Recently, a research group led by Prof. LIU Hongjun from Xi'an Institute of Optics and Precision Mechanics of the Chinese Academy of Sciences, proposed a novel design of nonlinear metamaterial to enhance the light-matter interactions and overcome the phase-matching limitations of traditional nonlinear optical parametric processes. The study was published in Nanoscale Horizons. 

Metamaterials have demonstrated significant potential for detecting mid-infrared information by frequency up-conversion. Although they can avoid phase-matching in nonlinear processes, mechanisms of metamaterials rely on ultrahigh-Q resonances with extremely narrow spectra linewidth that restrict their operation bandwidth.

In this study, researchers proposed a hyperbolic metamaterial design which takes the advantages of electromagnetic multipole coupling supported by gap-plasma modes. The metamaterial was a triangular pyramid shape and it was composed of Au-ZnO multilayer gap.

In specific, the intrinsic modes from triangular metamaterial can induce gap plasma modes, which are of great significance in mode matching. Therefore, the strong field localization at the tip of the triangles can be generated by the symmetry-breaking structure, and eventually excite multiple resonances over a larger spectral range.

The ultrabroadband mid-infrared nonlinear frequency up-conversion can be achieved in 3–5 μm with a pump light at 916 nm. The sum frequency process with a maximum conversion efficiency of 2.5 × 10⁵ can be realized using a femtosecond pulsed laser at a pump intensity of 13 MW cm^-2.

"The proposed approach provides a promising platform for developing ultrabroadband nonlinear coupling of light and matter, and provides a new opportunity for metasurface nonlinear frequency conversion technology," said Prof. LIU Hongjun.