The development of nanophotonics in recent years has provided new opportunities to explore the interaction between free electrons and metamaterials. This advancement helps overcome limitations in THz vacuum electron devices, extending the frequency range, enhancing radiation intensity, controlling phase polarization, manipulating spatial distribution, and even developing entirely new forms of vacuum electron devices. Based on the control and enhancement of free electron radiation through metamaterials, we aim to break through the limitations of vacuum electron devices, creating efficient, tunable vortex waves, frequency combs, and mechanisms for enhancing radiation. This endeavor addresses key challenges in vacuum devices for the terahertz range.