Generation of terahertz waves based on nonlinear frequency conversion with stimulated Raman adiabatic passage

In recent years, high-power, tunable terahertz (THZ) radiation sources have become the key areas of research in the world. The method of THZ waves by nonlinear optical difference frequency generation (DFG) has the advantages of wide tuning, high power, room temperature operation, and compact structure. However, the conversion efficiency of the current difference frequency method is low, which needs a trade-off between conversion efficiency and tuning range. We apply the nonlinear optical cascade difference frequency conversion theory based on stimulated Raman adiabatic passage (STIRAP) and propose a theoretical scheme to generate THZ waves. Numerical simulation investigates the cascaded difference frequency process of generating THZ waves with the help of the nonlinear medium lithium niobate (LN) crystal. The theoretical analysis shows that the maximum quantum conversion efficiency from signal laser to THZ waves is 43.2 % when the wavelength of the tuned signal laser varies between 1.044 - 1.065 µm with the fixed two pump laser wavelengths constant. The tunable THZ waves of 0.48 - 5.0 THz can be obtained and the maximum output intensity of THZ waves is 2.17 MW/cm², and the method is robust to temperature variations. It also provides a novel idea for the cascaded difference frequency generation of THZ waves.