Stimulated polariton scattering in an Yb laser pumped lithium niobate based continuous-wave singly resonant optical parametric oscillator

Abstract

The effective refractive index of a dielectric medium modifies in the infrared spectral band due to a significant contribution from the vibrational modes by virtue of the presence of heavy ions. The coupling between electromagnetic (EM) modes and transverse optical (TO) vibrational modes is the underlying principle behind such a modification. This interaction manifests through the generation of polariton modes that can be observed in the Raman spectrum. However, they do not coincide with the peaks associated with the Raman scattering process. In the present work, we show that the polariton modes can be excited in a lithium niobate based continuous-wave singly resonant optical parametric oscillator (SRO) pumped by a single-frequency, near-infrared Yb-doped fiber laser. The stimulated polariton scattering process is triggered by a strong interaction between the mid-infrared resonant SRO signal beam and TO phonon modes of a lithium niobate crystal, which leads to generation of Stokes mode(s). Such mode(s) can be observed as side peak(s) in the SRO signal spectrum. The generated low energy polariton mode exhibits a dominant EM character, and this provides a plausible route to generate narrow linewidth far-infrared (FIR) or terahertz radiation by utilizing the high intra-cavity signal power in the SRO. In addition, the off-axis FIR generation process assists in reducing the thermal load on the lithium niobate crystal at high pump powers.