AI 中文总结
该研究揭示麦克斯韦横向性锁定结构光四极跃迁的纵向梯度，通过分解光场梯度张量发现不同Δm通道的选择性响应，证明纵向光学结构对结构光四极跃迁的关键作用。
AI 中文摘要
电四极吸收由光场梯度驱动，而非仅由光场振幅驱动，这一区别对结构光至关重要，其中傍轴近似下微小的纵向场可产生主导阶纵向梯度。对于全矢量拉盖尔-高斯模式，麦克斯韦横向性将该纵向梯度锁定于主导傍轴场的横向结构梯度，因此这是一种普遍的电磁约束，而非可独立消除的依赖聚焦的修正项。将无迹对称光场梯度张量分解为球分量后，可发现强烈的通道选择性响应：Δm=0跃迁在保留阶次下需要纵向梯度；Δm=±1通道除了横向场载流子尺度纵向导数外，还会收到标量和矢量非傍轴修正；Δm=±2通道在相同阶次下保持横向性。这些结果表明，即使纵向场振幅本身傍轴近似下很小，纵向光学结构仍可作为不可或缺的主导梯度贡献进入结构光四极跃迁。
英文摘要
Electric quadrupole absorption is driven by optical field gradients, not by field amplitudes alone. This distinction is crucial for structured light, where a paraxially small longitudinal field can generate a leading-order longitudinal gradient. For fully vectorial Laguerre-Gaussian modes, Maxwell transversality locks this longitudinal gradient to the transverse structured gradient of the leading paraxial field. This locking is therefore a general electromagnetic constraint, not a focusing-dependent correction that can be removed independently. Decomposing the symmetric-traceless optical field-gradient tensor into spherical components reveals a strongly channel-selective response: the $Δm=0$ transition requires the longitudinal gradient at the retained order, the $Δm=\pm1$ channels receive scalar and vectorial nonparaxial corrections on top of the carrier-scale longitudinal derivative of the transverse field, and the $Δm=\pm2$ channels remain transverse at the same order. These results show that longitudinal optical structure can enter structured-light quadrupole transitions as an indispensable leading-gradient contribution, even when the longitudinal field amplitude itself is paraxially small.
