We propose a novel scheme for terahertz (THz) wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves (HSWs). The repeated frequency conversions are accomplished by oscillations of Stoke waves in resonant cavity (RC) where low-order Stokes waves (LSWs) are converted to high-order Stokes waves again and again. The continuous frequency conversions are accomplished by optimized cascaded difference frequency generation (OCDFG) where the poling periods of optical crystal are aperiodic leading to the frequency conversions from low-order Stokes waves to high-order Stokes waves uninterruptedly and unidirectionally. Combined with the repeated and continuous frequency conversions, optical-to-terahertz energy conversion efficiency (OTECE) exceeds 26% at 300 K and 43% at 100 K with pump intensities of 300 MW/cm2.

We propose a nonlinear optical frequency conversion method based on cascaded difference frequency generation. Feasible designs are presented that enable the cascaded optical waves. They are initially generated by cascaded difference frequency generation with two pump waves, then transferred to high-order Stokes waves by optimized cascaded difference frequency generation, and finally congregated at a few spectral lines. We demonstrate wide frequency tuning characteristics and high conversion efficiency characteristics of the nonlinear frequency convertor by providing a theoretical framework. The frequency tuning from 291 THz to 175 THz and energy conversion efficiency exceeding 30% are achieved by numerical calculations. The nonlinear frequency convertor exhibits physics distinctly different from conventional three-wave mixing parametric amplification or difference frequency generation. We argue that the nonlinear frequency convertor based on cascaded difference frequency generation is promising for achieving efficient and robust frequency conversion.

In this work, we propose a novel dual optical frequency comb (DOFC) generation scheme based on dual cascaded difference frequency generation (DCDFG). Feasible designs are introduced that enable the two sets of cascaded optical waves, initially generated by DCDFG in an aperiodically periodically poled lithium niobate (APPLN) crystal with a pump wave and two signal waves, then transferred to high-order Stokes waves by oscillations of cascaded Stokes waves and the optimization of phase mismatching of each-order DCDFG; finally, a DOFC was constructed. We demonstrate a high-performance DOFC with characteristics of high repetition frequency difference, tunable repetition frequency difference, high flatness, and a tunable spectral distribution range by providing a theoretical framework. We argue that the scheme proposed in this work is promising for achieving a high-quality DOFC.