Strong optical chirality in asymmetric notched waveguide arrays

Achieving robust intrinsic chirality within planar photonic platforms is crucial for the development of compact sensing and information-processing systems. However, conventional designs often rely on complex three-dimensional structures or partial etching, which present significant nanofabrication challenges. Here, we demonstrate giant linear and nonlinear optical chirality in an all-dielectric metasurface composed of asymmetric notched waveguide arrays. By introducing a controllable longitudinal geometric shift between opposing semicircular notches on each waveguide, we explicitly break the structural mirror symmetry to produce a sharp, strong chiral resonance. This resonance originates from a bound guided mode folded into the radiative continuum to form quasi-guided modes (QGMs) via period doubling, while its chirality is fundamentally governed by the specific multipolar topology induced by the asymmetry. Notably, this chirality spans a wide wavevector range of the QGM band, thereby ensuring large circular dichroism (CD) in the spectrum across varying incident angles. Numerically, we achieve a near-unity third Stokes parameter, S 3 , and a large CD up to 1. Experimental validation on a silicon-on-sapphire platform reveals a stable CD of 0.42 and a high-Q factor of ∼800, in reasonable agreement with numerical predictions. Furthermore, we leveraged these high-Q resonances to enhance third-harmonic generation (THG), observing a pronounced nonlinear CD of 0.78. Our findings establish asymmetric notched waveguide arrays as a powerful and fabrication-friendly platform for integrated chiral light sources and nonlinear photonic devices.