【JACS】3.6微米手性微球实现角度选择性光谐振，首创无形变圆周径向激光发射
文章标题：Angle-Selective Optical Resonance and Circular Radial Lasing from a Chiral Polymeric Microsphere
通讯作者：Osamu Oki, Yohei Yamamoto
文章概要
引言
在传统光学认知中，球形结构由于其高度的几何对称性，通常在光学性质上表现出各向同性。然而，当微观各向异性的分子自组装成微球，其内部的折射率分布被打破时，这种各向同性便不复存在。此前，虽然已有研究发现具有双极或扭曲双极分子构型的微球内部存在独特的双折射现象，但球体表面特定分子排列对整体光学功能的影响一直未能被清晰阐明。这主要是由于传统的流体液晶液滴性质不稳定，容易在外力或电场下发生形变，难以进行精确、重复的光学测量。为了解决这一难题，筑波大学的科研团队巧妙地利用手性发光共轭聚合物进行自组装，成功构筑了固态手性高分子微球。这种固体微球不仅结构极其稳定，能够克服流体液滴的漂移和变形缺陷，还完美承载了表面的拓扑缺陷与手性纹理，为探索拓扑表面与光子学谐振之间的深层互动提供了绝佳的平台。
Figure 1. (a) Schematic representations of topological orders of molecules: A point defect on an LC film (left), and radial (center) and bipolar LC droplets (right). Orange rods represent LC molecules. (b) Schematic illustration for circular radial lasing from a solid twisted-bipolar (TB) microsphere. α denotes the azimuth angle.
主要实验及结论
研究人员首先合成了一种具有给体-受体交替结构的手性共轭聚合物，并通过气相扩散法使其自组装成直径在1.5至4.5微米之间的规整微球。扫描电镜与偏振光学显微镜的观测清晰地证实了这些手性微球具有独特的扭曲双极（TB）构型。为了揭示长期以来只能通过理论模拟推测的表面分子排列，团队创新性地引入了偏振相关光致发光（PL）成像技术。由于该共轭聚合物具有沿聚合物主链方向发射线性偏振光的特性，且光激发和发射仅发生在微球表面不到100纳米的极浅深度内，科研人员首次在实验上直接观测并重建了微球表面的分子偏振图谱。结果表明，在微球的拓扑缺陷周围，聚合物主链呈现出完美的逆时针（CCW）旋涡状排列，其主链在跨越赤道时与双极轴形成了约50度的交角，为扭曲双极构型提供了最为直接的结构证据。
Figure 2. (a) Molecular structures of (S,S)-PFBT. (b) SEM micrographs of (S)-MS. The inset shows histograms of d. (c) POM and optical (inset) micrographs of (S)-MS. Inset scale bar is 5 μm. (d) Schematic representation of the experimental configuration of polarization-dependent PL imaging. The magenta line depicted in the microsphere represents the TB axis of (S)-MS. β denotes the angle between the z-axis and the TB axis. (e, f) Polarization mapping (top) and the schematic representation of the molecular arrangement at the surface of (S)-MS (bottom), reconstructed from the series of polarization-dependent PL images collected at β = 0° (e) and β = 90° (f). In the polarization mapping, the color of each pixel indicates the polarization direction that exhibits the maximum PL intensity upon rotation of the analyzer. In (f), γ denotes the angle between the y-axis and the PFBT main chain crossing the equator.
基于这一明确的表面旋涡分子排列，研究团队构建了分析模型，深入探讨了其对微球表面回音壁模式（WGM） 谐振波长的调控规律。微球表面相切排列的聚合物主链偶极发射主要耦合到横电（TE）模式，而表面旋涡状的分子走向意味着不同方位角上的有效折射率呈现出周期性渐变。光谱模拟预测，谐振波长会随着方位角的变化而发生显著移动。紧接着，团队利用显微光致发光系统和高光谱相机（HSC）成像对单个手性微球进行了全方位空间光谱表征。实验结果与理论高度吻合，手性微球在特定的第一和第三象限表现出极强的、极具方向性的回音壁模式谐振发射，并且随着波长递增，谐振亮斑在微球边缘展现出清晰的逆时针角位移。作为对照实验，无定形阿 chiral 微球展现出的是全向均匀谐振，而对映异构体微球则展现出完全镜像的顺时针谐振特性，这强有力地证明了手性渐变折射率光学路径是导致角度选择性谐振的根本原因。
Figure 3. (a) Schematic representation of (S)-MS whose bipolar axis is set along the y-axis. (b, c) α-Dependent _n_av (b) and λWGM (c) simulated with parameters _n_e = 1.80, _n_o = 1.55, d = 4.4 μm, and γ = 50°. The l-values indicate the mode numbers of the respective resonance.
在成功实现角度选择性谐振的基础上，研究人员进一步探索了手性微球的激光发射行为。实验使用波长为405纳米的飞秒脉冲激光作为泵浦源，并采用圆偏振光以确保微球受到各向同性的均匀激发。当激发功率超过约25微焦耳每平方厘米的阈值时，单个直径为3.6微米的手性微球发射光谱急剧变窄，半峰全宽缩减至1.6 nm，表现出强烈的非线性增长特征，确证了多模回音壁激光的振荡发射。更为震撼的是，角度相关激光测量表明，受到表面逆时针旋涡状聚合物偶极主导的受激辐射影响，微球高功率放大的激光能量被特异性地引导至特定的方位角方向。这种在完全保持完美球形、无需改变几何形状的前提下实现的圆周径向激光发射，打破了传统微腔激光各向同性出射的限制。
Figure 4. (a) Schematic representation of the experimental setup for HSC imaging. POM and optical (inset) micrographs show (S)-MS (d = 4.4 μm) placed at β = 90° on a substrate. The magenta arrow represents the bipolar axis of (S)-MS. (b) λ-Dependent PL micrographs of (S)-MS in the λ range of 561–563 nm. (c) PL spectra collected from the entire microsphere (i), one of the diametrically opposite bright spots in Q1 and Q3 (ii), and a position around the topological defect (iii) as represented in (b). (d) Plots of the α-dependent PL intensities integrated from the α-dependent PL spectra of (S)-MS. The orange arrow depicts the direction of the transition dipole moment. (e) Plots of the experimental α-dependent λWGM (green open circle) overlaid with the calculated α-dependent λWGM (red curves, parameters: _n_e = 1.75, _n_o = 1.52, d = 4.35 μm, and γ = 55°).
Figure 5. (a) μ-PL spectra of (S)-MS upon increasing the pumping power of the fs-pulsed laser at 405 nm. The colored numbers on the right of each PL spectrum indicate the pump power density (μJ cm–2). The inset shows the plot of the integrated PL intensity versus the pump power. (b) Schematic representation of the experimental configuration of the α-dependent lasing measurements. (c) α-Dependent WGM lasing spectra of (S)-MS. The red and blue colors of PL spectra represent that PL is collected at Q1 and Q3, and at Q2 and Q4, respectively. (d) Polar plot of the PL intensity at the main lasing peak (559 nm) as a function of α.
总结及展望
这项研究成功展示了拓扑表面纹理与光物质相互作用的迷人魅力，完美地将手性超分子化学与微纳光子学融合在一起。通过精准调控共轭聚合物的分子手性与自组装拓扑结构，不仅在固体微球表面实现了旋涡分子排列的直接可视化，更发展出了具有空间选择性光放大和角度自定向输出功能的全新微腔激光器。这种无需改变微腔几何形状便能操控光场行为的创新策略，为未来开发新型集成光子学器件、高方向性微纳激光光源以及多维防伪技术开辟了全新的道路。这种化学合成、精细自组装与前沿光学原理的深度碰撞，也正宣告着“光化学（Optochemistry）”这一新兴交叉研究领域的蓬勃兴起。