###### Abstract  
Advanced applications require transmit-array lenses featuring low profiles, covering 360\({}^{\circ}\) phase modulation, and, potentially, polarization conversion. Subwavelength metamaterial elements with multilayers manipulate electromagnetic waves with predesigned phase shifts, resulting in the transmit-array lens's low profile and light weight. Conventional designs separate the transmit-array lens and polarization converter with two functional devices and suffer from high profiles and complicated mechanical assembly. In order to resolve these issues, a subwavelength lens element combining the functions of phase modulation and polarization conversion is proposed. The metamaterial lens element consists of phase modulation, linear-circular polarization conversion modules, and a metallic via structure electrically connecting these two modules. The multifunctional lens element modulates the transmitted phase with a three-bit phase shift and operates in the frequency range from 24.5 to 27 GHz, with less than \(-10\) dB reflection coefficient, 1 dB transmission loss, and a 2.5 dB axial ratio. A transmit-array lens antenna with the proposed element is implemented to validate the abilities of beam focus and polarization conversion. The gain of the lens antenna reaches 26.3 dBi, with a 3 dB beamwidth of 6.7\({}^{\circ}\) and \(-18\) dB sidelobe level, and the axial ratio of the converted circular polarization is below 1 dB. The aperture efficiency of the antenna is 45.2%. transmit array; lens; phase modulation; polarization conversion 20221  
implemented with a high gain of 33.8 dBi, 1 dB bandwidth of 9.2%, and aperture efficiency of 51.2% [10]. The transmit-array antenna utilizes a half-wavelength unit-cell element and modulates the transmitted wave phase by rotating the element. In order to realize the polarization conversion function, a separate metamaterial polarization converter is commonly implemented. A fixed-beam transmit-array and metamaterial polarization converter are combined to realize the high gain and linear-to-circular polarization conversion. Several metamaterial polarization converters have been investigated for linear-to-circular polarization conversion [16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. A cross-polarization converter uses metalized vias to convert linearly polarized incident electromagnetic wave into its orthogonal-corresponding electromagnetic wave [16]. Its conversion efficiency is 98.6% at 8.79 GHz when the incident angle is normal. A dual-band dual-linear-to-circular polarization converter can convert x-polarization to left-handed circular polarization at a lower band and right-handed circular polarization at a higher band [17]. A polarization converter to convert a linearly polarized incident wave into an outgoing circularly polarized wave is proposed in [18], which is constructed by a two-dimensional array of thin cavities with etched slots. The polarization converter features an extremely low insertion loss of around 0.1 dB and a high polarization conversion efficiency of 0.97. A wideband polarization converter is designed in [19], where a multilayer frequency-selective surface structure is employed to achieve a broad operating frequency range. By rotating the proposed polarization converter, the incident wave can transform in four cases: left- and right-handed circular polarization waves and horizontal and vertical polarization waves. In this paper, we proposed a transmit-array lens element that can realize both phase modulation and polarization conversion functions at the same time. The lens element consists of phase modulation, linear-circular polarization conversion modules, and a metallic via structure connecting these two modules. When the lens unit works at 24.5-27 GHz, it can convert linear polarization wave into circular polarization wave with phase coverage of 360\({}^{\circ}\), reflection coefficient lower than \(-\)10 dB, transmission loss of 1 dB, and an axial ratio of 2.5 dB.