Zou Zheguang, Lirette Robert, Zhang Likun
National Center for Physical Acoustics and Department of Physics and Astronomy, University of Mississippi, University, Mississippi 38677, USA.
Phys Rev Lett. 2020 Aug 14;125(7):074301. doi: 10.1103/PhysRevLett.125.074301.
We present acoustic modeling, measurements, and interpretation of the angular momentum carried in an ultrasonic vortex beam that is obliquely reflected off a flat water-air interface. The experimental measurements observe the theoretically predicted reversals of phase rotation, topological charge, and orbital angular momentum in a reflected vortex beam in direct analogy to optical phenomena. The spatial and temporal evolution of the linear and angular momentum during the reflection are determined by calculating the velocity field from two-dimensional scanned pressure fields. A conversion of the angular momentum indicates a radiation torque along the oblique reflecting surface. We understand this radiation torque originates from the break of rotational symmetry with respect to the incident plane for normal components of the energy flux and linear momentum density at the reflecting surface. Our study provides mechanical evidence on the effect of a flat surface on the reflection of vortex beams and gains insight into the underlying physics, impacting non-contact manipulation of objects and communication.
我们展示了对超声涡旋光束中携带的角动量进行的声学建模、测量及解释，该超声涡旋光束从平坦的水 - 空气界面斜反射。实验测量观察到反射涡旋光束中相位旋转、拓扑电荷和轨道角动量的理论预测反转，这与光学现象直接类似。通过从二维扫描压力场计算速度场，确定了反射过程中线性和角动量的空间及时间演化。角动量的转换表明沿斜反射面存在辐射扭矩。我们认为这种辐射扭矩源于反射面处能量通量和线性动量密度的法向分量相对于入射平面的旋转对称性破坏。我们的研究为平面表面对涡旋光束反射的影响提供了力学证据，并深入了解了潜在的物理原理，这对物体的非接触操纵和通信具有重要意义。
