9. Summary of THz radiation sources

THz sources have come a long way since the 2000s, both in terms of performance and ease of use. In continuous operation and at 1 THz, the milliwatt is available with many compact sources, and larger devices such as molecular lasers reach the hundred mW mark. Major trends are emerging. Below 500 GHz, electronic sources are very powerful, delivering tens of mW. For the time being, they can deliver signals at frequencies above 1 THz, but at very low power levels (μW). For frequencies above 2 ~ 2.5 THz, QCL lasers are dominant, but their power drops dramatically below 2 THz. We can imagine that, in the near future: on the one hand, electronic sources will exceed the THz threshold, delivering intense signals; on the other, new concepts based on QCL lasers (intra-cavity frequency doubling, QCL MIR-pumped molecular lasers) will undoubtedly enable the latter to emit beams at and below 1 THz, while...