The field of nonlinear optics has grown substantially in past decades, leading to tremendous progress in fundamental research and revolutionized applications. Generally, the optical nonlinearity for a light wave at frequencies beyond near-infrared can be only observed with very high peak intensity, as in most materials only the electronic nonlinearity dominates while ionic contribution is negligible. However, some researches have shown that the ionic contribution to nonlinearity can be much larger than the electronic one in microwave experiments. For terahertz (THz) waves, the excited stimulated phonon polaritons may assist to substantially trigger the ionic nonlinearity of the crystals by strong coupling, but how the phonon polariton contributes and behaves for the THz frequency was remains unknown.

This problem is clearly studied by Qiang Wu and Jingjun Xu’s group in TEDAIAP, where they experimentally demonstrated a giant second-order optical nonlinearity at THz frequency, orders of magnitude higher than that in the visible and microwave regimes. They also proposed a nonlinear modification to the Huang equations to explain the observed nonlinearity enhancement. This work brings about an effective approach to achieve high nonlinearity in ionic crystals, promising for applications in THz nonlinear technologies.

This study is online published as a research paper entitled “Giant enhancement of THz-frequency optical nonlinearity by phonon polariton in ionic crystals” in Nature Communication in June 26, 2021.

Article Info:

Yao Lu, Qi Zhang, Qiang Wu*, Zhigang Chen*, Xueming Liu, and Jingjun Xu*, Giant enhancement of THz-frequency optical nonlinearity by phonon polariton in ionic crystals, May 2021, Nature Communications 12(1):3183, DOI:10.1038/s41467-021-23526-w