Boosting Optical Nonlinearity of van der Waals Materials with High-Order Multipoles

With their high nonlinear optical susceptibilities, a variety of 2D materials have the potential for integrated nonlinear photonic devices. For the nonlinear optical conversion efficiency, their atomic-level thickness inevitably results in low absolute magnitude, which can be resolved by combining them with plasmonic metasurfaces. However, the field enhancement of plasmonic metasurfaces is hindered from further improvement by the overwhelming electric dipole enhancement mechanism. Here, high-order multipoles are constructed to break the limits. The high-order multipole enhancement mechanism is realized with anapole states, in which the dominant electric quadrupole or electric octupole brings smaller mode volume and higher field enhancement. Theoretically, the averaged enhancement factor is unprecedentedly large, ≈3 × 10⁶ while a record-high second-harmonic generation enhancement of ≈8 × 10⁵ fold is experimentally demonstrated for a WS₂ monolayer laid on the structures. The maximum conversion efficiency of ≈0.3%, occurs when stacking four layers of WS₂ monolayer onto the structures. Such a near-field enhancement route can take effect up to the thickness of ≈5 × 10⁴ layers of WS₂ monolayer, in which it turns into a pure bulk case. The work provides a clear pathway towards remarkable electromagnetic field enhancements, unparalleled light-matter interactions, and high-performance ultra-compact devices.