Laser-Drilled All-Dielectric Huygens’ Transmit-Arrays as 120 GHz Band Beamformers

An all-dielectric Huygens’ transmit-array is proposed and experimentally demonstrated in the 120-GHz-band. The proposed transmit-array is fabricated using a high-precision laser-drilling process providing high fabrication precision and accuracy. A non-uniform Huygens’ surface is next placed on top of a planar high gain slot-array antenna for far-field beamforming. Full-wave simulations are first used to demonstrate the beamforming capability by generating difference-pattern beams as an example. A uniform transmit-array was then fabricated and characterized to investigate the effect of the transmit-array on the antenna performance, along with showing very low fabrication tolerances and minimal dimensional variation due to the laser drilling process. Next, a phase-gradient transmit-array for beam-tilting is experimentally demonstrated as a practical example of beamforming. The proposed all-dielectric structure is the first Huygens’ structure to be demonstrated at these frequency bands, and represents an attractive alternative to conventional metallic Huygens’ metasurfaces based on standard printed circuit board (PCB) solutions.


I. INTRODUCTION
Electromagnetic metasurfaces are 2-D structures that consist of arrays of sub-wavelength resonators [1]. A wide range of electromagnetic (EM) wave manipulations can be achieved by careful design of the geometric shapes and material properties of these resonators. For example, metasurfaces have been used for applications such as controlled reflection [2]- [4], polarization control [5]- [7], refraction [8]- [10], absorption [11]- [13], and electromagnetic illusions and cloaking [14], [15], using variety of resonator configurations. An important class of metasurface is a Huygens' Metasurface, which exhibits orthogonal and co-located electric and magnetic dipoles [16]- [18]. By exciting both dipole moments, a Huygens' metasurface can be synthesized to achieve wide variety of EM wave transformation The associate editor coordinating the review of this manuscript and approving it for publication was Kuang Zhang. with zero back-scattering, thus providing phase control in transmission.
Metasurfaces, and in particular Huygens' metasurfaces, can be broadly classified into two categories based on their design: metallic metasurfaces and all-dielectric metasurfaces. Metallic metasurfaces are composed of unit cells that consist of metallic patterns, most commonly implemented on a dielectric substrate using standard printed circuit board (PCB) processes, at microwaves [19]- [27]. On the other hand, all-dielectric metasurfaces, as the name suggests, are composed of unit cells based solely on dielectric materials [17], [28]- [42]. All-dielectric metasurfaces are preferred over metallic metasurfaces because they typically exhibit lower Ohmic losses as frequencies become higher, as typically observed at optical frequencies [43]. Various design techniques exist to design such surfaces, where for instance, all-dielectric metasurface have been modeled using deep neural networks [44] and analytical modeling of spherical particles [45], while metallic metasurfaces are modeled using typical circuit analysis [46].
Metallic metasurfaces are more common at microwave frequencies due to the mature PCB-based fabrication, however as frequency increases towards mm-waves and beyond 100 GHz, realizing narrow line-widths and gaps using standard PCB fabrication becomes challenging, in addition to associated Ohmic losses. Consequently, some efforts have been made to realize photonics inspired all-dielectric metasurface structures at microwaves [2], [28], [29], [31] and mm-wave frequencies [47], as an alternative to metallic resonator-based metasurfaces. Such structures are based on mechanically drilling typical substrate materials, as opposed to standard PCB processes, to form dielectric resonators connected using bridges. However, such structures based on single layer dielectrics do not scale to mm-wave frequencies due to mechanical vibrations induced during the drilling process, leading to large fabrication errors [48]. Consequently, a double layer all-dielectric structure with double cross bridged unit cell has recently been proposed in [47] at 60 GHz IEEE 801.11ad to mitigate this problem.
On the application side, with the upcoming sixth generation (6G) communication networks [49] being expected to see a high demand for antenna beamforming techniques above 100 GHz, there is an interest in devising novel ways to enable general wave transformations. It can either be in the form of novel antenna structures or achieving beam shaping using external phase plates realized using metasurfaces, for instance, where Huygens' metasurface can be used for engineering the near-field distribution of directive, high gain antennas [50]. Of particular interest in this work is operating in the near-field zone of large-scale antenna arrays used in the Gigabit Access Transponder Equipment (GATE) for short-range communication [51]. These antennas are designed using a complex and expensive process of diffusion-bonding of metallic plates so normally they are designed with a near-uniform phase to achieve broadside radiation in the far-field. There are two options to achieve other patterns shapes and directions. The first choice involves redesigning the antenna which is a challenging process. A second alternative is to use a metasurface operating in the near-field of the antenna to generate the desired far-field pattern [47]. The second alternative appears more practical and allows for diverse beamforming without modifying the antenna or to modify characteristics of already deployed antenna units.
In this context, we explore the realization of an all-dielectric Huygens' metasurface in the 120 GHz, and to use them for engineering the near-field distribution of diffusion-bonded planar slot array antennas for far-field beamforming. We use a laser drilling process to realize bridge-connected dielectric resonators that provide accurate prototyping of small resonator dimensions with no mechanical vibrations, thereby exhibiting very low fabrication tolerances. Laser drilling enables a single layer realization of all-dielectric structures in the 120 GHz band, whereby a single bridge is sufficient using a single layer dielectric only. We experimentally demonstrate these all-dielectric structures which are co-designed with planar slot array antennas, using them as transmit-arrays with beamforming capabilities. We further note that this is the first implementation of a Huygens' metasurface at these frequencies beyond 120 GHz, where the latest Huygens metasurfaces have been demonstrated up to 83 GHz only based on multi-layer metallic configuration [52], [53].
The paper is organized as follows. Sec. II presents the proposed Huygens' unit cell using full-wave driven-mode simulation to demonstrate the Huygens' resonances and the typical EM performance. Sec. II further compares the performance of the proposed all-dielectric unit cell with a metallic unit cell based on metallic patch resonators. Sec. III presents a full-wave demonstration of the transmit-array by operating in the near-field of a directive slot-array antenna to generate difference-pattern beams, as an illustrative example. Sec. IV presents an experimental demonstration of a uniform transmit-array to characterize its performance. A phase-gradient transmit-array is next demonstrated using beam-tilting as an example that can be achieved using the proposed structure. Conclusions are finally provided in Sec. V.

II. PROPOSED ALL-DIELECTRIC HUYGENS' STRUCTURE A. HUYGENS' UNIT CELL
The proposed all-dielectric Huygens' unit cell is shown in Fig. 1(a). The unit cell is a dielectric shell disk with two interconnecting bridges to its neighboring disks. The presence of the bridges restricts the operation of the unit cell to polarizations orthogonal to the bridges (y− polarized waves in this illustration). This unit cell was implemented at 60 GHz with machine drilling [54] but had poor performance due to mechanical vibrations when increased fabrication errors leading to inaccurate designs. A solution to the mechanical instability was recently proposed by using a dielectric unit cell with four interconnects, but this required an additional dielectric layer to cancel reflections from the extra interconnects, leading to a bulky design [47].
Here we propose using laser-drilling instead of mechanical drilling, which has not been explored for all-dielectric metasurfaces at these frequencies. Laser-drilling uses high precision laser beams, and the process is free from any mechanical vibrations. Consequently, it leads to high accuracy in the fabricated design which is paramount for high performance beamforming at high frequencies.
Finite-Element-Method (FEM) High Frequency Structure Simulator (HFSS) was used to demonstrate the EM performance of the unit cell. Fig. 1(b) shows the typical reflection and transmission response with a normally impinging plane waves (i.e. plane wave propagating in the z− direction and polarizing in the y−direction). Two nulls are observed in the reflection plot, which correspond to zero back-scattering. These two frequencies (120.2 and 126.6 GHz) are the Huygens' resonances where the electric and magnetic dipoles are excited [19].
To demonstrate the Huygens' resonances, the driven-mode field plots are shown in Fig. 1(c) at 120.2 and 126.6 GHz. The field plots clearly show the electric field (y−axis) and magnetic field (x−axis) excited in the unit cell at both frequencies. The two fields are orthogonal, co-located, and occur at the same frequency, hence the Huygens' condition is met.
The dimensions of the unit cell were then varied to demonstrate the full 360 • phase variation achieved with low reflection. The ratio a/b or ellipticity, τ , of the unit cell is the main dimension that controls the transmission phase. The diameter of the inner hole, d in and the bridge widths, w, were used to optimize the performance for low reflection. Fig. 1(d) shows the transmission magnitude where the transmission loss is maintained below 1 dB across the bandwidth. Fig. 1(e) shows the reflection magnitude, where it is below −10 dB in a wide bandwidth from about 117 to 139 GHz. Finally, Fig. 1(f) shows the transmission phase for each unit cell, whereby varying the ellipticity a full 360 • phase variation can be achieved.

B. ALL-DIELECTRIC VS METALLIC CELL
Next, the proposed all-dielectric structure will be compared with a metallic unit cell [46] as shown in Fig. 2(a). The metallic-PCB unit cell consists of two metallic patch resonators on either side of a dielectric substrate. The outer and inner ellipticity of the patch resonators are degrees of freedom to control the transmission phase while achieving good matching. Fig. 2 The metallic and all-dielectric unit cells have comparable loss performance at the Huygens' resonances around 130 GHz. The all-dielectric cell however offers larger matching bandwidth and better transmission efficiency compared to the metallic one. Moreover, the all-dielectric cell features greater phase variation with frequency, which is also reflected in the larger transmission group delay response, as shown in Fig. 2(e). Besides, in terms of fabrication, the metallic unit cell is more challenging to fabricate at 120 GHz due to the high tolerance on the PCB fabrication requiring very small line-width of the elliptical resonator. Hence, the laser-drilled VOLUME 8, 2020 FIGURE 2. Comparison between a metallic-PCB unit cell [46] and the proposed all-dielectric unit cell. a) Illustration of the metallic-PCB unit cell based on metallic patch resonators and the proposed all-dielectric unit cell. The metallic-PCB unit cell was implemented using a substrate with the following properties: r = 2, tan δ = 0.0021, h = 0.184 mm, copper conductivity, σ = 5.8 × 10 7 S/m, and has the following dimensions: = 2 mm, elliptical patch outer ellipticity, τ out = 1.5, inner ellipticity (i.e. hole), τ in = 0.5, and copper thickness, t = 15 µm. The all-dielectric unit cell was implemented on Rogers' RO3006 ( r = 6.5, tan δ = 0.0020, h = 25 mil = 0.635 mm), and has the following dimensions: a/b = τ = 2, w = 0.37 mm, and d in = 0.22 mm. Simulation results were obtained by exciting both unit cells with a normally impinging plane-wave to obtain b) reflection, transmission c) magnitude and d) phase, and e) group delay.
all-dielectric unit cell is an attractive solution to implement Huygens' surfaces above 100 GHz.

III. FULL-WAVE TRANSMIT-ARRAY DEMONSTRATION A. HUYGENS' TRANSMIT-ARRAY ELEMENT
The proposed Huygens' unit cell will be used to engineer the near-field of a directive slot array antenna operating in the 120-GHz-band [55]. Fig. 3 shows the FEM-HFSS model of the Huygens' element consisting of one slot sub-array unit cell and four Huygens' unit cells. The distance between the top of the antenna unit cell and the Huygens' unit cell is s = 5 mm, which was found to give an optimum matching performance. By repeating this composite slot sub-array and Huygens cell in 2D, the near-field phase may be spatially engineered. The proposed Huygens' surface thus acts a transmit-array.
The slot sub-array unit cell consists of a feeding waveguide which carries the power to a cavity. The coupling slots couples the power to the four slots which subsequently excite four radiation slots with linear polarization. The transmit-array is designed such that one unit cell covers one radiation slot, so that the slot array period fixes the transmit-arrays periodicity. The polarization of the antenna is the in y−direction (i.e. perpendicular to the Huygens' unit cell bridges). The antenna is designed with a uniform near-field phase, hence the Huygens' unit cell simulated with a normally impinging plane wave can be directly used with the antenna unit cell without any modification.

B. TRANSMIT-ARRAY BEAMFORMING
Full-wave simulations are next used to demonstrate the operation of the transmit-array on top of the slot array antenna. As an example of beamforming, a transmit-array was designed to achieve difference-pattern beams from one directive beam. Such a surface has one-half of the unit cell with a certain transmission phase, φ and the other half are out of phase (i.e. φ ± 180). When a wave is incident on a difference-pattern surface, a null will be created at the point of phase change due to the destructive interference of the fields. The null will cause the main beam of a directive antenna to split into two beams (i.e. difference-pattern beams). One application of difference-pattern transmit-array increasing the spatial coverage of directive antennas by splitting the main beam into two beams as was demonstrated in [47]. Fig. 4(a) shows an illustration of a 1-D array of the difference pattern structure. Fig. 4(b) shows the FEM-HFSS model of a 1-D array of the slot array unit cell and the difference-pattern structure. The phase variation was imposed in the H-plane of the antenna for best performance, which can be attributed to weak coupling between the slots in the H-plane. Periodic boundaries are used to achieve the full size of the array needed, which in this case is a 20 × 20 array of the transmit-array and an 8 × 8 array of the slot antenna unit cell (total 16 × 16 slots). Extra unit cells of the transmit-array are added around the antenna to reduce edge effects. Fig. 4(c) shows the near E-field phase at 126 GHz and at 8 mm above the antenna with and without the difference-pattern transmit array. As can be seen, the antenna has a near-uniform phase distribution, and when the transmit-array is added the desired phase discontinuity of 180 • is achieved. Fig. 4(d) and (e) further show the far-field radiation patterns of the antenna with and without the transmit-array at 126, 127, and 128 GHz. It is demonstrated that the transmit-array causes the main antenna beam at 0 • to split into two beams, as desired.
Next, experimental demonstration of the proposed Huygens' transmit-array will be presented for uniform and phase-gradient transmit-arrays.

IV. EXPERIMENTAL DEMONSTRATION A. LASER-DRILLING PROCESS AND SPECIFICATIONS
Laser-drilling is the process of using focused high energy laser beams to cut, drill, or mill a material. In the process, the material is removed by a combination of melting and evaporation [56]. It provides high accuracy, fast drilling rates, and is capable of drilling smaller hole sizes than standard PCB fabrication [57]. Table 1 shows the specifications of the laser-drilling process used to fabricate the proposed design. The most important parameter that influences the design stage is the taper angle of the laser beam. For best accuracy and agreement with simulation, a taper angle of 2.25 • from the top to the bottom of the substrate was taken into consideration in the FEM-HFSS designs.
To investigate the effect of the transmit-array on the antenna and the accuracy of the laser drilling process, a uniform structure was first fabricated, where the same unit cell was used throughout the array. Fig. 5(a) & (b) show the slot array antenna and the uniform transmit-array, respectively. To verify fabrication accuracy, the unit cells were measured under the microscope as shown in Fig. 5(c). Table 2 shows the design, average measured, and standard deviation for two dimensions, a and w. Dimension a has an average error of 9 µm with a standard deviation of 13 µm, while dimension w has an average error of 2 µm with a standard deviation of 14 µm. These errors are within the position accuracy of 10 to 15 µm shown in Table 1, and thus represent very low dimensional tolerances along with excellent repeatability.
The antenna reflection and far-field radiation characteristics were first measured. The transmit-array was then placed on top of the antenna with plastic washers in between to get a separation of 5 mm, and fastened using four plastic screws in the corners. The antenna together with the transmit-array was then measured. Fig. 5(d) shows the measured reflection of the antenna only, and the antenna with the transmit-array in measurements and simulation. Generally, the transmit-array has negligible effect on the reflection except above about 128 GHz, which was also predicted by full-wave simulations. The −10 dB matching bandwidth of the antenna and the transmit array is from 119 to 128 GHz. Fig. 5(e) shows the measured gain of the antenna only and the antenna with the transmit-array. In the matched bandwidth of 119 to 128 GHz, it is shown that transmit-array causes has a maximum gain drop of 2 dB at 128 GHz, which is indicative of its dissipation losses. Fig. 5(f) shows the far-field radiation pattern of the antenna in measurements and simulation, showing almost identical performance, which indicates a good phase uniformity across the surface and fabrication robustness. Fig. 5(g) further shows the far-field radiation pattern of the antenna with the transmit-array in measurements and simulations, where the shape is comparable around the main lobe.

B. PHASE-GRADIENT TRANSMIT-ARRAY
Next, a phase-gradient transmit-array was fabricated to experimentally demonstrate beam-tilting as an example of beamforming using the proposed Huygens' unit cell. To achieve beam-titling from the antenna, a phase-gradient from 0 to 360 • must be imposed in the near-field within a given spatial distance, which can be achieved using unit cells of the proposed Huygens' cells with varying ellipticities as shown in Fig. 1(f). Fig. 6(a) shows a 1-D array of the phase-gradient transmit-array where the ellipticity decreases and phase increases from left to right. The phase-gradient transmit-array was placed on top of the slot array antenna and then near-field phase at 129 GHz is shown in Fig. 6(b). It is shown that a phase-gradient, k x0 = 2π/32 mm = 196 rad/m is achieved. The beam-tilt angle achieved using this phase-gradient can be calculated using where k 0 is the free-space phase constant.  Fig. 7(a) shows the fabricated phase-gradient transmitarray. To verify fabrication accuracy, three unit cells were selected and measured under a microscope as shown in Fig. 7(b). Table 2 shows the design, average, and standard deviations of the dimensions a and w. Just like the uniform transmit-array, high fabrication accuracy is achieved. The reflection of the antenna with the phase-gradient was then measured as shown in Fig. 7(c). Reflection below −10 dB is obtained from 120 to 132 GHz. Fig. 7(d)-(e) show the measured far-field radiation patterns of the antenna with the transmit-array at 128, 129 and 130 GHz, where a clear beam-tilt is observed at each frequency, comparable with the simulation results.
The theoretical tilt angles can be calculated at 128, 129, and 130 GHz as follows: θ 128 GHz = sin −1 (k x0 /k 0,128 GHz ) = 4.20 • , θ 129 GHz = sin −1 (k x0 /k 0,129 GHz ) = 4.17 • , and θ 130 GHz = sin −1 (k x0 /k 0,130 GHz ) = 4.14 • . Comparable tilt angles were achieved in measurements: θ 128 GHz, meas. = 5 • , θ 129 GHz, meas. = 4.2 • , and θ 130 GHz, meas. = 4 • . Note that the side-lobe levels (SLL) are high (≈ −4 dB) in both simulations and measurements, as shown in Fig. 7(d)-(f). One of the reasons for the high SLL is spurious diffraction orders which occur due to the simple design of the phase-gradient transmit-array [58]. In addition, the large unit cell size also leads to poor discretization of the phase-gradient, as shown in Fig. 6(b). In the proposed design, the Huygens' unit cell size was restricted to 2 × 2 mm to match the size of the slot array antenna unit cell. To reduce the SLL, the phase discretization can be improved by making the unit cell smaller using a higher permittivity dielectric, for instance [47]. Furthermore, from a more theoretical point of view, the side-lobes can completely be eliminated by using a bianisotropic unit cell to suppress spurious diffraction orders, which has been demonstrated in [58] for example, using a three-layer metallic bianisotropic unit cell. Therefore a complete elimination of the side-lobes will require devising a   more complex all-dielectric unit cell exhibiting bianisotropy, compared to the simple one used here in this work.
The discrepancies between measurement and simulation results can be due to various factors. First, the dielectric constant and the loss tangent of the substrate are not known at the design frequencies, where even a small deviation from the design values can cause significant variation between simulation and measurements. In addition, it was assumed the substrate properties were constant with frequencies, which may not have been the case, especially considering the high frequency of operation of the proposed designs. Another issue is misalignment between the antenna and the transmit-array structure. The antenna/transmit-array structure was simulated under ideal alignment and flatness conditions, hence any deviation from that will cause significant impact on the measurements. In spite of these unknown factors, the transmit-array provides a good performance.

V. CONCLUSION
In this paper, a novel laser-drilled all-dielectric Huygens' transmit-array was proposed and experimentally demonstrated in the 120-GHz-band by operating in the near-field of a slot array antenna. The proposed structure is an array of dielectric shell disks connected with neighboring disks with dielectric bridges. The all-dielectric unit cell was demonstrated to excite Huygens' resonances hence achieving EM wave transformations with zero back-scattering. The proposed unit cell was compared with a metallic unit cell based on metallic patch resonators, and it exhibits comparable loss performance, with the added advantage of fabrication accuracy achieved using laser-drilling. A uniform array of the proposed unit was first fabricated to characterize its effect on the antenna gain and estimate the fabrication tolerance using the laser drilling process. It was found to cause a maximum drop in gain of about 2 dB. A phase-gradient transmit-array was then fabricated to successfully demonstrate beam-titling as an example of beam-forming in the 120-GHz-band. Table 3 finally shows a comparison between the proposed structure and other Huygens' metasurface (HMS) designs at microwave frequencies, including metallic PCB-based as well as all-dielectric metasurfaces. The majority of metasurface designs in literature consist of two or more metallic layers on a dielectric and are fabricated using the standard PCB process [19], [46], [59]- [62]. On the other hand, all-dielectric designs at microwave frequencies are typically fabricated using CNC-machining [47], [63], with the proposed design being the first of its kind that uses laser-drilling to achieve high-precision fabrication. The proposed laser drilled all-dielectric Huygens' transmit-array may thus represent an attractive alternate to a standard PCB based fabrication for passive antenna beam-forming above 100-GHz for beyond 5G communication networks.