Tunable few-cycle coherent terahertz radiation with watt-level power from relativistic femtosecond electron beam

We experimentally demonstrated the generation of watt-level, few-cycle coherent terahertz radiation with tunable frequency from 0.3 THz to 0.8 THz. The radiation is produced by an ultra-short relativistic electron bunch when passing through a magnetic undulator. The electron bunch is emitted from a thermionic gun and compressed to approximately 250 fs by an alpha magnet. With an average power up to 1.2 W, the easy-to-implement scheme enables various new applications.