Metasurfaces offer a unique playground to tailor the electromagnetic field at subwavelength scale to control polarization, wavefront, and nonlinear processes. Tunability of the optical response of these structures is challenging due to the nanoscale size of their constitutive elements. A long-sought solution to achieve tunability at the nanoscale is all-optical modulation by exploiting the ultrafast nonlinear response of materials. However, the nonlinear response of materials is inherently very weak, and, therefore, requires optical excitations with large values of fluence. We show that by properly tuning the equilibrium optical response of a nonlocal metasurface, it is possible to achieve sizable variation of the photoinduced out-of-equilibrium optical response on the picosecond timescale employing fluences smaller than 250 μJ / cm², which is 1 order of magnitude lower than previous studies with comparable reflectivity variations in silicon platforms. Our results pave the way to fast devices with large modulation amplitude.

An active ultrafast formation and modulation of dual-band plasmon-induced transparency (PIT) effect is theoretically and experimentally studied in a novel metaphotonic device operating in the terahertz regime, for the first time, to the best of our knowledge. Specifically, we designed and fabricated a triatomic metamaterial hybridized with silicon islands following a newly proposed modulating mechanism. In this mechanism, a localized surface plasmon resonance is induced by the broken symmetry of a C2 structure, acting as the quasi-dark mode. Excited by exterior laser pumps, the photo-induced carriers in silicon promote the quasi-dark mode, which shields the near-field coupling between the dark mode and bright mode supported by the triatomic metamaterial, leading to the dynamical modulation of terahertz waves from individual-band into dual-band PIT effects, with a decay constant of 493 ps. Moreover, a remarkable slow light effect occurs in the modulating process, accompanied by the dual-transparent windows. The dynamical switching technique of the dual-band PIT effect introduced in this work highlights the potential usefulness of this metaphotonic device in optical information processing and communication, including multi-frequency filtering, tunable sensors, and optical storage.

超构表面是指由亚波长结构构成的纳米光学天线阵列。在合适的激发条件下，纳米天线可以产生共振，实现近场增强，进而增强非线性光学效应。相较于传统非线性光学晶体，超构表面集成度较高，有利于实现小型化的高效非线性光源。由于光只传播亚波长的距离，针对非线性谐波产生等应用，超构表面具有无需考虑相位匹配的优势。此外，超构表面具有亚波长的空间分辨率，通过对结构单元的设计和排列，可以实现对非线性谐波的相位、偏振和振幅的灵活调控。该综述针对超构表面在光学频率转换、非线性波前调控以及超快全光调控等领域的国内外近期工作进行了总结，并对非线性超构表面在走向实际应用中面临的挑战和进一步的发展方向进行了展望。