Functional Multispectral Control Ali Forouzmand and Hossein Mosallaei* Metamaterials Laboratory, Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts 02115, USA *

[email protected]

ABSTRACT. There is a growing demand in the field of metasurface to design and implement functional multispectral devices capable of complex beam conversion and high capacity optical information processing. Control over the size of aperture footprint, fabrication complexity, and fundamental cross-talk poses great challenges toward the realization of these multifunctional multispectral optical devices. Here, we demonstrate a systematic design strategy and a full roadmap toward implementation of a novel class of nanoantenna arrays based on the multilayer shared-aperture concept which can simultaneously multiplex multiwavelength into a single optical device platform. The idea is supported by engineering two dual-layer metasurface-based designs with capability of superdistinct operating channels (lie in thermal infrared and visible spectra) and super-close operating channels (both lie in visible spectrum) to simultaneously and independently perform anomalous or similar wavefront manipulation at two predesigned wavelengths. We leverage transparent conducting oxide (TCO)-dielectric and plasmonic-dielectric composite multilayer nanostructures to realize the aforementioned designs, respectively. The challenges such as coupling effects among the different wavelengths, compactness, fabrication feasibility, and material frequency dispersion are carefully addressed by careful selection of constituent materials and geometrical shape of resonators, array ACS Paragon Plus Environment

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architecture, and optimization of structural parameters of inclusions. As a proof of concept, we have designed two dual-wavelength holographic metalenses generating images located in-plane and out-plane at two selected wavelengths. The proposed technique is in particular of interests in the fields of data storage, information processing, and display.

Keywords: Metasurfaces, shared aperture antennas, multi-function, multi-spectral, metahologram

Nowadays, the graded-pattern plasmonic/dielectric planar structures are known as one of the wellestablished category of metasurfaces (MTSs) which have been studied in-depth and employed for various novel optical applications like optical information processing and analog/digital computation1,2, wave-front engineering in reflection/transmission modes3-6, ultra-thin flat lenses, compact optical vortex beam generation7,8, aberration-free and dual-polarity metalenses9,10, and metaholograms11-21. In these optical devices, the control over the transmitted/reflected beam can be achieved by the realization of the desired abrupt phase discontinuity through spatially varying the physical sizes, constituent materials, and orientation of the subwavelength meta-atoms.22-24 The functional graded-pattern metasurfaces can be mainly divided into two categories namely transmittarrays and reflectarrays. A single-layer plasmonic metasurface faces with several challenges and restrictions like low transmission efficiency (the maximum possible transmission efficiency for an ultra-thin layer is 25%) and lack of full phase-change coverage due to supporting single magnetic or electric geometrical resonance.25,26 In addition, their operation is inherently possible in a single narrow bandwidth and their overall efficiency degrades drastically beyond it. By leveraging the concept of Pancharatnam-Berry (PB) and geometrical rotation of non-symmetric shape meta-atoms (e.g., V- and Cshaped), full phase control can be obtained through the introduction of cross-coupling between two linearly polarized incident beams.27-29 Moreover, the full phase-agility can be realized via all-dielectric elements (e.g., nanodisk, nanosphere, and nanocuboids) which intrinsically support both electric and magnetic resonances.30-32 In order to surmount the low level of transmission efficiency in single-layer MTSs, one possible approach is multi-layer transmitarrays composed of cascaded layers of several 2 ACS Paragon Plus Environment

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metasurfaces.33-35 In these multi-layer structures, layers complement each other to attain a single specific functionality for the optical device. Furthermore, the reflection-type metasurfaces have high efficiency in which the impinging wave can be mainly reflected meanwhile experiencing the required phase-delay. Although there are significant advances in the design of single-functional metasurfaces and overcoming their limitations like enlargement of dynamical operating range (broadband functionality), the concept and potentials behind multi-functional metasurfaces with multi-spectral capability have not been investigated in-depth. The recent progresses in the metasurface technology and nanofabrications provide an emerging platform to increase the data capacity and realization of ultracompact multipolarization and multi-wavelength optical devices. So far, three different approaches are proposed in order to design a unique metasurface with capability of distinct functionalities at different wavelengths including segmentation36-39, the brute-force searching40-42, and multi-objective optimization43-45. In segmentation method, the aperture is divided into N separate sub-apertures which individually control N different input wavelengths. Therefore, the maximum average efficiency is 1/N at each wavelength. In Ref. 46, a phase-modulated multicolor metasurface has been designed by integration of three plasmonic subpixels with different sizes into the metasurface. Subpixels separately control red, green, and blue (RGB) lights (λ=405, 532, and 658 nm). The successful realization of this approach needs low in-plane coupling, enlargement of the antenna aperture, and low spectral separation between operating wavelengths. In other words, wider operating bandwidth needs larger subpixel which causes more crosstalk and reduction of data density. In brute-force technique, the necessity of building a library and searching for an efficient building block which poses the multi-wavelength responses make the design more challenging as the number of functions and wavelengths increase. In Ref. 47, a one-dimensional achromatic metasurface has been proposed which operates in multi-wavelengths in telecommunication regime (λ=1300, 1550, and 1800 nm). The proposed unit-cell includes two closely-spaced rectangular dielectric resonators which have been optimized through cumbersome simulations by tailoring the coupling distance and width of each inclusion. The desired phase distribution for each wavelength cannot be independently and simultaneously tailored and the maximum wavelength ratio in this ACS Paragon Plus Environment

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achromatic-based design cannot be enhanced more than 1.5 due to the limitation of structure size and coupling between neighbor resonators. Finally, the optimization-based designs need to perform an evolutionary optimization over entire an array (full period of the metagrating) which has drawbacks such as computationally expensive, fabrication complexity, lack of physical insight and design robustness, and necessity of case-by-case design. Except the aforementioned achromatic metasurfaces, most of already demonstrated MTSs can have control over the phase distribution of reflected/transmitted beam for a single functionality within a single frequency band. In this article, we propose a robust technique based on the concept of multi-layer shared aperture antenna arrays48-50 which can bring compactness, multi-functionality, and multi-wavelength operation to the conventional graded-pattern MTSs. As a consequence, the proposed composite multilayer sharedaperture can offer the capability of simultaneous operation in different spectral wavelengths with similar or anomalous radiation patterns and wavefronts for functionalities like beam scanning and shaping. We will start by introducing the underlying mechanisms for the design of two types of bi-layer shared aperture topologies with capability of super-distinct operating channels (lie in thermal infrared and visible spectra) and super-close operating channels (both lie in visible spectrum) as shown schematically in Figure 1(a). Toward the realization of the aforementioned designs, we leverage TCO-dielectric and plasmonic-dielectric composite multi-layer structures, respectively. In TCO-dielectric composite, we utilize the dual-nature of highly-doped TCOs (i.e. behave as a dielectric in high-THz regime and act as a dispersive plasmonic in low-THz regime) to design a super-distinct dual-wavelength in which the offresonance basis of the top Si-made nanorods prevents from any undesired resonance coupling with the cross-shaped TCO-made nanoantenna located at the bottom layer and buried inside a magnesium oxide (MgO) insulator (Figure 1(b)). The corresponding operating wavelengths are 8.108 µm and 0.565 µm and the wavelength ratio is around 14.5. This wavelength ratio can be reduced to 1.5 by utilizing a plasmonic-dielectric composite which is composed of a two-layer nanostructure separated by an insulator and backed by a backmirror silver substrate (Figure 1(c)). The top layer consists of silver-made nanorod which operates over the linearly polarized (LP) incident beam at the operating wavelength of ACS Paragon Plus Environment

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750 nm (red color light) and the bottom layer is composed of Si-made nanorods which operates based on PB mechanism over the circularly-polarized (CP) incident beam at the operating wavelength of 500 nm (blue color light). The main challenges in the previously presented relevant works48,49 like lack of fullphase control for all subarrays, low-GHz/THz operation (non-dispersive materials), isolation, and alignment of subarrays have been addressed by the careful selection of the constituent materials, optimization of structural parameters, and array architecture. The proposed methodology enables the realization of independent phase distributions at two distinct wavelengths with flexible wavelength ratio which is significantly larger than previously published literatures36-50. The functional performance of each array at the predesigned operating wavelength channel will not be distorted by the geometrical variation of the other layer. This helps to eliminate the cross-talk issue for multi-functional and multiwavelength applications. Afterward, two types of high-performance dual-wavelength metaholograms for obtaining out-plane and in-plane imaging with high efficiency and polarization selectivity have been designed and investigated in-depth. The proposed roadmap and design scheme can be also in particular of interests due to the minimized physical area and the feasibility of direct implementation on an already existed conventional optical panel in order to enable desired multi-functionalities, which are beneficial for various applications such as display, security, data storage, and information processing and can pave the way toward realization of multi-wavelength imaging and sensing.

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Figure 1. (a) Illustration of electromagnetic spectrum from far-infrared to visible regime with indication of the promising candidates namely (b) TCO-dielectric and (c) plasmonic-dielectric composites to serve as super-distinct and super-close dualwavelength building blocks, respectively.

DESIGN RULES TOWARD REALIZATION OF DUAL-WAVELENGTH BUILDING BLOCK Thermal IR-Visible Design via TCO-Dielectric Composite. The schematic overview of the proposed two-layered TCO-dielectric structure to realize dual-wavelength thermal IR-visible control is sketched in Figures 2(a)-(d). It consists of an array of arbitrarily oriented Si-made nanorods in the top layer and an array of TCO-made crosses in the bottom layer buried inside MgO insulator51 and backed by a goldmade reflection mirror. All the constituent materials are fitted by a Lorentzian dispersive model to the experimentally measured data in Palik52 over wide frequency ranges in both thermal IR and visible spectra (Section 1S, Supporting Information). The top layer metasurface includes Si-made nanorods with lattice constant of Prod=280 nm (half of the operating wavelength, λ1=565 nm (f1=531 THz), to avoid undesired diffractions), width of wrod=80 nm, height of hrod=50 nm (the height of the Si-made nanorod is only λ1/11.3), and length of lrod=240 nm where the rotation of element ( θ r ) will lead to creating full phase agility at the green light regime. The bottom layer utilizes the TCO-made cross-shape array with width of wcross, height of hcross=270 nm, and pitch size of Pcross=1960 nm in which the gap size (gcross) can be modified to generate wide phase control over both LP incident beams (y- and z-directions) at λ2=8.108 µm (f2=37 THz) in the thermal IR regime. The high-index nature of the nanorod antenna which has low intrinsic ohmic loss in the visible spectrum will help to achieve high-efficiency in comparison with the reflection-type metal-insulator-metal (MIM) structures with gap-plasmon mechanism. Furthermore, the top layer will be ultimately thin at the thermal-IR regime which will negligibly influence the optical performance of the bottom layer. It should also be mentioned that TCOs can be highly doped to attain electrically conductive properties. This makes highly-doped TCOs as alternative plasmonic materials for metamaterial applications in the IR wavelength range.53-56 At the same time, they are transparent in the visible regime due to having large bandgap. TCO nanostructures ACS Paragon Plus Environment

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can be fabricated as nanospheres, nanorods, and three-dimensional structures.57-59 Although any kind of TCO such as indium tin oxide (ITO) and aluminum, gallium, and indium-doped zinc-oxide (AZO, GZO, and IZO) with carrier density in the range of 1020-1021 cm-3 can be utilized in the proposed unit-cell, we restrict our design to ITO as the most popular and widely-used TCO. The highly-doped ITO with the epsilon-near-zero property in the near-infrared regime will behave as a dielectric in short wavelength regime (visible spectrum) and will have plasmonic properties in long wavelength regime (thermal IR). The detailed information about the physical parameters of highly-doped ITO is provided in Section 1S of Supporting Information.55,60-62 In Figure 1(e), the dielectric constants of ITO and MgO insulator as a function of wavelength are plotted. It can be observed that there is a negligibly small mismatch (Re{εITO}- Re{εMgO}320°) and high reflection amplitude (>0.9) are obtained at the operating wavelength of 750 nm under illumination of z-polarized incident beam. The wavelength dependence of the reflection amplitude and phase for these cases are shown in Figures S4(a)-(b) of Supporting Information. In order to examine the effects of the rotation of the bottom layer on the reflection performance of the top layer at the red visible regime, the FDTD simulated results of the reflection amplitude are plotted in Figure 5(d) as functions of the gap size of Ag-made nanorod (gAg-rod=70, 80, and 100 nm) placed at the top layer and the rotation angle of the Si-made nanorods (θr=0°, 30°, and 45°) located at the bottom layer. It can be observed that the design is carried out in such a way that the effects of rotation of the bottom layer on the top layer is negligibly small for the weak resonances corresponding to the small gap-plasmon confinement. The phase of reflection coefficients behave in the similar manner thus are omitted here for the sake of brevity (Figure S4(c) in Supporting Information). By the increment of gAg-rod from 70 nm to 100 nm, the magnetic resonance shifts to higher frequencies and becomes stronger. For better understanding, the strength of excited magnetic resonance below the Ag-made nanorod antenna is examined for two cases of gAg-rod=70 nm and 100 nm and plotted in Figures 5(e)-(f), respectively. The higher confinement below the Ag-made nanorod makes the design more sensitive to the rotation of bottom layer. In Figures 5(g)(h), the total electric fields are investigated at the place of bottom layer (the rotation angle of Si-made nanorods is fixed at 45°) when the gap size of Ag-made nanorod is 70 nm and 100 nm at the operating frequencies of 394 THz and 468 THz, respectively. As expected, the magnitude of electric fields is much higher for the case of gAg-rod=100 nm due to stronger resonance of top layer. This can be concluded that by the increment of the gAg-rod, the reflection response of the plasmonic-dielectric supercell at the red light regime which should be tailored by the top layer will be more influenced by the bottom layer. Fortunately, almost 291.4° phase-change can be achieved by the variation of the gap size from 0 to 100 nm. In this range, the presented results confirm a negligible perturbation for reflection response of top layer without any dependency to the orientation of bottom layer.


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Figure 5. (a) Schematic illustration of the dual-band multilayer plasmonic-dielectric shared-aperture antenna. (b)-(h) Study the optical performance of top layer (Ag-made nanorod) at the red light spectrum under influence of bottom layer (Si-made nanorods) for z-polarized incident beam. (b)-(c) Amplitude and phase of the reflection coefficients of the proposed plasmonic-dielectric supercell in presence of the bottom layer (fixed at θr=0°) as a function of gap size (gAg-rod) of the Agmade nanorods at 400 THz. (d) Study of the effects of gap size of top layer and rotation of bottom layer on the reflection response of the super-cell within a range of frequencies around the red-color regime. (e)-(f) the normalized component of magnetic field (|Hy|) in x-z plane for two different gap sizes of 70 nm and 100 nm, respectively. (g)-(h) the total magnitude of electric field (|E|) at the place of bottom layer when the gap size of Ag-made nanorod is 70 nm and 100 nm and the rotation angle of Si-made nanorods is fixed at π/4.

In the blue light spectrum, the general optical responses of dual-wavelength plasmonic-dielectric composite is governed by the bottom-layer. Our investigations reveal the fact that if physical dimensions of the high-index dielectric inclusions vary (e.g., wSi-rod and LSi-rod), the performance of top-layer in the red light spectrum will be devastated. The possible approach to resolve this issue and negligibly impact the top-layer is utilizing the PB-based mechanism and controlling the rotation angle. It should also be mentioned that the extinction coefficient of silicon is much smaller than that of silver which enables us to push the operating wavelength down to 500 nm. The reflection amplitude and phase of the plasmonicACS Paragon Plus Environment

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dielectric composite unit-cell are shown in Figures 6(a)-(b) for two orthogonally polarized incident beams along the y- and z-axes with varying gAg-rod from 60 nm to 100 nm. It can be observed that the reflection responses are slightly modified by the physical modification of top layer which guarantee high isolation (low coupling effects) among parallel subarrays. The proposed unit-cell satisfies the PB criteria at the operating frequency of 600 THz and acts as a half-wave plate. Figures 6(c)-(d) represent the FDTD simulation results demonstrating the reflection amplitude and phase-change of the crosspolarized CP reflected beam at the operating wavelength of 500 nm in the presence of the top layer (the gap size of the Ag-made nanorod is fixed at 100 nm). As shown in Figures 6(c)-(d), the level of reflection amplitude is higher than 0.60 and total required phase-coverage is realized by only rotation of Si-made nanorods. The level of co-polarized CP reflected light is smaller than 0.235. The complementary information related to the co- and cross- polarized CP reflected beams versus frequency for all the rotation cases are shown in Figures S5(a)-(c) of Supporting Information. The ratio of reflection intensities of cross-polarized and co-polarized CP beams (extinction ratio) is calculated as 13.52 dB, 19.11 dB, 15.83 dB, and 8.77 dB for different rotation angles of θr=0°, 30°, 50°, and 90°. Furthermore, the investigation of extinction ratio of Si-made nanorods as functions of wavelength and different gap sizes of Ag-made nanorod is provided in Figures S5(e)-(h) of Supporting Information which not only demonstrates the high efficiency of Si-made nanorods at blue light regime but also clarifies the geometrical variation of top layer does not degrade the efficiency of bottom layer. The low interference among the parallel layers can also be revealed by the study of the near-field distributions at the place of corresponding resonances (591.7 THz and 607.5 THz regarding y- and z-polarized impinging beams, respectively) where the field is well-confined inside the Si-made nanorods and inplane and out-plane couplings between adjacent elements are weak. The nature of the aforementioned resonances are attested through the observed EM fields and vector profiles in Figures 6(e)-(j). The resonance mode is dominated by an electric dipolar mode under y-polarized illumination (Figures 6(e)(g)) and by a magnetic dipolar mode under z-polarized impinging field (Figures 6(h)-(j)). Figures 6(e)(f) demonstrate the excitation of an electric dipolar mode at the operating frequency of 591.7 THz where ACS Paragon Plus Environment

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the magnetic displacement currents circulate around the normal electric field (Ey, color bar). In contrast, the rotating tangential electrical components and the confined magnetic field (Hy, color bar) reveal the fact that the resonance which occurs at 607.5 THz is a magnetic dipole mode as shown in Figures 6(h)(i). In addition, the absolute value of the total electric fields are plotted at the center of the silicon nanorods for two orthogonal polarizations in Figures 6(g) and (j) where the electric field showing an antinode and node at the Si-made nanorod center (i.e. ED and MD modes). In order to address the misalignment issue which may occur in fabrication process, careful investigation is performed on the lateral deviation of Ag-made nanorod in y-direction up to ±100 nm and the results demonstrate negligibly small impact on the reflection response of both top and bottom layers at the red and blue light regimes. The details are provided in Figure S6 and Section 3S of Supporting Information. This level of deviation is much larger than the accuracy of state-of-art nanofabrication technology.64 It is a wellknown fact that larger coupling effects between the cascaded layers lead to the higher sensitivity to the relative lateral misalignment. In the case of negligibly low coupling interaction, the alignment issue will not be arisen and the functionality is promising without dependency on the lateral alignment between cascaded layers.66 Our studies on the plasmonic-dielectric composite demonstrate a higher sensitivity to the oblique incident beam in comparison with ITO-dielectric composite. Although the performance of Ag-made nanorod at the top layer can be well preserved as the oblique incident angle goes up to ±10° for TE polarization beam (electric field along the z-axis) at the operating frequency of 400 THz, the Si-made nanorod at the bottom layer has high sensitivity with respect to the oblique incident angle (limited angular tolerance) at the operating frequency of 600 THz. Therefore, its performance and half-wave plate response will be highly disturbed by slight tilting of CP oblique incident beam. This may be arisen due to the narrow-band operation, geometrical complexity of plasmonic-dielectric composite, and undesired optical interaction between subarrays under oblique incident beam.


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Figure 6. The performance evaluation of bottom layer (Si-made nanorods) at the blue light spectrum under influence of top layer (Ag-made nanorod). (a)-(b) Amplitude and phase of the reflection coefficients of the proposed plasmonic-dielectric supercell within a range of frequencies around the blue light spectrum in the presence of the top layer with various gap sizes from 60 nm to 100 nm. (c)-(d) The full-wave simulated reflection amplitude and phase of the cross-polarized CP reflected beam (RCP) as a function of the rotation angle of Si-made nanorods at λ2=500 nm when the incident beam is assumed LCP. The realized phase (dashed blue line) is in a promising agreement with the theoretical phase change based on PB mechanism (solid black line) in (d). Resonant field profiles for the ED resonance of Si-made nanorod at 591.7 THz: (e)-(f) crosssectional view (x-z plane) and (g) top view (y-z plane). (h)-(j)The same results in (e)-(g) for the MD resonance at 607.5 THz.

Dual-wavelength in-plane holographic shared-aperture metasurface. Here, the proposed plasmonicdielectric composite will be exploited to design a dual-wavelength multi-color hologram based on two primary colors (red and blue). The reconstruction principles are shown in Figure 7(a) in order to reproduce multicolor “NU” (i.e. the initials of Northeastern University, the affiliation of authors) at the distance of 10λ1=7.5 µm. First, the target image is divided into its red and blue components (“N” and “U”, respectively) as shown in Figures 7(b)-(c). The required phase distributions at the top and bottom ACS Paragon Plus Environment

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layers of the proposed plasmonic-dielectric composite in order to realize multicolor “NU” at the image plane are retrieved based on the CG algorithm method and shown in Figures 7(d)-(e). Then, these pixels should be substituted by the corresponding plasmonic-dielectric composite to provide the required phase shifts at both operating wavelengths. The building block of the top layer to construct the metahologram is a set of Ag-made nanorods consisting of 50×100 pixels with varying gap sizes operating over the zpolarized incident beam at the operating wavelength of 750 nm. The required phase profile shown in Figure 7(d) is encoded into an array of Ag-made nanorods where their gap sizes follow Figure 5(c). The bottom layer of this multi-color hologram includes an array of Si-made nanorods (100×100 pixels) angled with respect to the y-axis and provide different phase shifts based on PB tuning to modulate the cross-polarized CP beam. Low absorption nature of the high-index silicon enables us to create highly efficient hologram at short-wavelength light down to 500 nm. The rotation angle of Si-made nanorods placed at the bottom-layer should be varied as a function of position to provide the desired phase distribution to form “U” letter (shown in Figure 7(e)) at the distance of 7.5 µm and the operating wavelength of 500 nm. The simulated holographic images for a multicolor “NU” pattern is shown in Figures 7(f)-(g). In order to quantify the performance efficiency of this finite array, the ratio of reflected power to the incident power (i.e. reflection efficiency) is calculated as 87% and 63% for the red and blue colors, respectively. The design strategy which is leveraged in this multicolor in-plane metahologram imaging helps us to overcome the cross-talk limitation due to the fact that each layer is responsible to realize specific image in distinct wavelength. In addition, the accessible data capacity of subwavelength hologram can be enhanced at the same aperture footprint. The speckle noises and non-uniform amplitude distribution in the hologram plane are mainly attributed to two mains reasons. First, given to the available computational resources, we have simulated a finite size array. Second, the proposed holographic metalens is designed based on phase-only holography technique in which the lack of amplitude modulation may cause lower image quality. These issues can be resolved by the increment of size of meta-array which is the case in practical applications and utilizing more complicated design approaches14,75 like phase- and amplitude-modulated holography method. In conclusion, the proposed ACS Paragon Plus Environment

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methodology can improve the information capacity and single-band operation thus it can pave the way toward broad applications in compact color display chips, security, and data storage.

Figure 7. (a) Mechanism of multicolor holography via dual-wavelength plasmonic-dielectric composite to generate the target image of “NU” at the distance of d=7.5 µm. (b)-(c) The binary images of “N” and “U” with 50×100 pixels (top array) and 100×100 pixels (bottom array). (d)-(e) The required phase distributions calculated by the CG algorithm method to reconstruct the target holographic images of “N” and “U”. ny and nz correspond to the sampling in y- and z-directions. (e)-(g) The numerically calculated results with z-polarized incident red light (λ1=750 nm) and LCP polarized incident of blue light (λ2=500 nm) at the distance of 7.5 µm away from the proposed metalens.

It is worth mentioning that the proposed design technique is not only limited to the presented topologies and applications. The building blocks can be simplified/modified to be compatible with the desired application and measurement facilities. For example, it is possible to simply substitute the top layer of TCO-dielectric composite with a nanoantenna with varying geometrical parameters in y- and zdirections (e.g., Si-made nanopatch) which will operate on the LP beam similar to the bottom layer in the case of interest for same polarization excitation. It can also be scaled in order to cover other optical spectra and frequency ratios. As a potential future work, the possibility of implementation of extremely distinct operating channels like radio frequency (RF) and visible regimes is of particular interests; for instance to have an optical metatasurface on an RF terminal. Furthermore, the design strategy can be ACS Paragon Plus Environment

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utilized to add more capabilities (multi-functionality and multi-spectral operation) to an already existed single-band single-functional optical panel.

CONCLUSION

In this paper, we leverage a robust technique inspired by the composite multilayer shared-aperture concept in order to extend the conventional graded pattern metasurfaces to perform multifunctional responses at multiple wavelengths. In contrast with the recently proposed methods like segmentation, brute-force library searching, and multi-objective optimization, the proposed design strategy can offer several intriguing advances such as the wavelength ratio of the operating channels can be freely chosen from super-close (e.g., visible-visible) to super-distinct (e.g., thermal IR-visible), the possibility of independent design for each wavelength due to the low cross-talk and coupling effects, minimized aperture size, and simultaneous operation to serve anomalous or similar functionalities at the predesigned wavelengths. Two dual-layer metasurface-based designs namely TCO-dielectric and plasmonic-dielectric composites are designed with super-distinct operating channels (lie in thermal IR and visible with wavelength ratio of 14.5) and super-close operating channels (both lie in visible regime with wavelength ratio of 1.5), respectively. In two-layered TCO-dielectric patterns, the low coupling effect and possibility of simultaneous and independent phase control over the top and bottom layers are achieved by leveraging the dual nature of TCO materials and off-resonance operation of the top Si-made nanorods (PB mechanism). In two-layered plasmonic-dielectric composite, careful selection of constituent materials and optimization of structural parameters enable us to achieve high isolation and less sensitivity to alignment of subarrays. As a proof of concept, two dual-wavelength shared-aperture holographic metasurfaces are designed and studied with capability of generating two different holographic images located in different or similar distances from the metalens.

ACKNOWLEDGMENT. This work is supported by the U.S. Air Force Office of Scientific Research (AFOSR), #FA9550-14-1-0349. ACS Paragon Plus Environment

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SUPPORTING INFORMATION. The Supporting Information is organized into the following sections: 1. Optical Characteristics of Constituent Materials of Proposed Building Blocks, 2. Complementary Information about TCO-Dielectric Composite Building Block, and 3. Complementary Information about Plasmonic-Dielectric Composite Building Block.

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For Table of Contents Use Only

Composite Multilayer Shared Aperture Nanostructures: A Functional Multispectral Control Ali Forouzmand and Hossein Mosallaei* Metamaterials Laboratory, Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts 02115, United States *

[email protected]


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Figure 1. (a) Illustration of electromagnetic spectrum from far-infrared to visible regime with indication of the promising candidates namely (b) TCO-dielectric and (c) plasmonic-dielectric composites to serve as super-distinct and super-close dual-wavelength building blocks, respectively. 454x201mm (96 x 96 DPI)


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Figure 2. (a)-(d) Schematic demonstration of the topological orientation of the dual-band multilayer TCOdielectric shared-aperture antenna. (e) Comparison of the dielectric constants of ITO and MgO in the wide wavelength range of 300 nm to 1000 nm. (f)-(m) Study the optical performance of top layer (Si-made nanorods) at the green light spectrum with the full-matching condition (εITO=εMgO). (f)-(g) Simulated reflection amplitude and phase of the nanorod when the normal incident beam is polarized along the long axis (z-axis) and short axis (y-axis). (h)-(i) The reflection amplitude and phase of the cross-polarized reflected CP beam (RCP) as a function of the rotation angle θr under illumination of normal LCP incident beam. The FDTD simulated phase labeled by circle-shape markers compared with the theoretical prediction shown by the black solid line in (i). (j)-(k) The electric ﬁeld intensity (color bar) and the magnetic displacement current loop (arrows) at the electric dipole modes of the nanorod when the incident electric field is polarized along z-axis at the operating frequency of 516.3 THz. (l)-(m) The magnetic ﬁeld intensity (color bar) and the electric displacement current loop (arrows) at the magnetic dipole modes of nanorod when the incident field is polarized along the y-axis at the operating frequency of 545.2 THz. 440x400mm (96 x 96 DPI)


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Figure 3. The performance evaluation of bottom layer (TCO-made cross-shape nanoantennas) at the thermal IR spectrum under influence of top layer (Si-made nanorods) for y-polarized incident beam. (a)-(b) Maps of reflection response as functions of gcross and wcross of cross-shape nanoantenna. FDTD simulation results for the reflection (c) amplitude and (d) phase of the proposed unit-cell in the absence of top layer versus the gap size (gcross) for different widths of arms (wcross) of the cross nanoantenna at the operating frequency of 37 THz. (e) The effects of geometrical orientation of top layer on the reflection spectra of the bottom metasurface when wcross=800 nm, gcross=250 nm, hcross=270 nm, and Pcross=1980 nm. (f)-(g) The normal components of the electric and magnetic fields (|Ex| and |Hz|) are plotted at y-z plane and x-y plane, respectively. The impinging wave is propagating along x-axis and its electric field is oriented along the yaxis. (h)-(j) The total electric field distributions at the geometrical resonance of f=35.42 THz denoted by the vertical dashed line in (e) when the rotation angle of Si-made nanorods is 0°, 45°, and random. 668x427mm (96 x 96 DPI)


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Figure 4. (a) Schematic demonstration of light manipulation via the dual-wavelength out-plane holographic metalens for reconstructing two holographic images at two separate planes of 10λ1 and 2λ2 away from the metalens. The image planes are chosen at the distances of 10λ1 and 2λ2 above the shared aperture metasurface hologram where the virtual objects of "leaf" and "flame" are placed, respectively. (b)-(c) The binary images of the "flame" and "leaf" which consists of 50×50 and 350×350 pixels, respectively. (d)-(e) The calculated phase distributions of the electric field on the bottom and top layers of the bi-layer metasurface. (f) The numerically calculated amplitude of the electric field distributions at the image planes (10λ1 and 2λ2) at the operating frequencies of 531 THz and 37 THz under illumination of circularly and linearly polarized incident beams, respectively. 678x438mm (96 x 96 DPI)


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Figure 5. (a) Schematic illustration of the dual-band multilayer plasmonic-dielectric shared-aperture antenna. (b)-(h) Study the optical performance of top layer (Ag-made nanorod) at the red light spectrum under influence of bottom layer (Si-made nanorods) for z-polarized incident beam. (b)-(c) Amplitude and phase of the reflection coefficients of the proposed plasmonic-dielectric supercell in presence of the bottom layer (fixed at θr=0°) as a function of gap size (gAg-rod) of the Ag-made nanorods at 400 THz. (d) Study of the effects of gap size of top layer and rotation of bottom layer on the reflection response of the super-cell within a range of frequencies around the red-color regime. (e)-(f) the normalized component of magnetic field (|Hy|) in x-z plane for two different gap sizes of 70 nm and 100 nm, respectively. (g)-(h) the total magnitude of electric field (|E|) at the place of bottom layer when the gap size of Ag-made nanorod is 70 nm and 100 nm and the rotation angle of Si-made nanorods is fixed at π/4. 758x440mm (96 x 96 DPI)


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Figure 6. The performance evaluation of bottom layer (Si-made nanorods) at the blue light spectrum under influence of top layer (Ag-made nanorod). (a)-(b) Amplitude and phase of the reflection coefficients of the proposed plasmonic-dielectric supercell within a range of frequencies around the blue light spectrum in the presence of the top layer with various gap sizes from 60 nm to 100 nm. (c)-(d) The full-wave simulated reflection amplitude and phase of the cross-polarized CP reflected beam (RCP) as a function of the rotation angle of Si-made nanorods at λ2=500 nm when the incident beam is assumed LCP. The realized phase (dashed blue line) is in a promising agreement with the theoretical phase change based on PB mechanism (solid black line) in (d). Resonant field profiles for the ED resonance of Si-made nanorod at 591.7 THz: (e)(f) cross-sectional view (x-z plane) and (g) top view (y-z plane). (h)-(j)The same results in (e)-(g) for the MD resonance at 607.5 THz. 475x322mm (96 x 96 DPI)


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Figure 7. (a) Mechanism of multicolor holography via dual-wavelength plasmonic-dielectric composite to generate the target image of “NU” at the distance of d=7.5 µm. (b)-(c) The binary images of “N” and “U” with 50×100 pixels (top array) and 100×100 pixels (bottom array). (d)-(e) The required phase distributions calculated by the CG algorithm method to reconstruct the target holographic images of “N” and “U”. ny and nz correspond to the sampling in y- and z-directions. (e)-(g) The numerically calculated results with zpolarized incident red light (λ1=750 nm) and LCP polarized incident of blue light (λ2=500 nm) at the distance of 7.5 µm away from the proposed metalens. 730x374mm (96 x 96 DPI)


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Composite Multilayer Shared-Aperture Nanostructures: A Functional

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