Terahertz difference frequency generation in a longitudinally coupled LNOI/CPS waveguide: an analytical approach

To enable various important applications of terahertz (THz) technology, sources that are compact, low-cost, highly efficient, and tunable at room temperature are needed. THz-guided sources can provide an optimal platform for such features along with on-chip integrability. We analytically investigate the THz difference frequency generation in a longitudinally coupled lithium niobate on insulator (LNOI) and coplanar stripline (CPS) waveguides. The nonlinear interaction of two guided lasers inside the optical LNOI waveguide is modeled as a traveling-wave induced current density, generated due to the second-order nonlinear susceptibility of the lithium niobate. Such current density with THz difference frequency acts as an impressed source for terahertz generation that is coupled to the CPS as a guided wave. A semi-analytical approach, based on distributed-source transmission line theory, is applied to calculate terahertz signal power coupled to the CPS transmission line. In such an approach, the multilayer spectral domain method and complex-image technique are employed to compute the voltage induced along the CPS. We design a guided source structure that satisfies the modal phase-matching condition for efficient terahertz wave generation at 3 THz as a demonstration. The analytically obtained results are verified against full-wave simulations with excellent agreement. Our proposed model is general and can be used for fast design and optimization of a wide range of THz-guided sources based on the optical-to-terahertz conversion principle.