Near-Field Optical MIMO Communication with Polarization-dependent Metasurfaces

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Near-Field Optical MIMO Communication with

Polarization-dependent Metasurfaces

Shamsi Soleimani 1, Kasra Rouhi 1 and Ali Momeni 2

1 School of Electrical Engineering, Iran University of Science and Technology, Tehran, Iran

2 Laboratory of Wave Engineering, School of Electrical Engineering, Swiss Federal Institute of Technology in Lausanne

(EPFL), Lausanne, Switzerland

Abstract

The ability to control waves at the nanoscale has attracted considerable attention to ultrathin metasurface lenses

(metalenses) in optical imaging and encryption systems. We propose an approach to active tuning metasurfaces

by integrating an ultrathin layer of indium-tin-oxide (ITO) into a unit cell as an electro-optically tunable material.

A proposed design features two orthogonal wings that can independently manipulate waves with corresponding

orthogonal polarizations. The charge carrier concentration in the ITO accumulation layer is altered by modulating

the applied bias voltage. This bias voltage generates phase variations at terahertz frequencies for the reflected

transverse electric and transverse magnetic polarized waves. It is possible to move both focal points of a metalens

without any physical movement by varying the bias voltage. In addition, this paper explores the application of

virtually moving metalens for a novel multiple-input and multiple-output (MIMO) communication architecture.

We demonstrate a communication system based on single-point binary data communication and hexadecimal

orbital angular momentum (OAM) data communication. Then, orthogonal channels can be used for MIMO

communication with high capacity. The proposed design paves the way for high-speed communications as well

as polarization-controlled molecular imaging systems.

Keywords: Communication, Indium-tin-oxide (ITO), Lens, Metasurface, Multiple-input and Multiple-output

(MIMO), Tunable

I. Introduction

Metamaterials, artificially structured materials composed of subwavelength arrays of unit cells, can exhibit

extraordinary properties beyond those accessible by natural materials. They were initially proposed for challenging

fundamental laws and demonstrating negative refraction in the microwave regime. In subsequent research,

metamaterials were used as versatile platforms to manipulate electromagnetic waves throughout the spectrum

because of their extreme scalability. Metasurfaces are the equivalent version of two-dimensional metamaterials

proposed as an effective method for arbitrary manipulation of amplitude, phase, and polarization of

electromagnetic waves [1–5]. This unique feature of metasurfaces has recently attracted much attention due to its

small size, the possibility of integration, and high flexibility in manipulating the wavefront. In recent years, tunable

metasurfaces have attracted widespread attention because they provide significant opportunities for real-time wave

manipulation. They can be used in various applications, including vortex beam generation [6], asymmetric

arXiv: Near-Field Optical MIMO Communication with Polarization-dependent Metasurfaces S. Soleimani, K. Rouhi, A. Momeni

transmission and wave manipulation [7,8], holography [9], wave-based analog computing [10,11], spatial wave

control [12], and absorbers [13].

Terahertz lenses are widely used in spectrometers, terahertz communication systems, and millimeter and

submillimeter imaging systems. Generally, there are three types of terahertz lenses: ordinary, Fresnel, and

metasurface lenses [14]. In an ordinary terahertz lens, curved surfaces create a large space along with the thickness

of the lens. Indeed, ordinary lenses concentrate the waves by accumulating phase differences along the path. The

terahertz Fresnel lens is designed to reduce the thickness of ordinary lenses. On the other hand, in the metasurface

lens (metalens), an extra phase adds to the wave, leading to constructive interference on the focal point. A metalens

is a superficial lens that allows Fourier transform analysis. According to Fermat's principle, phase control can

modify the wavefront [15,16]. Therefore, metasurface can create the required phase change and develop ultra-thin

flat lenses with unique features and high performance. It also has high resolution and optimal performance for

ordinary lenses.

In structures that are not sensitive to polarization, only a single function is considered for one or both

polarizations. However, different behaviors can be extracted from orthogonal polarizations in an anisotropic unit

cell design [17]. If the function of two radiations with - and -polarization can be separated, different reflections

can be expected for each polarization, leading to independent beamforming. In [18], anisotropic transmissive

metasurfaces are presented that enable simultaneous and independent control of amplitude and phase responses of

two orthogonal polarizations. The transmission response of the suggested structure can give full phase coverage

with widely adjustable amplitude and negligible cross-polarized components. In [19], the transmission response

can be tuned to provide full phase coverage and minimal cross-polarized components. The designed metasurface

is made up of two layers of graphene arrays that can be switched between two states by biasing the two graphene

layers with the specified voltage and zero voltage, respectively. In this design, one state is for -polarized wave

manipulation, and the other is for -polarized incidence. In addition, the authors in [20] designed a metasurface

that can independently manipulate orthogonal linearly polarized terahertz waves by reconfiguring reflection

patterns. A series of graphene-strips-based unit cells form the basis of the proposed design. In addition, Zhu et al.

proposed a novel design method of aperture-multiplexing metasurfaces using a Back-Propagation Neural Network,

which can obtain independent wavefront control of orthogonally polarized electromagnetic waves [21]. For this

purpose, they suggested a metasurface based on a modified Jerusalem Cross structure, which decouples orthogonal

interactions by boosting the effective inductances of each of the two Jerusalem Cross branches. Due to the reduced

orthogonal couplings, the redesigned Jerusalem Cross structure can independently manipulate orthogonally

polarized waves. Furthermore, an anisotropic matrix metasurface consisting of asymmetric metal cross particles

with simultaneous dual-polarization anomalous reflections is proposed in [22]. There have also been several other

studies focused on the development of metasurfaces that are polarization-dependent, such as [23–34]. To the best

of our knowledge, there is no designed metasurface in the infrared spectrum capable of manipulating both

polarizations simultaneously.

In the past, wireless communication relied chiefly on electromagnetic plane waves [35]. There is also angular

momentum in electromagnetic waves, consisting of spin angular momentum (SAM) and orbital angular

momentum (OAM). As a wavefront with a spiral phase, the OAM has received a great deal of research attention

[36–38]. They can carry different modes (topological charges) independently. Beams with different OAM modes

are orthogonal and can be multiplexed/demultiplexed together. As a result, they can increase capacity without

relying on traditional resources like time and frequency. In future wireless communication networks, OAM with

multiple orthogonal topological charges is expected to bridge a new way to increase spectrum efficiency

significantly. Several experiments have recently demonstrated the feasibility of OAM wireless communications

[39,40]. According to [41], OAM multiplexing can achieve high capacity in mm-wave communications.

Additionally, OAM-based wireless communication research includes mode detection, mode separation, axis

estimation and alignment, mode modulation, OAM-beam convergence, etc. [35]. A significant increase in

spectrum efficiency can be achieved by combining multiple-input and multiple-output (MIMO) multiplexing with

arXiv: Near-Field Optical MIMO Communication with Polarization-dependent Metasurfaces S. Soleimani, K. Rouhi, A. Momeni

OAM. The spectrum efficiency of wireless communications can be improved by combining MIMO-based spatial

multiplexing and OAM multiplexing, as demonstrated in [40,42–44].

In this article, we survey the architecture of reconfigurable metasurface for imaging applications and wireless

nearfield communications. The reflected wave can be controlled separately for each polarization using the

polarization-dependent metasurface. We embedded the thin layers of indium-tin-oxide (ITO) wings perpendicular

to each other in our design. Hence, the embedded wings in - and -direction can interact only with - and -

polarized waves. Furthermore, the corresponding polarization can be controlled by tuning the voltage connected

to each wing without affecting cross-polarization. Several examples are presented to illustrate how the design can

be applied to lenses. The metasurface mimics the moving lens that can change its focal point without moving

physically. Then we demonstrate how MIMO communication in two orthogonal channels can increase the

transmission capacity of the communication system, as shown in Fig. 1. Our study demonstrates the advantages

of OAM-based wireless communications in transmitting information through the channel. Furthermore, we give a

novel OAM-modes-based orthogonal multiuser access framework and evaluate the obtained data efficiency in the

receivers.

II. Indium-Tin-Oxide

1. ITO Characteristic

Metasurfaces with active tunable properties can be used to extend the applications of devices. For example,

spatial light modulators, dynamic beam steering, reconfigurable imaging, and reconfigurable pulse shaping are

examples of externally controlling the phase and/or amplitude of the light that the individual antennas on the

metasurfaces emit, resulting in a dynamic wavefront transformation of light [45]. Active materials such as

transparent conducting oxides [46], graphene [47,48], phase-change materials (e.g., GeSbTe) [49], and liquid

crystals [50] are an example of tunable materials. The optical properties of these materials can be switched using

external stimuli.

Indium-tin-oxide (ITO), aluminum-zinc-oxide (AZO), gallium-zinc-oxide (GZO), and indium-doped-zinc-oxide

(IZO) as transparent conducting oxide materials (TCOs) were investigated to design metamaterials in the near-

infrared (NIR) spectral range and also as an option for plasmonic resonances engineering. Among these materials,

ITO is the most well-known TCOs deployed in electronic, optoelectronic, and mechanical applications. Utilizing

Fig. 1. The schematic design of the proposed metasurface for mode-division OAM MIMO communication by using polarization-

dependent metasurface based on Indium-Tin-Oxide.

arXiv: Near-Field Optical MIMO Communication with Polarization-dependent Metasurfaces S. Soleimani, K. Rouhi, A. Momeni

ITO as an active material with different amounts of carrier density can be useful in tunable devices. In this way, a

metasurface with this combination can be made adjustable by externally applying voltage.

Two distinct categories can be considered for the earlier applications of ITO. In the first group, a multilayer

metal-oxide-semiconductor (MOS) structure deposited on a silicon waveguide creates plasmonic modulators. In

these optical devices, the accumulation layer of ITO controls the plasmonic gap modes. In [51], an ultra-compact

“PlasMOStor” made up of an ITO-filled slot plasmonic waveguide has been presented. The second group consists

of a sandwiched insulator-ITO layer between two metal electrodes [39]. In metal-insulator-metal (MIM) structures,

the confined light experiences a significant phase shift due to Fabry-Perot-like resonance [52,53]. Consequently,

these features can be used to control the light-matter interaction. Substituting the insulator with an insulator-ITO

layer opens a window of opportunity for generating a reconfigurable antenna with tunable capability. In [54],

electrically tunable absorption has been experimentally investigated by depositing a thin layer of ITO over an array

of gold nanostrip antennas. It can be inferred from [55] that ITO integration can guide the impinging beam into a

MIM structure toward a desired diffracted angle at the selected single frequency by electrically modulating the

ITO via the field-effect technique. In this case, both pre-depositional and post-depositional processes and the

utilized technique control different structural, electrical, and optical properties of the ITO film (resistivity,

transmittance, refractive index, etc.). The most applicable and frequent technique is sputtering, among various

methods such as spray pyrolysis, screen printing, and chemical vapor deposition for ITO deposition. All the oxygen

content, the tin-to-indium ratio, growth temperature, doping impurity, sputtering power, and pressure help to

manage the sputtering process of ITO. Accordingly, different papers have reported the various values for the

dielectric function of ITO [54,56–59].

Utilizing a Drude function  

Γ can be helpful to have an accurate optical model for

conducting oxide ITO in which background permittivity (), collision frequency , and plasma frequency 

are selected according to the data fitting of the deposited ITO film experimental data. The plasma frequency is

related to the carrier density  by 

, where is the electron's effective mass, 

indicates the electron rest mass, and  shows the electron charge. Due to the considerable significance of plasma

frequency in the NIR regime, a range of   for the carrier concentration should be considered. In

this paper, the optical parameters of ITO layers are chosen as , and  [54,58].

It has been shown in [60,61] that gate voltage bias influences carrier density in the ITO layer based on the

Thomas-Fermi screening theory, in which the carrier density change is averaged over the entire ITO layer (

increase in the average carrier density [62]). Another approach is to consider an ultra-thin accumulation layer at

the insulator-ITO interface, where the refractive index changes with applied field intensity [59]. The ITO loss can

be influenced by various factors, such as the gate-induced injection of carriers into the ITO and the grain size of

ITO, which can be controlled by ITO deposition thickness and post-annealing temperature [63,64]. Additionally,

it was shown that increasing ITO grain size reduced the resistivity of ITO film [63]. The uniformity of electrical

and optical properties of ITO film and the level of sheet resistance is controllable during the fabrication process

[64,65].

2. Reconfigurable Unit Cell Design

Electromagnetic waves in the terahertz regime can excite prominent plasmonic resonances in ITO, but these

wave-ITO interactions need to be further enhanced for practical applications. Therefore, we design a unit cell that

greatly enhances wave-ITO interactions by employing the Fabry–Perot resonant principle. The proposed Fabry–

Perot resonant-based unit-cell consists of an ITO layer and alumina, which is placed between an optically thick

gold substrate (back mirror) and two upper orthogonal gold strips. So, when terahertz waves illuminate the top

layer, plasmonic resonances can be excited. In the bottom ground, the metallic material is embedded so that the

waves can be completely reflected. The proposed metasurface is constructed by periodically extending the ITO-

based unit cells along both - and -directions, as shown in the inset of Fig. 2(a). It's worth mentioning that two

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Near-FieldOpticalMIMOCommunicationwithPolarization-dependentMetasurfacesShamsiSoleimani1,KasraRouhi1andAliMomeni21SchoolofElectricalEngineering,IranUniversityofScienceandTechnology,Tehran,Iran2LaboratoryofWaveEngineering,SchoolofElectricalEngineering,SwissFederalInstituteofTechnologyinLausanne(EPFL)...

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