理想环偶极子（Anapoles）的非线性激发
Nonlinear Excitation of Ideal Anapoles
- Aalto University（阿尔托大学）
机构由 AI 辅助整理，请以论文原文为准。
AI总结：
本文利用精确电流多极展开和环偶极子条件设计理想环偶极子，通过二次谐波产生和非共振偏振调节实现宽谱激发，在半个波长尺寸结构中实现约10^-4的散射抑制。
AI中文摘要：
光学环偶极子（anapoles）作为一种在纳米尺度增强光与物质相互作用的 promising 平台，已引起广泛关注。理想环偶极子是一种不产生任何远场辐射的电磁场局域激发。在实际中，由于与高阶多极子相关的寄生散射场，环偶极子并非理想。在此，我们利用近期确立的精确电流多极展开和精确环偶极子条件，设计了一种理想环偶极子，其中直至电八极和磁八极的多极阶辐射被消除或强烈抑制，从而将总辐射功率降低数个数量级。我们还提出并研究了实际的光子结构，在这些结构中，这种环偶极子可通过均匀线偏振平面波经由涉及二阶磁化率张量非对角元素的二次谐波产生来激发。在这种情况下，通过调整入射场的偏振角，在非共振条件下达到环偶极子条件，这使得环偶极子能够在不同尺寸的散射体上于宽光谱范围内被激发。利用这种方法，我们在尺寸为半波长的光子结构中实现了约 $10^{-4}$ 量级的散射抑制。该方法为实现真正与周围环境隔离的局域电磁激发提供了可能性。
英文摘要：
Optical anapoles have attracted significant interest as a promising platform for enhancing light-matter interactions at the nanoscale. An ideal anapole is a localized excitation of electromagnetic fields that does not produce any far-field radiation. In practice, anapoles are not ideal due to the parasitic scattered fields associated with higher-order multipoles. Here, we use the recently established exact current multipole expansion and the exact anapole condition to design an ideal anapole in which radiation due to the multipole orders up to the electric and magnetic octupoles is either eliminated or strongly suppressed, reducing the total radiated power by orders of magnitude. We also propose and study realistic photonic structures in which such anapoles can be excited by a uniform linearly polarized plane wave through second-harmonic generation that involves the off-diagonal elements of the second-order susceptibility tensor. In this case, the anapole condition is reached off-resonantly by adjusting the polarization angle of the incident field, which allows for the anapoles to be excited over a wide spectral range in scatterers of different sizes. Using this approach, we achieve scattering suppression on the order of $10^{-4}$ in photonic structures that are half a wavelength in size. The method offers the possibility to realize localized electromagnetic excitations that are truly isolated from the surroundings.
