Optical metasurfaces with high quality factors (Q-factors) of chiral resonances can boost substantially light-matter interaction for various applications of chiral response in ultrathin, active, and nonlinear metadevices. However, current approaches lack the flexibility to enhance and tune the chirality and Q-factor simultaneously. Here, we suggest a design of chiral metasurface supporting bound state in the continuum (BIC) and demonstrate experimentally chiroptical responses with ultra-high Q-factors and near-perfect circular dichroism (CD = 0.93) at optical frequencies. We employ the symmetry-reduced meta-atoms with high birefringence supporting winding elliptical eigenstate polarizations with opposite helicity. It provides a convenient way for achieving the maximal planar chirality tuned by either breaking in-plane structure symmetry or changing illumination angle. Beyond linear CD, we also achieved strong near-field enhancement CD and near-unitary nonlinear CD in the same planar chiral metasurface design with circular eigen-polarization. Sharply resonant chirality realized in planar metasurfaces promises various practical applications including chiral lasers and chiral nonlinear filters.

具有高品质因子（Q 因子）手性共振的光学超表面可以显着增强光物质相互作用，用于超薄、有源和非线性超器件中手性响应的各种应用。然而，目前的方法缺乏同时增强和调整手性和 Q 因子的灵活性。在这里，我们建议设计一种支持连续谱 (BIC) 束缚态的手性超表面，并通过实验证明在光学频率下具有超高 Q 因子和近乎完美的圆二色性 (CD = 0.93) 的手性响应。我们使用具有高双折射的对称减少的元原子支持具有相反螺旋度的缠绕椭圆本征态极化。它为通过打破面内结构对称性或改变照明角度来实现最大平面手性提供了一种方便的方法。除了线性 CD 之外，我们还在具有圆本征极化的相同平面手征超曲面设计中实现了强近场增强 CD 和近酉非线性 CD。在平面超表面中实现的强烈共振手性有望实现各种实际应用，包括手性激光器和手性非线性滤波器。