Single‑cycle terahertz (THz) pulse sources combining high average power and high repetition rate represent a cutting‑edge platform for advanced spectroscopy, imaging, and communication applications. Optical rectification in nonlinear crystals, pumped by near‑infrared ultrafast lasers, is one of the most promising routes to efficient THz generation and THz frequency combs synthesis. However, as the repetition rate increases, maintaining sufficient peak power becomes increasingly challenging, leading to a drop in conversion efficiency.

In this work, researchers from IFN-CNR Milano within the I-PHOQS Infrastructure, in collaboration with the University of Milan, Politecnico di Milano, Ruhr‑Universität Bochum, and Helmut‑Schmidt‑Universität, report the first demonstration of THz generation by optical rectification inside a passive enhancement cavity reaching intracavity powers above 1.9 kW. A commercial ytterbium‑doped fiber laser operating at 93 MHz is coupled to an enhancement cavity and it pumps a 50 µm‑thick lithium niobate crystal inside, producing single‑cycle THz pulses with about 2 THz bandwidth and average output powers on the order of the milliwatt. In the frequency domain, a THz frequency comb is generated. The comb teeth are separated by 93 MHz, and the power per mode is on the order of hundreds nW.

This novel, cavity‑enhanced approach overcomes the intrinsic limitations of traditional THz sources by enabling high conversion efficiency at large repetition rates, while relying on compact and scalable laser technology. Such a platform opens the door to next‑generation THz systems for high‑resolution spectroscopy, biomedical imaging, and advanced wireless communications.

These results are published in APL Photonics (April 16, 2025) as an Editor’s pick.