Abstract：
Dynamic control for a strong circular dichroism (CD) response is essential in engineering applications such as polarization manipulation, sensing, and imaging. Here, we propose and experimentally demonstrate a broadband tunable dual CD response via bound states in the continuum (BICs) in two-dimensional topologically protected metasurfaces composed of all-dielectric Si chiral grating structures that generate a pair of mixed and degenerated BIC mode and circular dichroic mode (CDM) as an additional degree of freedom in CD manipulation. It is found that a singular CD peak of nearly 100% at 1.6 μm can be achieved by CDM when BIC is hidden under normal incidence, while the CD peak can be split into two in which peak wavelengths can be precisely and linearly tuned over a bandwidth of 180 nm by the incident angle when the BIC mode is excited under oblique incidence. Additionally, dynamic modulation of output polarization states from linear to circular can be arbitrarily achieved at the split CD peaks by controlling the incident angle when asymmetry perturbations on chiral gratings are introduced due to the decoupling of various polarization states at Γ point by BIC to different positions in K space. The proposed chiral grating metasurface exhibits unique angle-sensitive tunable CD spectral characteristics, making it ideal for hyperspectral and spin-selective wavefront shaping, and holds significant promise in various applications such as optical security, angle sensors, chiral lasers, nonlinear filters, and other active chiral optical devices.
摘要：
强烈的圆二色性（CD）响应的动态控制在工程应用中至关重要，例如偏振操纵，传感,和成像。这里,我们提出并通过实验证明了在二维拓扑保护的超表面中通过连续介质（BIC）中的束束态实现宽带可调谐双CD响应，该超表面由全电介质Si手性光栅结构组成，该结构产生一对混合和退化的BIC模式和圆形二向色模式（CDM）作为CD操纵中的附加自由度。发现当BIC在垂直入射下隐藏时，通过CDM可以在1.6μm处实现近100％的奇异CD峰，而CD峰可以分为两个，其中当BIC模式在倾斜入射下激发时，峰值波长可以通过入射角在180nm的带宽上精确线性调谐。此外,由于BIC将Γ点处的各种偏振状态解耦到K空间中的不同位置，因此在引入手性光栅上的不对称扰动时，可以通过控制入射角来在CD峰值处任意实现从线性到圆形的输出偏振状态的动态调制。所提出的手性光栅超表面表现出独特的角度敏感可调谐CD光谱特性，使其成为高光谱和自旋选择性波前整形的理想选择，并在各种应用中拥有重要的前景，如光学安全，角度传感器,手性激光器,非线性滤波器，和其他有源手性光学器件。
