Waveform control of terahertz light via electron beam longitudinal phase-space manipulation

Synthesizing intense terahertz (THz) waveforms with user-defined electric field profiles represents a pivotal challenge for controlling matter at ultrafast timescales. While relativistic electron beams can serve as a versatile source of coherent radiation, conventional schemes lack the ability to deterministically sculpt the temporal structure of the emitted light. Here, we experimentally demonstrate a paradigm for generating tailored THz waveforms by manipulating the longitudinal phase space of the electron beam. By controlling the nonlinear longitudinal correlation of an energy-chirped electron beam, we generate a microbunch train with a chirped density modulation. When coupled to a tapered undulator, this structured beam produces coherent radiation, whose spectral-temporal properties are tunable via adjustment of the undulator taper. Autocorrelation measurements confirm the emission of chirped pulses spanning 8–13 THz, with spectral characteristics that align with simulations of the beam structure. Our approach, establishing a direct link between beam dynamics and light-field synthesis, opens a pathway toward customizable high-field waveforms for advanced coherent control experiments.