▎ 摘　　要

Graphene has shown potential in terahertz (THz) polarization modulation due to highly tunable optoelectronic properties, fast photoelectric response, and ease of integration. However, the performance of polarization converters based on graphene metasurfaces is often limited by the achievable carrier mobility of large-area graphene. In this paper, a flexible and tunable broadband transmissive THz cross-polarization converter based on a hybrid metal-graphene metasurface is proposed. It is composed of two metal grating layers with a graphene-loaded 45-degree antenna array sandwiched between them. The THz response of the antenna can be tuned by adjusting the graphene Fermi level, which further alters the cross-polarization conversion efficiency (CPCE) of the device. The average CPCE can be continuously tuned from 80.3% to 4.5% within a broadband from 0.6 to 2.0 THz, and the average modulation depth of the whole band is 94.2%. The mechanisms of this highly efficient polarization conversion and dynamic modulation are explained with a transfer matrix method and an equivalent circuit model. Furthermore, the proposed structure has a low requirement on the graphene mobility, which is only 500 cm(2)/(V center dot s) here. This work provides a new approach to highly efficient tunable cross-polarization conversion over a broadband in THz, which will promote the application of graphene-based polarization modulators in THz sensing, imaging and communication.