Ugras Thomas J, Yao Yuan, Robinson Richard D
School of Applied and Engineering Physics, Cornell University, Ithaca, New York, USA.
Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York, USA.
Chirality. 2023 Nov;35(11):846-855. doi: 10.1002/chir.23597. Epub 2023 Jun 18.
Chiral materials with strong linear anisotropies are difficult to accurately characterize with circular dichroism (CD) because of artifactual contributions to their spectra from linear dichroism (LD) and birefringence (LB). Historically, researchers have used a second-order Taylor series expansion on the Mueller matrix to model the LDLB interaction effects on the spectra in conventional materials, but this approach may no longer be sufficient to account for the artifactual CD signals in emergent materials. In this work, we present an expression to model the measured CD using a third-order expansion, which introduces "pairwise interference" terms that, unlike the LDLB terms, cannot be averaged out of the signal. We find that the third-order pairwise interference terms can make noticeable contributions to the simulated CD spectra. Using numerical simulations of the measured CD across a broad range of linear and chiral anisotropy parameters, the LDLB interactions are most prominent in samples that have strong linear anisotropies (LD, LB) but negligible chiral anisotropies, where the measured CD strays from the chirality-induced CD by factors greater than 10 . Additionally, the pairwise interactions are most significant in systems with moderate-to-strong chiral and linear anisotropies, where the measured CD is inflated twofold, a figure that grows as linear anisotropies approach their maximum. In summary, media with moderate-to-strong linear anisotropy are in great danger of having their CD altered by these effects in subtle manners. This work highlights the significance of considering distortions in CD measurements through higher-order pairwise interference effects in highly anisotropic nanomaterials.
由于线性二向色性（LD）和双折射（LB）对其光谱有伪影贡献，具有强线性各向异性的手性材料很难用圆二色性（CD）进行准确表征。从历史上看，研究人员在穆勒矩阵上使用二阶泰勒级数展开来模拟传统材料中LDLB相互作用对光谱的影响，但这种方法可能不再足以解释新兴材料中的伪影CD信号。在这项工作中，我们提出了一个用三阶展开来模拟测量的CD的表达式，该表达式引入了“成对干涉”项，与LDLB项不同，这些项不能从信号中平均掉。我们发现三阶成对干涉项可以对模拟的CD光谱做出显著贡献。通过对广泛的线性和手性各向异性参数下测量的CD进行数值模拟，LDLB相互作用在具有强线性各向异性（LD，LB）但手性各向异性可忽略不计的样品中最为突出，在这些样品中，测量的CD偏离手性诱导的CD的因子大于10。此外，成对相互作用在具有中等到强手性和线性各向异性的系统中最为显著，在这些系统中，测量的CD增加了两倍，随着线性各向异性接近其最大值，这个数字还会增加。总之，具有中等到强线性各向异性的介质很可能会因这些效应而使其CD发生微妙的变化。这项工作强调了在高度各向异性的纳米材料中通过高阶成对干涉效应考虑CD测量中的失真的重要性。
