Optically pumped terahertz sources

High-power terahertz（THz） generation in the frequency range of 0.1-10 THz has been a fast-developing research area ever since the beginning of the THz boom two decades ago, enabling new technological breakthroughs in spectroscopy, communication, imaging,etc. By using optical（laser） pumping methods with near-or mid-infrared（IR） lasers, flexible and practical THz sources covering the whole THz range can be realized to overcome the shortage of electronic THz sources and now they are playing important roles in THz science and technology. This paper overviews various optically pumped THz sources, including femtosecond laser based ultrafast broadband THz generation, monochromatic widely tunable THz generation, single-mode on-chip THz source from photomixing, and the traditional powerful THz gas lasers. Full descriptions from basic principles to the latest progress are presented and their advantages and disadvantages are discussed as well. It is expected that this review gives a comprehensive reference to researchers in this area and additionally helps newcomers to quickly gain understanding of optically pumped THz sources.