# Chiral Dielectric Metasurfaces for Highly Integrated, Broadband Circularly Polarized Antenna
## 1 Introduction  
The quest for antennas that are cost-effective, compact, and efficient for a broad bandwidth with a specific radiation pattern relies on the ability to manage millimeter-wave (mm-wave) electromagnetic field interactions. To transfer mm-wave antennas from research laboratories to the market of mobile infrastructures (for 5G communication) and satellite communication, it is crucial to address polarization mismatches and suppress multipath interferences. This may in principle be reached with circularly polarized (CP) antennas working at different ranges in the GHz regime [1; 2; 3; 4; 5; 6; 7], but two major limitations must be addressed. The first is to reach high-quality CP radiation with high gain within a broad bandwidth (BW). The quality of a CP electromagnetic wave is defined in terms of its axial-ratio (AR), i.e., the ratio between the major and minor axes of the polarization ellipse. The closer AR is to unity (0 dB) the higher the CP quality, and in practice electromagnetic fields have been considered to be CP for AR below 3 dB [1; 2; 3; 4; 5; 6; 7]. As for the second drawback, one has to reduce the size and Ohmic losses (inherent metallic inclusions) of mm-wave antennas to produce them in high-throughput, integrable platforms. Dielectric resonator antennas (DRAs) made with high-permittivity (\(\epsilon\sim 10\cdots 10^{2}\)) materials (e.g., ceramics) can concentrate and radiate CP electromagnetic fields with small losses [8]. However, the integration of these CP-DRAs into modern wireless systems is hampered by the need for multiple resonators and a multi-feed mechanism to achieve wide bandwidths [9; 10; 11; 12; 13]. Another possibility to address these limitations is to employ chiral systems in the design of CP antennas [14; 15]. Chirality refers to the handedness of an object which cannot be superimposed with its mirror image as in left- (LCP) and right-handed circularly polarized(RCP) light. The interaction of chiral electromagnetic fields with a chiral scatterer depends on this handedness. This may be used to simultaneously improve the handedness of an electromagnetic source while enhancing its electromagnetic field intensity, which can be reached through the proper engineering of suitable chiral structures [16]. In fact, chirality concepts have been used to control electromagnetic wave propagation in the microwave and optical domains [17; 18; 19], which serves as inspiration for CP antenna design.  
In this communication, we demonstrate theoretically a new concept to produce integrated, highly efficient mm-wave CP antennas in which we employ metasurfaces [20; 21]. These types of surfaces were proven useful for several applications, including gain enhancement [22] and phase rectifying [23]. In contrast to previous antenna designs using bulky metallic-based chiral metasurfaces [14; 15], we exploit the pairing of electric and magnetic resonances of high-permittivity (\(\epsilon=16\)) cylindrical resonators with slanted slots at the center. An analogous concept in nanophotonics yielded an enhanced broadband near- and far-field chiroptical activity [18]. To reach optimized radiation patterns with high gain and wide AR-BW, we used a 2 \(\times\) 2 arrangement of slotted cylinders. Through numerical calculations we show that this concept can be readily applied in antenna arrays by using a metal cavity surrounding each unit cell. The results for the system with metal cavity show promising improvements in gain and AR amplitudes and bandwidths. For comparison purposes, simulations were made using two commercial software packages for antenna design, namely CST Studio and ANSYS HFSS, and the results were in excellent agreement. CST Studio uses a time domain (transient) solver based on the finite integration technique (FIT) with a hexahedral mesh, while HFSS employs the finite element method (FEM) with tetrahedral mesh elements. As we shall demonstrate, antennas made with a chiral dielectric metasurface (CDM) are promising for broadband and efficient CP mm-wave radiation.