无处不在的光子轮
The ubiquitous photonic wheel
摘要：
圆偏振电磁平面波携带围绕波的传播方向顺时针或逆时针旋转的电场。根据这种旋转的旋向性，它的\emph{longitudinal}自旋角动量密度与光的传播平行或反平行。然而，也有光波不是简单的平面，并且携带围绕垂直于传播方向的轴旋转的电场，从而产生\emph{transverse}自旋角动量密度。这种电场结构被戏称为“光子轮”。最近已经表明，光子轮是常见的光学，因为它们发生在由波导限制的电磁场中，在强聚焦光束中，在等离子体波和倏逝波中。本文建立了电磁波{沿明确方向传播，携带}横向自旋角动量密度的一般理论。我们表明，取决于{这些波的形状，}自旋密度可以是垂直于\emph{mean}线性动量（全局横向自旋）或线性动量\emph{density}（局部横向自旋）。我们发现后一种情况一般只发生在无衍射光束，如贝塞尔光束。此外，我们还引入了\emph{meridional}斯托克斯参数的概念来量化横向自旋密度。为了说明我们的理论，我们将其应用到贝塞尔光束和倏逝波的典型情况。这些结果为理解、产生和操纵具有横向自旋角动量密度的光束开辟了一条新的途径。
英文摘要：
A circularly polarized electromagnetic plane wave carries an electric field that rotates clockwise or counterclockwise around the propagation direction of the wave. According to the handedness of this rotation, its \emph{longitudinal} spin angular momentum density is either parallel or antiparallel to the propagation of light. However, there are also light waves that are not simply plane and carry an electric field that rotates around an axis perpendicular to the propagation direction, thus yielding \emph{transverse} spin angular momentum density. Electric field configurations of this kind have been suggestively dubbed "photonic wheels". It has been recently shown that photonic wheels are commonplace in optics as they occur in electromagnetic fields confined by waveguides, in strongly focused beams, in plasmonic and evanescent waves. In this work we establish a general theory of electromagnetic waves {propagating along a well defined direction, which carry} transverse spin angular momentum density. We show that depending on the shape of {these waves, the} spin density may be either perpendicular to the \emph{mean} linear momentum (globally transverse spin) or to the linear momentum \emph{density} (locally transverse spin). We find that the latter case generically occurs only for non-diffracting beams, such as the Bessel beams. Moreover, we introduce the concept of \emph{meridional} Stokes parameters to operationally quantify the transverse spin density. To illustrate our theory, we apply it to the exemplary cases of Bessel beams and evanescent waves. These results open a new and accessible route to the understanding, generation and manipulation of optical beams with transverse spin angular momentum density.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
