# Grating-like Terahertz Metasurface for Large-Deflection-Angle Beam Manipulations
## 1 Introduction  
The terahertz band refers to the electromagnetic wave between microwave and far-infrared waves, which is commonly defined from 0.1 to 10 THz [1]. Over the past few decades, the promising application prospects of terahertz technology in wireless communication [2], remote sensing [3], imaging [4, 5], biomedicine [6], and moving-target detection [7] have drawn increased research interest. As the essential components, terahertz electromagnetic devices such as beam-steering antennas [8], polarization converters [9], and high-Q resonators [10] have been widely reported. Among these, multifunctional beam manipulations, especially beam steering, have received much interest [11, 12]. Beam-steering technologies usually include mechanical scanning, phased array, frequency scanning, and multi-feed technologies [11]. In the terahertz band, frequency-scanning technology may have the advantages of high response speed, low power consumption, and a simple system compared with the other three mechanisms. Recently, a frequency-scanning antenna was designed for the W band for real-time imaging. The imaging system operating at 79 GHz was composed of a frequency-scanning array, a parabolic reflector, and a post-processing module, which proved its feasibility for low-cost imaging applications [13]. A frequency-sweep reflector antenna working at 235-330 GHz was proposed based on the direction-of-arrival estimation mechanism [14]. The diffraction enhancement method is applied to design the antenna structure to attain high gain and directivity, and the measured results show that the diffraction beam steers from \(-36.6^{\circ}\) to \(-11.03^{\circ}\) during the designed operating frequency. A two-dimensional frequency-scanning planar-integrated array based on the leaky wave principle and true-time-delay phase-shifting feeding network was designed and manufactured, and consequently, a scanning range of \(22.7^{\circ}\)\(\times\)\(60^{\circ}\) is achieved by sweeping the frequency between 325 and 400 GHz [15]. Moreover, metasurfaces, especially reconfigurable and programmable metasurfaces, have provided considerable freedom for manipulating terahertz beams [16; 17; 18; 19; 20; 21]. Phase-change materials and electrically tunable materials were extensively exploited for developing dynamically controlled metasurface elements. For instance, by varying the temperature, a frequency-tunable metasurface filter and ultra-wideband absorber based on the phase-change material vanadium dioxide were designed, respectively [22; 23]. A dual-band terahertz absorber has been designed using a graphene-based metamaterial resonator, and tunable absorption performances can be attained by changing the chemical potential of graphene [24]. Nematic liquid crystal (LC) with a birefringence effect has also drawn a lot of attention. The LC-based metal-dielectric-metal structure has been designed and fabricated to modulate the reflection phase of electromagnetic waves fully electronically in millimeter frequencies [25]. A transmissive terahertz amplitude modulator working at 421.2 GHz was implemented based on an LC-loaded plasmonic metamaterial, and a large modulation depth of up to 96% can be obtained by applying different bias voltages to the LC layer [26]. Tunable terahertz beam-steering was realized by using LC-coupled reconfigurable metasurfaces [27]. In terms of programmable metasurfaces, LC-based dynamically programmable reflective and transmissive metasurfaces were achieved at 672 GHz and 408 GHz, respectively [28; 29], and the programmable modulation of terahertz beams can be performed. Furthermore, a wide-angle beam scanning device using a 1-bit field programmable terahertz metasurface was developed with a working frequency covering 630-650 GHz and a scanning beam ranging from 20\({}^{\circ}\) to 60\({}^{\circ}\)[30]. However, terahertz frequency-scanning metasurfaces have not been widely investigated, especially for large-deflection-angle beam control.