Quasi-Guided Modes Enhance Tunable Nonlinearity in Novel Developed Metasurface

In a study published in Laser Photonics Reviews, Prof. LIU Hongjun from the Xi’an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences and the collaborators developed an epsilon-near-zero (ENZ) nonlocal metasurface structure. This design achieves localized field enhancement with a high Q-factor across a broad range of wave vectors, while improving angular stability and spectral tunability of the nonlinear response.

With the continuous integration of nanophotonics and nonlinear optics, achieving high-efficiency, low-power and broadband nonlinear responses has become a hot research topic. However, the nonlinear enhancement effect of traditional quasi-bound states in the continuum (Q-BIC) structures is confined to specific wave vectors and wavelengths, with strong sensitivity to structural changes, severely limiting both practical utility and tunability.

Researchers designed a novel nonlocal metasurface architecture. By patterning periodic perturbations of silicon nanodisks on an ENZ thin film, they employed Brillouin zone folding to shift the guided mode spectrum above the light cone. This enabled the excitation of high-Q quasi-guided modes (Q-GMs) that strongly couple with ENZ modes, resulting in anticrossing splitting up to 260 meV. Such strong coupling significantly enhanced the localized field intensity, boosting the nonlinear response.

In the experiment, two interleaved silicon disk arrays were fabricated on a 23 nm thick indium tin oxide (ITO) film using electron beam lithography. Structural perturbations folded the first Brillouin zone, shifting guided modes into the radiative continuum and enabling high-Q, angle-insensitive Q-GMs. 

Z-scan measurements showed a nonlinear refractive index of 3.8 × 10^-13m²/W, about three orders higher than pure ITO, and stable across incident angles. In addition, a physical model combining a two-temperature approach with nonparabolic band calculations revealed a rapid ENZ response of around 450 fs, supporting ultrafast all-optical modulation.

"This metasurface supports dynamic wavefront control, tunable filtering, and nonlinear conversion, offering a new paradigm for ENZ-based nonlocal photonic device platforms," said Prof. LIU from XIOPM.

Fig. Design of the structure and linear response. (Image by Dr. SHI Wenjuan)