# Metalens Antenna Based on an Ultrathin Dual-Polarized Double-Layer Huygens' Metasurface
## 1 Introduction  
Metasurfaces, as two-dimensional metamaterials, have been extensively studied since they were first proposed in 2011 [1]. Metasurfaces are typically composed of subwavelength scattering elements made of metals or dielectrics with specific shapes and arrangements, which offer the advantages of thin thickness, easy fabrication, and low production cost [2, 3]. By locally adjusting the responses of each meta-atom, metasurfaces have the remarkable abilities to control the electromagnetic amplitude, phase, and polarization, which have been applied in widespread fields such as optics, communications, and sensing [4, 5, 6, 7, 8, 9, 10, 11, 12]. The Huygens' metasurface is an artificial electromagnetic surface designed based on the Huygens' principle [13], exhibiting the powerful ability to control incident waves accurately. The meta-atom of a Huygens' metasurface consists of electric and magnetic resonators, which can simultaneously provide electric and magnetic current responses to full control of the electromagnetic fields. When the induced magnetic current and intrinsic electric current are balanced, the Huygens' resonance is excited. Utilizing this effect, nearly unity transmission amplitude and full 360\({}^{\circ}\) phase coverage can be achieved. Therefore, Huygens' metasurfaces exhibit great application value and broad prospects in fields such as microwave communication, wireless power transfer, radar systems, and antenna design [14, 15, 16, 17, 18, 19, 20]. Huygens' metasurfaces also play a crucial role in metalens antennas. Traditional transmitarray (TA) employing multilayer frequency-selective surface (FSS) enables fullphase control, breaking the constraints of conventional optics laws [21,22]. However, such a TA generally requires a multilayer structure to achieve phase control, resulting in incompact structure and difficult fabrication. Double-layer Huygens' metasurfaces offer a compact and cost-effective alternative. Various Huygens' metasurfaces with double-layer structure have been developed for metallic lens antennas and anomalous refraction applications, including anti-symmetric metallic elements [23], symmetric dual-polarized metallic elements [24,25], and electric dipole pairs [26]. Several double-layer Huygens' metalens antennas with distinct characteristics have been reported in recent years. For instance, a dual-polarized Huygens' metasurface has been proposed to achieve high-gain transmission [23,27,28], and [PERSON] et al. reported a highly transmissive Huygens' metasurface with a thickness of only 0.033 times of wavelength [29]. Moreover, a two-functional metasurface has been demonstrated to independently manipulate differently polarized beams [30,31]. These designs provide advantages in terms of phase control, operational bandwidth, and functional integration, offering improved solutions for applications such as waveform shaping, focusing, and asymmetric reflection [32,33,34]. In this work, we propose a new ultrathin double-layer anisotropic meta-atom based on the principle of Huygens' resonance. The meta-atom features an ultrathin thickness while maintaining high transmission. Two types of metalens based on the dual-polarization and two-channel characteristics of the meta-atom are designed. Our work provides a more flexible solution for the design of transmissive devices, allowing different types of metasurfaces to be realized using the same meta-atom. Moreover, the proposed Huygens' metasurface features a double-layer, ultrathin structure that not only significantly reduces fabrication complexity and cost but also enhances integration potential, thereby facilitating device miniaturization and lightweight development. Simulating results demonstrate that this meta-atom holds strong potential and feasibility for the design of various electromagnetic devices. This work is organized as follows: Section 2 introduces the design principles and structure of the meta-atom, and demonstrates the Huygens' resonance phenomenon.