Chen Hao, Chen Xiao, Zhong Haoshuang, Xu Shuqiao, Zhang Guoyong, Yang Zicheng, Chu Bo, Zhan Peng, Wang Shuming, Chen Zhuo, Wang Zhenlin
Opt Express. 2025 Jul 28;33(15):32881-32889. doi: 10.1364/OE.570432.
C-points in momentum space correspond to circularly polarized states, with broad applications in light-matter interactions, chiral lasing, and nonlinear optics. However, achieving a circularly polarized state at the Γ point requires relocating C-points to this position, which necessitates breaking both in-plane and out-of-plane symmetries. Besides, the chiral responses are typically confined to a narrow range of incident angles. Here, by including periodic groove perturbations on both sides of the grating slit, guided-mode resonances (GMRs) with a high quality factor (∼10) that propagate parallel to the grating slit are created, which possess two pairs of C-points off the Γ point. By adjusting the spacing between the grooves on both sides of the grating slits, one pair of C-points with the same handedness gradually evolves toward the Γ point and is eventually located on a high-symmetry axis, resulting in near-circularly polarized states over an extended momentum range along that axis, while preserving the in-plane -symmetry. Furthermore, we achieve spectrally tunable field-enhanced circular dichroism (CD) as well as nonlinear CD exceeding 50 dB. The proposed structure also exhibits strong robustness against fabrication imperfections, offering practical feasibility for chiral photonic applications.
动量空间中的C点对应于圆偏振态，在光与物质相互作用、手性激光和非线性光学等领域有广泛应用。然而，要在Γ点实现圆偏振态，需要将C点重新定位到该位置，这就需要打破面内和面外对称性。此外，手性响应通常局限于很窄的入射角范围。在此，通过在光栅狭缝两侧引入周期性的凹槽扰动，创建了具有高品质因数（约为10）且平行于光栅狭缝传播的导模共振（GMRs），其在远离Γ点处具有两对C点。通过调整光栅狭缝两侧凹槽之间的间距，一对具有相同手性的C点逐渐向Γ点演化，最终位于一条高对称轴上，从而在沿该轴的扩展动量范围内产生近圆偏振态，同时保持面内对称性。此外，我们还实现了光谱可调谐的场增强圆二色性（CD）以及超过50 dB的非线性CD。所提出的结构对手工制造缺陷也具有很强的鲁棒性，为手性光子应用提供了实际可行性。
