In this work, we propose a high-efficiency terahertz (THz) wave-generation approach by cascaded difference frequency generation (CDFG) with an aperiodically poled lithium niobate (APPLN) crystal at cryogenic temperature. The APPLN crystal with desirable poling periods along the crystal length determines phase mismatches of each-order CDFG, resulting in a decrement of phase mismatches in cascaded Stokes processes and an increment of phase mismatches in cascaded anti-Stokes processes simultaneously. This is in contrast with previous works on CDFG, where the THz wave was generated with irreversible phase mismatches in cascaded Stokes and anti-Stokes processes. The variations of phase mismatches enhance the evolution of optical spectra in cascaded Stokes processes and restrain the evolution of optical spectra in cascaded anti-Stokes processes, yielding unprecedented energy efficiencies in excess of 30% from optical waves to THz waves with APPLN. The unprecedented energy efficiencies in this work are theoretical results without THz wave absorption at a temperature of 10 K. Compared with the maximum THz intensity from traditional CDFG using periodically poled lithium niobate, the maximum THz intensities are enhanced by 2.5 and 2.8 times using APPLN with stepwise changing and gradually changing poling periods, respectively.

Optically pumped nonlinear frequency down conversion is a proven approach for monochromatic terahertz (THz)-wave generation that provides superior properties such as continuous and wide tunability as well as laser-like linewidth and beam quality. Phase-matching (PM) is an important connection between the pump sources and nonlinear crystals and determines the direction of energy flow (as well as the output power). In past decades, a variety of peculiar PM configurations in the THz region have been invented and are different from the traditional ones in the optical region. We summarize the configurations that have been applied in nonlinear THz-wave generation, which mainly fall in two categories: scalar (collinear) PM and vector PM (including macroscopic noncollinear PM and microscopic vector PM). The development of this technique could relax the matching conditions in a wide range of nonlinear crystals and pump wavelengths and could finally promote the improvement of coherent THz sources.

Cation-embedding and oxygen-vacancy co-modified VO_x thin-sheet-microspheres assembled with nanoparticles were designed for zinc-ion batteries with a high rate capacity.