Abstract：Terahertz waves are electromagnetic waves with frequencies between 0.1 and 10 THz, between microwave and infrared, with strong penetration, high resolution, no ionizing radiation, and other advantages, which have a wide range of application prospects in the fields of safety detection, medical diagnosis, material analysis and so on. The demand for Terahertz technology development and application of terahertz wave multifunctional modulation devices is growing. The traditional medium in the terahertz band response is weak; the metasurface, because of its ability to modulate the terahertz wave at the wavelength scale and in the processing and design of low-cost, small volume and other advantages by the researchers are widely concerned. Terahertz functional devices with metasurface have been reported to have more or less single functions and are limited by a single polarisation wave. Researchers propose that the program is mainly to change the geometric morphology and arrangement of the sub-wavelength metasurface structure to introduce the transmission phase or the geometric phase through the rotating unit structure to achieve the electromagnetic wave parameter tuning. However, a single tuning method still exists due to poor tunability and so on, and therefore, the design of the metasurface electric field tuning device based on the synergistic tuning of the transmission and geometric phase theory seems more prospective and meaningful. In this paper, a new diagonal double-cross structure metasurface is introduced, which consists of a top metal pattern, an intermediate dielectric layer, and a bottom metal plate and combines the synergistic effect of the transmission phase and geometrical phase to complete the independent tuning of the left circularly polarised wave and the right circularly polarised wave. At the incident of left/right circularly polarised waves with a frequency of 1.1 THz, the metasurface exhibits a variety of functions, including vortex beams, beam splitting, vortex wave splitting, and focusing beams with different topological charges. This innovative structural design provides a new idea for researching multi-functional and multi-polarisation terahertz modulation devices. It offers potential application scenarios in the field of terahertz wireless communication. Future research can extend it to the microwave and optical fields by changing the metasurface dimensions.
