(RF) Transparent Meta-Glass
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Nanophotonics 2020; 9(12): 3889–3898 Research article Mahdi Safari, Yuchu He, Minseok Kim, Nazir P. Kherani and George V. Eleftheriades* Optically and radio frequency (RF) transparent meta-glass https://doi.org/10.1515/nanoph-2020-0056 this study paves the way for the design of new visibly Received January 25, 2020; accepted April 22, 2020; published online transparent metamaterials and artificial dielectrics. June 18, 2020 Keywords: 5G communication; metamaterials; metasurfaces; Abstract: We propose a radio frequency (RF) and visibly radomes; transparency. transparent composite metasurface design comprising newly developed transparent multilayer conductive coat- ings. Detailed experimental and theoretical analysis of the 1 Introduction RF/visible transparency of the proposed meta-glass is provided. The proposed nature-inspired symmetrical Recent years have witnessed a growing interest in thin honeycomb-shaped meta-glass design, alters the electro- metallic nano-films that can simultaneously offer high magnetic properties of the glass substrate in the RF spec- electrical conductivity and optical transparency, owing to trum by utilizing visibly transparent Ag-based conductive their promising potential in realizing various optically coatings on each side. Furthermore, the competing effect of transparent electromagnetic (EM) devices and low-emis- the Ag thickness on optical and RF transparency is dis- sivity materials. In this regard, conductive oxides (e.g., cussed. We show that using multilayer dielectric-metal indium tin oxide (ITO), aluminum-doped zinc oxide (AZO)) coatings, specifically 5-layered spectrally selective coat- and dielectric-metal-dielectric (DMD) designs (usually ings, RF transparency of the meta-glass can be enhanced comprising silver nano-films) have been extensively stud- while preserving visible transparency. Herein we demon- ied [1–18] for a variety of applications. These include low- strate high transparency meta-glass with 83% and 78% emissivity optical coatings [1–3, 16, 19, 20], transparent peak RF and optical transmission at 28 GHz and 550 nm, conductive oxides (TCOs) in photovoltaic devices [2–4], respectively. The meta-glass yields enhanced RF trans- transparent circuit components [4], antennas [5], meta- mission by 80% and 10% when compared to low-emissivity materials [7–10, 17, 20, 21], and various flexible devices and glass and bare glass, respectively. The meta-glass design sensors [11, 12, 18]. Moreover, there has been a focus on the presented here is amenable to a variety of 5G applications design and fabrication of visibly transparent low-emissive including automobile radar systems. This work provides a radio frequency (RF) absorbers for IR, acoustic and RF superior alternative to the standard indium-tin-oxide (ITO) bistealth applications [18–22], where researchers exploit transparent material which is becoming scarce. Moreover, the low conductivity of thin films (e.g., ITO) for high RF absorption. Despite their successful integration in a multitude of device constructs, the limited electrical con- *Corresponding author: George V. Eleftheriades, Department of ductivity offered by these material systems along with their Electrical and Computer Engineering, University of Toronto, Toronto, scarcity (e.g., ITO) prevents the realization of transparent ON, M5S 3G4, Canada, E-mail: [email protected]. https:// EM devices that can also operate in the high radio fre- orcid.org/0000-0001-7987-3864 quency (RF) range (GHz–THz). Mahdi Safari, Yuchu He and Minseok Kim: Department of Electrical To overcome these constraints, spectrally selective and Computer Engineering, University of Toronto, Toronto, ON, M5S 3G4, Canada, E-mail: [email protected] (M. Safari), coatings have been proposed recently that can offer high [email protected] (Y. He), [email protected] electrical conductivity in the microwave regime while also (M. Kim). https://orcid.org/0000-0003-1014-0771 (M. Safari). offering high optical transparency [15, 23]. As such, they https://orcid.org/0000-0001-6765-9844 (Y. He) serve as a promising steppingstone in the development of Nazir P. Kherani: Department of Electrical and Computer Engineering, transparent EM devices, particularly for next generation 5G University of Toronto, Toronto, ON, M5S 3G4, Canada; Department of communication applications. These 5G communication Materials Science and Engineering, University of Toronto, Toronto, ON, M5S 3E4, Canada, E-mail: [email protected] technologies which are based on microwave frequencies Open Access. © 2020 Mahdi Safari et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 3890 M. Safari et al.: Optically and RF transparent meta-glass extending up to 100 GHz require the consideration of en- structures from being ideal candidates for the design of RF ergy-efficient designs in light of the higher RF ohmic losses metamaterials even though they provide transparency in in this frequency range. the visible region. Among the various potential applications associated In this work, we propose and experimentally demon- with 5G communication, transparent meta-glasses that strate a meta-glass that significantly enhances the RF maximize both the optical and RF transparencies are of transmission at the operating frequency of 28 GHz and particular interest given that RF reflection from ordinary which is amenable for various 5G applications such as ra- glass is high due to its high permittivity in the RF region, domes in automobile radar systems. The prototypical especially for transverse electric (TE) polarization due to meta-glass structure offers increased RF transparency and the lack of a Brewster’s angle. Further, RF reflection is minimal ohmic losses by simultaneously inducing both exacerbated by the wide use of low-emissivity coatings on electric and magnetic responses such that the intrinsic windows (low-emissivity glass), where these coatings can impedance of the meta-glass is matched to that of free completely block the RF signal and preclude the use of space. The underlying physics defining the balance be- communication devices [16, 24]. It is from the recognition of tween RF and optical transparency is examined in detail. In these constraints that we take inspiration to develop the the following section, we present the theory and design of idea of a meta-glass that maximizes both optical and RF the RF transparent meta-glass. We then show how RF transparencies. transparency is constrained by the competing properties of To realize such a meta-glass, we hereby utilize a optical transparency and electrical conductivity of the cascade of dielectric (D) and metal (M) layers at the nano-thin silver film layer(s). Subsequently, we present nanometre thickness scale to design a composite of met- and discuss the RF and optical response of the designed asurfaces on a glass substrate. High optical transparency and fabricated meta-glass structure. We conclude by is achieved via optimal utilization of multilayers (m) of pondering opportunities in the field of transparent meta- oxide-free dielectric aluminum nitride (AlN) and nano- materials. thin silver (Ag) films, while the metasurfaces are obtained by design to realize low RF reflections and minimize ohmic losses. In particular, these metasurfaces comprise 2 Theory of arrays of subwavelength-sized unit cells that interact with an incident RF wave such that the structure is The aim of this study is to realize an optically transparent impedance matched to the surrounding medium, thereby meta-glass that also offers a wide-band, wide-angle, high minimizing the RF reflections. The prototypical meta- RF transparency near 30 GHz. We commence by demon- glass structure proposed herein opens the prospect of strating the design of the highly visible and electrically a manifold of new optically transparent metamaterial conductive optical coating (Figure 1c). We then utilize this designs. optical coating as a conductive surface to design a meta- In contrast, it is important to recognize that over the surface which offers high RF transparency near 30 GHz. last decade various concepts on optical transparency have Here below, we elaborate on the theory behind the been explored to advance the design of visibly transparent optical transparency of multilayered dielectric-metal (m- metamaterial structures [25]. Structures with balanced loss DM) optical coatings and the physics of RF transparency of and gain, such as those embracing parity-time symmetry, the honeycomb-shaped composite metasurface structure have been proposed to attain low scattering response from (Figures 1 and 2). We analyse the design procedures for a metamaterial layer or a single meta-atom using active optical and RF transparency separately given the huge elements [26, 27]. The potential of designing transparent difference between optical and RF wavelengths. meta-atoms using only passive elements has also been explored [27]. Exceptional properties of such photonic structures have been studied at length [28]. A recent study 2.1 Design of the multilayered optical shows that densely packed metallic nanospheres and coating nanorods act as artificial dielectrics in the optical region and it is possible to fabricate thick transparent metallic To obtain high optical transparency as well as high elec- structures [29]. However, the proposed approaches lack trical conductance, we consider spectrally selective coat- simplicity and present fabrication challenges that make ings