声束的自旋和轨道角动量
Spin and orbital angular momenta of acoustic beams
摘要：
本文分析了均匀介质中单色声波场的自旋角动量和轨道角动量。尽管是纯纵向（无卷曲），非均匀声波一般具有非零自旋角动量密度所造成的局部旋转的矢量速度场。我们发现，局域声波的自旋积分消失与纵向声子的自旋为0的性质。我们还表明，螺旋度或手征密度消失相同的声场。作为一个例子，我们考虑非傍轴声学贝塞尔光束进行定义良好的整数轨道角动量，以及非零的本地自旋密度，与横向和纵向分量。我们描述了声贝塞尔光束中的非平凡偏振结构，并指出了一些可观察到的现象，如非零能量密度和一阶涡旋光束中心的纯圆横向偏振。
英文摘要：
We analyze spin and orbital angular momenta in monochromatic acoustic wave fields in a homogeneous medium. Despite being purely longitudinal (curl-free), inhomogeneous acoustic waves generically possess nonzero spin angular momentum density caused by the local rotation of the vector velocity field. We show that the integral spin of a localized acoustic wave vanishes in agreement with the spin-0 nature of longitudinal phonons. We also show that the helicity or chirality density vanishes identically in acoustic fields. As an example, we consider nonparaxial acoustic Bessel beams carrying well-defined integer orbital angular momentum, as well as nonzero local spin density, with both transverse and longitudinal components. We describe the nontrivial polarization structure in acoustic Bessel beams and indicate a number of observable phenomena, such as nonzero energy density and purely-circular transverse polarization in the center of the first-order vortex beams.
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