Cascaded second-order processes for the efficient generation of narrowband terahertz radiation

The generation of high-energy narrowband terahertz radiation has gained heightened importance in recent years due to its potentially transformative impact on spectroscopy, high-resolution radar and more recently electron acceleration. Among various applications, such terahertz radiation is particularly important for table-top free electron lasers, which are at the moment a subject of extensive research. Second-order nonlinear optical methods are among the most promising techniques to achieve the required coherent radiation with energy > 10 mJ, peakfield > 100 MV m$^{−1}$, and frequency between 0.1 and 1 THz. However, they are conventionally thought to suffer from low efficiencies < ∼10$^{−3}$, due to the high ratio between optical andterahertz photon energies, in what is known as the Manley-Rowe limitation. In this paper, we review the current second-order nonlinear optical methods for the generation of narrowbandterahertz radiation. We explain how to employ spectral cascading to increase the efficiency beyond the Manley-Rowe limit and describe the first experimental results in the direction of aterahertz-cascaded optical parametric amplifier, a novel technique which promises to fully exploit spectral cascading to generate narrowband terahertz radiation with few percent optical-toterahertz conversion efficiency