Laser-free and low-jitter Electro-Optic Sampling of FEL-based THz radiation by intrinsic ultrafast laserlike-pulses at FLASH

The FLASH1 THz beamline at DESY produces intense multicycle THz undulator radiation, along with quasi-single cycle edge radiation. Characterizing the temporal profile of THz radiation generated by free-electron lasers (FELs) is challenging due to unavoidable timing jitter between the THz pulse and an external laser synchronized to the FEL master clock. Although Electro-Optic Sampling (EOS) is a powerful diagnostic technique for the FEL THz sources, this jitter limits the temporal resolution and spectral bandwidth, necessitating timing jitter correction. To address this, we introduce a laser-free and low-jitter diagnostic method, Afterburner EOS (ABEOS), which uses broadband laserlike-pulses generated with the THz radiation as a probe. ABEOS results demonstrate good consistency with calculations across two probe configurations: a sequence of 10 pulses and a single pulse. Furthermore, comparisons with conventional methods of scanning EOS with an external laser, single-shot EOS (SSEOS), and FTIR show strong and reliable agreement. This technique offers ‘laser-free and low-jitter’ THz waveform characterization as a simplified, accurate, and reliable alternative to conventional THz diagnostics, validated up to 2 THz with potential for application at higher frequencies.