Raman Shifting induced by Cascaded Quadratic Nonlinearities for Terahertz Generation

We introduce a new regime of cascaded quadratic nonlinearities which result in a continuous red shift of the optical pump, analogous to a Raman shifting process rather than self-phase modulation. This is particularly relevant to terahertz generation, where a continuous red shift of the pump can resolve current issues such as dispersion management and laser-induced damage. We show that in the absence of absorption or dispersion, the presented Raman shifting method will result in optical-to-terahertz energy conversion efficiencies that approach $100\%$ which is not possible with conventional cascaded difference-frequency generation. Furthermore, we present designs of aperiodically poled structures which result in energy conversion efficiencies of $\approx 35\%$ even in the presence of dispersion and absorption.