Wavefront Control of Millimeter Waves With a VO2-Based Reconfigurable Meta-Reflectarray

In this paper, a novel architecture of a reconfigurable metasurface is proposed and numerically assessed for wavefront control in wireless systems working in the millimeter-wave range. We show that a metasurface made of engineered silicon bricks covered with a thin film of vanadium dioxide provides anomalous reflection with an efficiency of 93% at 60 GHz and a relative bandwidth of 15%. After applying a thermal stimulus that triggers the transition of vanadium dioxide from the insulator phase to the metallic phase, the metasurface abruptly changes its response and provides specular reflection. The device shows high efficiency and broadband operation in both VO2 states, allowing dynamic control of millimeter waves. Therefore, it finds applications in imaging, sensing, and intelligent wireless communications in 5G and 6G systems, especially for beam-shaping and beam-steering. Moreover, the proposed reconfigurable metasurface has a simple configuration that lends itself to standard fabrication tools, and it has a subwavelength size for integrated and compact communication systems.

while maintaining large directivity. There has been a growing interest in developing reconfigurable devices for millimeter waves in the last few years. In reference [10], a high-gain, frequency-reconfigurable metasurface antenna has been proposed in which vanadium dioxide (VO 2 ) allows to change the operational bandwidth. A reconfigurable metasurface working at 100 GHz was shown to be capable of tunable steering of electromagnetic waves [11]. A digital coding metasurface has been designed with a nematic liquid crystal to realize different reflection phases tuned by DC-bias voltage [12]. A reconfigurable dielectric resonator antenna with a 360 • adjustable pattern was proposed in [13]. In that work, reconfigurability was obtained by symmetrically placing six electromagnetic bandgaps (EBG). Another reconfigurable structure has been reported in [14], which is based on a wideband millimeterwave Fabry-Pérot cavity (FPC) antenna with switched-beam radiation at a frequency of 60 GHz [14]. Nevertheless, the reconfigurable devices proposed so far [10], [12], [13], [14] are antennas, and, in most cases, they are not capable of forming or deflecting a millimeter-wave beam; also, in reference [11], the deflection efficiency is limited to 50%.
In addition, in reference [15], a tunable thermal radiation metasurface using a graphene-metal hybrid structure has been designed for infrared anticounterfeiting and information encryption with applications in energy harvesting and thermal management. Other advanced graphene-based structures are proposed and experimentally investigated in reference [16] to detect hemoglobin biomolecules based on analysis of the transmittance of the structure. Another recently proposed device is an all-optical plasmonic flip-flop metasurface designed in the THz frequency range, which can be controlled by tuning the chemical potential of graphene [17]. Programmable manipulation of terahertz beams was proposed with a reflective hybrid graphene-metal coding configuration, combining C-shaped metallic split-ring resonators with graphene [18].
Reconfigurability of the electromagnetic response of metasurfaces may be achieved with different materials. One of the tuning mechanisms is provided by conductive oxides. For example, an active metasurface based on indium-tinoxide (ITO) was proposed in [19]. Phase transition materials (PTM) are activated by applying an external DC-bias voltage or a temperature stimulus [20], [21], [22]. Changing the crystallographic phase of germanium antimony telluride (GST) leads to terahertz wave control and modulation capabilities on the picosecond time scale [23]. Another example based on liquid crystals is a flexible millimeter antenna array for 5G networks [24]. 4-N , N -dimethylamino-4'-N 'methyl-stilbazolium tosylate (DAST) [25] provides a method of tailoring directional light scattering from a low-loss metasurface. Although these approaches [20], [21], [22], [23], [24], [25] lead to some degree of reconfigurability, in some configurations they may show low operational speed in the millimeter-wave range [24], [26], limitations due to feasibility [25], and, in some cases, limited performances [20], [23]. To solve these significant challenges, we have selected VO 2 as PTM, which allows fast tuning and deep modulation depths, and undergoes an abrupt and reversible switch between an insulator state to a metallic one. Indeed, VO 2 has a reversible phase transition between a high-resistivity insulating state at room temperature and a low-resistivity metallic state at a critical temperature of 68 • C [10], [11], [27], [28]. Additionally, VO 2 is the most promising thermochromic material in terms of feasibility, stability, and commercialization due to its cost efficiency and performance, especially in the millimeter band [10], [11], [27], [28].
The most critical problem that has not been addressed by previous works [10], [11], [12], [13], [14] is designing a simple, yet effective, architecture of reconfigurable metasurface for millimeter waves. Here, we propose and numerically investigate a novel structure for reconfigurable wavefront engineering of millimeter waves based on a meta-reflectarray made of silicon resonators covered by a VO 2 thin film for applications in beam-shaping and beam-steering in intelligent wireless communication systems, high-resolution imaging, sensing and also MIMO antennas for controlling the direction of millimeter-wave propagation. Also, this structure can be employed as a thermally-controlled switch. The proposed design, thanks to the VO 2 phase transition, provides highly efficient anomalous reflection when VO 2 is in the insulating state. In contrast, it switches to a highly efficient specular reflector when VO 2 switches to the metallic state. The switching is achieved by inducing a change the VO 2 film temperature. Compared to previous works on VO 2 -based reconfigurable devices working in the millimeter-wave range, the structure proposed here shows significant improvements: it proposes a novel and simple architecture to control reflection by tuning the temperature of the VO 2 layer, it can be easily fabricated thanks to its simple configuration, it shows high efficiency and broadband operation in both the VO 2 states.

II. PROPOSED STRUCTURE
The proposed structure is illustrated in Fig. 1. We designed a meta-reflectarray made of silicon resonators covered by a VO 2 film. The structure switches the angle of the reflected wave when the operating temperature increases above the critical temperature of the VO 2 phase transition (T C = 68 • C). The substrate is made of copper (Cu), and the thickness of the VO 2 ''skin'' layer is 50 µm, larger than the penetration depth of the metallic state of VO 2 in the millimeter-wave range , which is about 15 µm based on the resistivity data (2.6 × 10 −5 -5 × 10 −4 .m [30], [31]). When VO 2 is in the insulator state, it has a relative permittivity of 9. In this situation, Si and VO 2 have similar electromagnetic behavior [32], [33]. Therefore, the thin layer of VO 2 plays a marginal role in the interaction with incident waves since its thickness is much smaller than the wavelength in the medium (λ ∼ 0.5 mm). In the insulator state of VO 2 , at temperatures below T C , the supercell of the metasurface, shown in Fig. 1(b), is designed to provide anomalous, off-normal reflection with an angle of 30 • under plane-wave illumination at normal incidence. However, after the thermally-induced phase transition occurs, the VO 2 layer behaves as a metallic film that screens electromagnetic waves and prevents them from reaching the silicon metasurface underneath. In the metallic state of VO 2 , the structure reflects all the incident waves in the specular (normal) direction.

III. BACKGROUND THEORY AND DESIGN
To realize beamforming at room temperature, the metasurface should provide a phase discontinuity φ(x), calculated by the generalized Snell's law [34], [35]: where θ r and θ i are the angles of reflected and incident waves, and k 0 is the free-space propagation constant of the plane wave. The electromagnetic wave impinges at an angle θ i = 0 • , and the reflected wave should be in the desired anomalous direction. Thus, considering k 0 = 2π/λ where λ is the wavelength in the vacuum, the phase profile of the reflected wave should be equal to: Eq (2) is the target phase profile that is required to deflect a normally-incident input plane wave at any arbitrary angle θ r . To achieve the desired phase profile φ(x), one of the lateral dimensions of the Si bricks is fixed to a = 1 mm, and the other, b, is changed from 0.02 to 1.43 mm. Changing the lateral dimension of the silicon brick allows covering the phase of reflection between -180 • and +180 • , thus, any phase profile can be provided to deflect millimeter waves at any arbitrary angle. For illustrative purposes, we set the deflection angle θ r = 30 • so that the phase profile of the reflected wave should be equal to: The desired phase profile in eq. (3) is achieved with the supercell reported in Fig. 1(b), after applying the design procedure detailed below. The procedure's main goal is to achieve maximum reflection amplitude (ideally 100%) and simultaneously obtain full control over the phase of the reflected wave [34], [36].
To characterize the behavior of silicon resonators with variable size, and in particular, to calculate the phase of the reflection coefficient as a function of the resonator shape and size, we perform full-wave simulations in the frequency domain with the commercial software CST Microwave Studio ® using the finite-difference frequencydomain method (FDFD). The unit cell, containing one resonator with a specific shape and size, has lateral size equal to p along the x and y-axis, and periodic boundary conditions are set on the boundaries perpendicular to this axis. This is a standard choice in designing metasurfaces [37]. In the simulations, a plane wave is normally incident on the metasurface with an electric field polarized in the direction of the x-axis. Based on the experimental data in ref [32], there is a negligible variation in silicon relative permittivity at the millimeter wave frequency range, thus, we assumed a constant relative permittivity of 11.7. For VO 2 , we consider a constant relative permittivity equal to 9 in the insulator state (below T C ), and for high temperature, we use the frequency-dependent Drude model, ε = ε ∞ − ω 2 p /(ω 2 − jωγ ), with ε ∞ = 12, plasma frequency ω p = 1.4 × 10 15 rad/s, and collision frequency γ = 5.75×10 13 1/s in the metallic state [33], [38], [39], [40]. Also, the Drude model, fitted to the experimental data reported in [41], is adopted for the frequency-dependent permittivity of copper. The parameters used in the Drude model are ε = ε ∞ − ω 2 p /(ω 2 − jωγ ), with ε ∞ = 4.68, ω p = 1.32 × 10 15 rad/s, and the collision frequency is γ = 10.5 × 10 13 1/s. To investigate the proposed device in both states of VO 2 , we consider two different simulations at low temperature (below T C ) and high temperature (above T C ). The phase of the reflection coefficient is reported in the inset of Fig. 2 as a function of b, the lateral dimension of the silicon brick.
Using the data of the inset of Fig. 2, the size of the silicon bricks in the supercell (see Table. 1) has been chosen to provide the target phase profile of eq. (3), as reported in Fig. 2. The real phase profile, limited by the discretization effect of the metasurface, perfectly overlaps the target one leading VOLUME 11, 2023 to the deflection of a normally-incident wave at an angle of 30 • . As mentioned, there is no Ohmic loss in the resonators, and, due to the use of a metallic backplane, transmission is forbidden, resulting in virtually unitary reflectance.
To accurately study the performance of the VO 2 -covered silicon resonators in terms of phase control, the effective index of the fundamental mode propagating in the z direction was calculated as a function of the lateral size b of the silicon brick (keeping the size a constant). Using periodic boundary conditions, a finite-element eigenmode solver (COMSOL) was adopted to solve the eigenmode problem in the xy-plane. Figure 3a illustrates how the electric field spatial distribution in the xy-plane varies as a function of the lateral size of the Si brick (b). The size-dependent enhancement of the electric field inside and around the Si resonator is associated with a size-dependent change in the effective mode index. The variation of the effective index produces, in turn, a variation of phase accumulated by the reflected wave, which can be calculated as φ = 2k 0 n eff h Si . For this calculation, we assume that the eigenmode propagates for a round-trip (with total length 2h Si ) inside the brick along the z direction. The phase as a function of b, retrieved from the effective index n eff extracted by modal analysis is reported in Fig. 3b, and it shows good qualitative agreement with the full-wave simulations of Fig. 2.

IV. PERFORMANCES OF THE RECONFIGURABLE METASURFACE
Next, we have simulated the supercell of the metasurface designed according to the procedure outlined in the previous section: it consists of six resonators, which are repeated periodically with a period of 6p, as in Fig. 1. We assume an x-polarized plane wave impinging at normal incidence, and therefore we adopt periodic boundary conditions for the supercell, whose lateral size is 6p along the x-axis. This is a good approximation of a beam with a spot size significantly larger compared to the wavelength and to the supercell. The numerical results from the FDFD simulations in CST are presented in Fig.4 for VO 2 in the insulator state and Fig. 5 for VO 2 in the metallic state.
To investigate the detailed performance of the metasurface, we first calculated the reflectance in the millimeter wave bandwidth assuming VO 2 in the insulator state; the results are depicted in Fig. 4. The dominant contribution to reflection is in the wave directed at 30 • , which shows an efficiency 56512 VOLUME 11, 2023 Authorized licensed use limited to the terms of the applicable license agreement with IEEE. Restrictions apply.  of more than 80% in the range of frequencies 57-66 GHz, therefore providing a relative bandwidth of about 15% of anomalous reflection. Moreover, the x-polarized secondary lobes (i.e., other discrete diffraction orders), appearing at 0 • and −30 • , are lower than 20%; furthermore, the y-polarized reflectances are almost zero in any direction and, therefore, not reported in Fig. 4. In particular, the device efficiency at 60 GHz reaches 93%, which is remarkably higher in comparison to reconfigurable devices previously reported [42], [43].
In stark contrast, when the temperature of the structure increases over 68 • C, the VO 2 skin layer converts from insulators to metal, preventing an incident wave from reaching the silicon resonators. Thus, each part of the supercell creates the same phase profile causing the metasurface to reflect incident waves normally to the structure, in the specular direction. The wave reflectance has been calculated in the wide bandwidth between frequencies of 45GHz and 75GHz, as shown in Fig. 5. This graph indicates that the metasurface reflects millimeter waves in the specular direction with an efficiency close to 100%, while the other reflectance terms are almost zero. As shown in Figs. 4 and 5, when VO 2 switches from insulator to metal, the reflectance at 30 • decreases to zero, causing the main power of reflection to be normal to the structure. Therefore, changing the temperature of the VO 2 layers provides a degree of tunability of deflection of millimeter waves. Table 2 shows the simulation parameters of the proposed reconfigurable metasurface based on VO 2 at both low and high temperatures.
For the realization of the proposed structure, a silicon layer with a thickness of 1.7 mm, equal to the thickness of silicon  bricks, should be deposited on a copper layer with a thickness of 0.5 mm. Then, by performing lithography, the metasurface pattern with silicon resonators is transferred to the photoresist layer. After that, through etching, the pattern is created on the supercell of the metasurface. In the next step, a thin layer of VO 2 is covered the silicon bricks using a sputtering process. Temperature control can be achieved by passing an electric current through the back metal and exploiting the ohmic loss of copper. The critical problem in this process, which limits the tunability, is the fact that in a fabricated sample, VO 2 probably will not completely cover the vertical walls of the Si bricks. This can be mitigated by designing bricks with a conical shape.
A comparison of our structure with respect to other reconfigurable devices previously reported in the millimeter-wave range [10], [23], [42], [43], [44], [45], [46], [47] is summarized in Table 3, which highlights the strengths of the proposed metasurface configuration namely: large bandwidth, and high efficiency. The operational speed of this device is dependent on switching mechanisms for the VO 2 layer: the first mechanism is ultrafast (100 femtoseconds), which is photo-induced, and that is only possible using high-intensity and short pulses as stimuli [48], and the second one is a slower one (nanosecond), purely thermal, which can be induced with electrical current, or heat [49].

V. CONCLUSION
In this paper, we presented a novel and simple architecture of a reconfigurable metasurface with engineered silicon bricks covered with a thin layer of VO 2 as a phase transition material, which provides wavefront engineering of an incident beam in the millimeter wave range. The device reflects the incident wave in an anomalous direction at temperatures at which VO 2 is an insulator. At temperatures above 68 • C degrees, VO 2 switches reversibly to the metallic state, and the metasurface reflection occurs in the specular direction. Our numerical results indicate that thermal tuning of the metasurface is possible while maintaining an 80% efficiency over a relative bandwidth of 15% around 60 GHz, where it reaches an efficiency of 93%. The wideband operation in the millimeter-wave range, the high efficiency in anomalous reflection (low-temperature state of VO 2 ), and specular reflection (high-temperature state of VO 2 ) make the proposed device an appropriate candidate for high-resolution imaging, sensing, MIMO antennas for beam steering in high data rate and intelligent wireless communication systems, especially in beyond-5G and 6G cellular systems.
MOHAMMAD ALI SHAMELI was born in Isfahan, Iran, in 1993. He received the B.Sc. degree in electrical engineering from the Isfahan University of Technology, in 2015, and the M.Sc. and Ph.D. degrees in the fields and waves of electrical engineering from the University of Tehran, Tehran, Iran, in 2017 and 2021, respectively. He is currently a Postdoctoral Fellow with the University of Padova, Padua, Italy, where he is a member of the Photonics and Electromagnetics Group. His research interests include electromagnetics, photonics, and metamaterials. Open Access funding provided by 'Università degli Studi di Padova' within the CRUI CARE Agreement VOLUME 11, 2023