Higher-order cascaded processes in continuous-wave optical parametric oscillators

Continuous-wave optical parametric oscillators (OPOs) combine single-frequency operation with a huge tuning range. Thus, they have emerged as working horses for spectroscopy in the near and mid infrared over the last decades (Dunn and Ebrahimzadeh, 1999). However, in the terahertz range prominent nonlinear materials like lithium niobate have strongly damped phonon resonances leading to huge absorption. Consequently, the generation of terahertz waves by optical parametric oscillation requires around 100 W of near-infrared pump light exceeding the power delivered by standard single-frequency lasers.