We demonstrate that tight focusing of a circularly polarized Gaussian beam in optical tweezers leads to spin-momentum locking—with the transverse momentum density (Poynting vector) being helicity-dependent, while the transverse spin angular momentum density becomes independent of helicity. We further use a stratified medium in the path of the trapping beam in our optical tweezers setup to enhance the magnitude of the transverse momentum and the electric field intensity in the radial direction with respect to the beam axis and cause them to significantly overlap. This overlap allows us to experimentally observe the circular motion of a birefringent particle, trapped off-axis, in response to an input circularly polarized fundamental Gaussian beam carrying no intrinsic orbital angular momentum (OAM). The circular motion is dependent on the helicity of the input beam so that we can identify it as the signature of the elusive Belinfante spin in propagating light beams obtained in our optical tweezers configuration. Our results can be extended to beams carrying intrinsic OAM leading to simple routes for achieving complex manipulation of micro-machines or other mesoscopic matter using optical tweezers.

中文翻译：

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直接观察光镊中光的自旋相关动量的影响

我们证明，圆偏振高斯光束在光镊中的紧密聚焦会导致自旋动量锁定-横向动量密度（Poynting矢量）与螺旋度有关，而横向自旋角动量密度与螺旋度无关。在我们的光学镊子装置中，我们还在捕获光束的路径中使用分层介质，以增强横向动量的大小和相对于光束轴的径向方向上的电场强度，并使它们明显重叠。这种重叠使我们能够实验性地观察到双折射粒子的偏轴运动，该双折射粒子响应于输入的不带固有轨道角动量（OAM）的圆偏振基本高斯光束而发生偏轴运动。圆周运动取决于输入光束的螺旋度，因此我们可以将其识别为以光学镊子配置获得的传播光束中难以捉摸的Belinfante自旋的特征。我们的结果可以扩展到携带固有OAM的光束，从而导致使用光学镊子实现微机或其他介观物质的复杂操纵的简单路线。