Multi-cycle terahertz generation in lithium niobate wafer stacks via mid-infrared pumping

Near-infrared laser-pumped optical rectification (OR) using quasi-phase matching (QPM) in lithium niobate (LN) is widely employed to generate multi-cycle terahertz (THz) pulses, which, however, suffer from low efficiency. Here, we demonstrate that mid-infrared pumping is an effective approach to increase the efficiency of multi-cycle THz generation. By using a 2.3-µm laser to pump a QPM macro-crystal composed of ten x-cut lithium niobate wafers, with their ferroelectric Z axis alternately rotated by π, a laser-to-THz conversion efficiency up to ∼0.4% has been achieved at room temperature, more than twice the efficiencies attained with near-infrared pumping. Electro-optic sampling reveals the generation of five-cycle THz pulses at 0.15 THz for 350-µm-thick wafers and 0.22 THz for 250-µm-thick wafers. Such mid-infrared laser-pumped OR in QPM wafer stacks provides an efficient, controllable, and scalable method for generating intense multi-cycle THz pulses suitable for diverse narrow-bandwidth applications.