Raman Shifting Induced by Cascaded Quadratic Nonlinearities for Terahertz Generation

Abstract

Cascaded quadratic optical nonlinearities are well known for producing effective third-order nonlinear optical effects such as self-phase modulation or self-steepening. As a result, they have been extensively applied in areas such as mode-locking and pulse compression. In this article, a regime of cascaded quadratic nonlinearities involving highly phase-matched second-order interactions is introduced, which produce an effective third-order nonlinearity analogous to Raman shifting rather than the typical case of self-phase modulation. This results in a continuous red-shift of the optical pump frequency rather than spectral broadening. This phenomenon is particularly relevant to terahertz generation, where a continuous red-shift of the pump frequency resolves current issues of dispersion and laser-induced damage. In the absence of absorption or dispersion, the presented method results in optical-to-terahertz energy conversion efficiencies that approach 100%, which is not possible with conventional cascaded difference-frequency generation. Designs of aperiodically poled lithium niobate structures are presented, which result in energy conversion efficiencies of 35% even in the presence of dispersion and absorption. The presented work thus addresses an important bottleneck in terahertz generation, which paves the way for the development of compact particle accelerators, X-ray free-electron lasers, advanced electron-beam diagnostics, and various experiments in condensed-matter physics and chemistry.