Two-dimensional terahertz beam manipulations based on liquid-crystal-assisted programmable metasurface

Terahertz communications have attracted more and more attention and research interest owing to the significant advantages, such as high speed, low delay, and ultra-wideband.^1–3 Moreover, terahertz wireless communication is considered to be one of the key candidate technologies for 6G.⁴ However, with the increase in frequency, the propagation of electromagnetic waves in space faces challenges of serious path loss, such as scattering and reflection effects of obstacles and remarkable atmospheric absorption, resulting in low communication coverage and energy utilization efficiency. Recent studies have proved that reconfigurable intelligent surfaces (RIS) or intelligent reflection surfaces (IRS) can significantly improve signal coverage, strengthen signal quality, and enhance spectral efficiency, through reconstructing the electromagnetic environments based on dynamic beamforming technology.^5,6 For instance, by applying distributed RIS deployment scheme, the communication interruption problem caused by mobile humans can be effectively overcome in terahertz indoor communication scenario.⁷ A hybrid beamforming scheme based on multi-hop RIS is also proposed to improve the signal coverage range in the terahertz band.⁸ 

RIS was proposed based on the fundamentals of the metasurface. As an artificially designed two-dimensional (2D) electromagnetic material, metasurface can control the amplitude, phase, polarization, and orbital angular momentum of electromagnetic waves, realizing RCS reduction,⁹ abnormal beam deflection,¹⁰ polarization conversion,^11,12 vortex beam generation,^13,14 imaging,¹⁵ and other functions, showing the powerful ability to manipulate electromagnetic waves. By loading active devices, programmable or reconfigurable metasurfaces have been designed further to achieve dynamic regulation of electromagnetic waves.^16–18 By comparing with conventional beamforming techniques, such as mechanical scanning and phased array, metasurface-based schemes possess a higher response rate and are free of complex feed networks. However, in the terahertz band, active semiconductor devices, such as the PIN diode and varactor, are hardly applicable due to the size and working frequency limitations. Recently, tunable materials, such as liquid crystal (LC),^19–21 micro-electro-mechanical systems (MEMS),^22–24 ferroelectric thin films,^25,26 graphene,^27–29 phase change materials, such as germanium-antimony-telluride (GST)³⁰ and VO₂,^31–34 have been extensively reported for dynamic terahertz wave modulations.

As a birefringent material, the orientation of LC molecules can be altered under the control of an external electric field, resulting in electrically tunable effective dielectric constant, which can be exploited to design terahertz tunable and reconfigurable devices.^35,36 Programmable metasurfaces based on LC have also been widely investigated at the terahertz band. A reflective 1-bit programmable metasurface with a maximum beam deflection angle of 32° is achieved under oblique incidence conditions.³⁷ Furthermore, a reflective programmable metasurface covering the beam scanning range of 20°–60° is realized under normal incidence condition.³⁸ The transmissive programmable metasurface is also demonstrated to achieve the terahertz beam manipulation.³⁹ It should be noted that the aforementioned programmable metasurfaces are independently electrically controlled for each column, which can only realize beam scanning in one plane. In contrast, 2D programmable metasurface can realize terahertz beam manipulation in arbitrary directions. Nevertheless, the independent control of each metasurface element will lead to complex drive circuits, especially for large-scale metasurface arrays.

In this work, two LC-based reflective-type 1-bit 2D programmable metasurfaces that are operating around 94 and 220 GHz, respectively, are designed and manufactured by combining the lithographic processing and printed circuit board (PCB) manufacturing technology, enabling beam orientation adjustment in half space. We remark that it is a broadly applicable solution toward reflective-type 2D beam programming in the terahertz band, free of complex drive circuits, and also exhibits low cost and high reliability compared with on-chip technologies. Our work further enhances the ability of programmable metasurface to tailor terahertz waves, which has important application prospects in terahertz wireless communication, imaging, and other fields.

The sketch of the proposed programmable metasurface is shown in Fig. 1. As can be found, the metasurface element is composed of seven layers, from top to bottom, respectively: quartz, meta-atom, polyimide (PI), LC, PI, feed element, and FR4 substrate. The thickness of quartz is 200 μm, and the bottom surface is manufactured with meta-atom patterns (complementary split rings, made of 300 nm thick gold) using the photoetching technology. 100 nm-thick PI layers are used to achieve the pre-orientation of LC molecules without an external electric field. The feed element is a gold-coated copper patch with a thickness of 18 μm, fabricated on an FR4 substrate of 2 mm thickness. A multi-layer PCB process is applied to fabricate the feed array, and different depths of metalized vias are fabricated to connect the corresponding feed elements of different regions. Next, metal wires of width about 120 μm are proceeded to connect each via to the external pad. Then, LC with a thickness of 60 μm is filled and packaged between the quartz and FR4 substrate. It is worth noting that the designed feed elements have the dual functions of reflecting terahertz waves and loading bias voltage at the same time. While the whole meta-atom layer will also behave as a reference ground, which can provide a uniform biased electric field between the meta-atom and feed element owing to the large ratio of metal area. Thus, a metasurface that can realize independent control of each element is obtained. In this work, the designed metasurface array consists of 32 × 28 elements in total, in which every 2 × 2 elements are controlled independently as a super cell so that there are 16 × 14 controllable metasurface super cells. Since the period length of the elements working at 94 and 220 GHz is designed as 900 and 400 μm, respectively, and then the corresponding sizes of the designed feed elements are 1680 and 680 μm (120 μm gap for isolation between different patches). In order to avoid accelerated aging and potential device damage caused by charge accumulation in the liquid crystal layer under the condition of DC bias, a square wave of 1 kHz and ±10.5 V is generated by DAC (TI DAC60096 EVM) in an external trigger mode as the control voltage, and consequently, the designed metasurface can attain real-time control of far-field radiation patterns by inputting different coding patterns.

Figures 2(a) and 2(b) present the designed metasurface element and the corresponding simulation results of amplitude and phase under different dielectric constants of LC. As shown in Fig. 2(a), the element structure is obtained by etching a split ring on a square metal sheet, which enables the realization of a desired phase shift and a high reflectance. Referring to the previous studies, the LC dielectric constants used in this work are selected to be 2.55 and 3.65 for two different states, respectively, with a dielectric loss of 0.02.⁴⁰ Under the conditions of the dielectric constant of 2.55 (0 V) and 3.65 (±10.5 V square wave), the amplitudes of the developed metasurface element are approximately equal to 0.93, while the phase difference is close to 180° around 94 GHz [see Fig. 2(b)]. The phase responses under the two states are defined as 0 state and 1 state, respectively; thus, 1-bit programmable metasurface element is obtained.

Figures 3(a) and 3(b) show the fabricated metasurface working near 94 GHz and the corresponding measurement environment. The metasurface is placed vertically, and absorbing material is applied around the metasurface. In this scheme, the receiving antenna and the metasurface are orthogonally placed on a rotatable table and remain relatively static, with a distance of about 10 times the operating wavelength, to satisfy the far-field measurement condition. The linearly polarized wave generated by a transmitter is incident to the metasurface with an angle covering −90°–90°. To verify the 2D beam scanning capability, measurements in column coding mode are performed in the xoz and yoz planes, respectively. It should be noted that in the xoz plane, there are 14 columns that can be coded, while in the yoz plane, there are 16 columns, as shown in Fig. 3(c). A similar measurement scheme is applied to verify the 2D beam scanning effect around 220 GHz using the THz time-domain spectroscopy (TDS) system, as shown in Figs. 3(d) and 3(e). Two sets of THz-TDS experimental instruments are utilized to measure the far-field beam distribution of the metasurface in the xoz plane and yoz plane, respectively. During the measurement, the transmitter and the metasurface remain stationary, while the receiver rotates in the horizontal plane. Due to the shielding of the transmitter, the beam scanning angles are tested from 20° in the yoz plane and 22.5° in the xoz plane, respectively. To keep the beam steering in the horizontal plane, as the column coding direction switching from y axis to x axis, the metasurface, transmitter, and receiver rotate for 90° simultaneously along the z axis, while the relative orientations remain unchanged.

Figures 4(a)–4(c) and 4(d) present, respectively, the yoz-plane and xoz-plane measured far-field scattering patterns of the metasurface controlled by different coding sequences. As can be found, the designed metasurface working near 94 GHz has a range of beam scanning angles covering roughly 20°–60° in both the yoz and xoz planes (in smaller angles, there is the blocking of the receiver) through changing the coding sequences in real-time. Hence, the designed metasurface has a wide-angle and high-performance beam manipulation capability. Furthermore, as shown in Figs. 4(a)–4(c), with the working frequency increasing from 97 to 104 GHz, the beam scanning performance is well maintained, which indicates that the metasurface possesses broadband characteristics. We also observe that, in the yoz plane, the reflected beam is basically symmetric at 97, 100, and 104 GHz. However, in the xoz plane, the measured beams intensity exhibits asymmetry to some extent, especially when the reflection angle is about ±20°, possibly due to the fact that in the xoz plane, the receiver and coaxial lines have a more significant blocking effect on the right-side beams [see Fig. 3(b)]. In addition, the measured beams become wider, and the intensity decreases compared to the measurement results in the yoz plane, which can be attributed to less coding length. By comparing with the simulation value, the experimental working frequency of the metasurface appears blue shift, which may be caused by the error of lithographic pattern and quartz thickness. Overall, the measured results prove that the developed metasurface possesses relatively wideband and wide-angle 2D beam manipulation capability.

Next, a 2D programmable metasurface is designed and fabricated near 220 GHz to verify the broadband applicability of the proposed method in this work. Figure 5(a) shows the structure of the metasurface element, whose period length is reduced to 400 μm, while the thickness of the LC layer is still 60 μm. For the incident x-polarized plane wave, the designed metasurface element satisfies the 1-bit phase coding condition as well around 220 GHz, as shown in Fig. 5(b). Figures 5(c) and 5(d) show the simulated and measured far-field results of the metasurface in the yoz plane and xoz plane, respectively. In the yoz plane, the beam distribution measured under different coding sequences are in good agreement with the simulation results, and the beam scanning angles approximately cover 25°–60°. The effect of beam steering is also demonstrated in the xoz plane. Nevertheless, some angular deviation can be observed and all the measured angles are smaller than the corresponding simulated results, which can probably be ascribed to the reading error introduced by the rotary table during the measurement. In general, the experimental results verify the 2D programmable ability of this method again.

By comparing with some previous research works, our work combines lithography processing and PCB manufacturing to construct a wideband and wide-angle terahertz 2D programmable metasurface, which exhibits powerful capacity in terahertz wave manipulation. The proposed metasurface construction method shows a wide applicable frequency band, low complexity, low cost, and high reliability. Simulation and experimental results show that the metasurface developed by this method has superior performance in terahertz beam steering, which implies the important application potentials in RIS for reconstructing the propagation path of terahertz waves and improving the quality of terahertz wireless communications. Vortex beam generation is also a spotlight recently. Based on 2D programmable metasurface, dynamic generation of vortex beam can be realized by switching the coding patterns in real time, which will further improve the communication capacity.^41,42

For the proposed 1-bit 2D beam tailoring metasurface, the dielectric loss can be reduced by decreasing the thickness of the LC layer. Moreover, symmetric beams are usually generated by a 1-bit metasurface, and, hence, some related studies on quasi-2-bit or multi-bit programmable metasurfaces were performed to further improve the efficiency.⁴³ Considering the 60 μm-thick LC layer, the programmable metasurface may encounter the limitation of low response speed. In addition to reducing the thickness of LC layer, the use of ferroelectric nematic LC or blue-phase LC is also possible to improve the response speed of the device.^44,45 Furthermore, if it is feasible to combine the above LCs with metasurface, the terahertz space-time programmable metasurface is expected to be realized, which will have wider application prospects in terahertz wireless communications and simplified-architecture transmitters.^46–48 

In summary, an approach for constructing terahertz reflective programmable metasurfaces that can achieve independent regulation of each metasurface element is proposed. Based on this construction method, two 1-bit LC-integrated metasurfaces are designed and measured near 94 and 220 GHz, respectively, to validate the 2D beam scanning capability. The beam steering range can cover 20°–60° and 25°–60° for the two metasurfaces, respectively, which confirms the feasibility and universality of the method. In addition, the applicable frequency regime of this method can be further extended to lower or higher frequencies. Our work provides greater freedom to manipulate terahertz waves and has broad application prospects for terahertz wireless communications, holographic imaging, and vortex beam generation.

This work was supported by the National Key Research and Development Program of China (Nos. 2018YFB1801505, 2017YFA0700201, 2017YFA0700202, and 2017YFA0700203), National Natural Science Foundation of China (No. 62288101), 111 Project (No. 111-2-05), and the Fundamental Research Funds for the Central Universities (No. 2242023K5002).