Advantages of pulse-train excitation in narrow-band terahertz generation: mitigation of undesired nonlinear effects

In this work, we have studied the limitations of narrowband multi-cycle (MC) terahertz (THz) generation via optical rectification (OR) in periodically poled lithium niobate (PPLN) crystals. Detailed investigation of the transmitted beam profile, THz conversion efficiency (CE), and parasitic second-harmonic generation (SHG) strength as a function of incident pump beam size showed that Kerr-lensing is a significant bottleneck in the efficiency scaling of MC THz generation. We have also demonstrated that compared to the usage of a single pump pulse, excitation of the PPLN crystal via a pulse train, not only boosts up THz CE and narrows down the bandwidth of the achieved THz beam but also helps to mitigate the effect of undesired nonlinearities, such as Kerr-lensing and parasitic SHG.