Abstract
AbstractThis paper investigates the singular optics of nonparaxial light beams in the near field when the light behaves as a tractor beam. New insights into the optical pulling force, which is usually represented by integrating the stress tensor at a black box enclosing the object, are interpreted by the optical singularity of the Poynting vector. The negative nonconservative pulling force originates from the transfer of the azimuthal Poynting vector to the longitudinal component partly owing to the presence of a scatterer. The separatrice pattern and singularity shifts of the Poynting vector unanimously exhibit a differentiable near‐field distribution in the presence of optical pulling force. A new method is established to calculate the near‐field optical force using the differential Poynting vector in the far field. The results obtained provide a clear physical interpretation of the light–matter interaction and manifest the significance of singular optics in manipulating objects. image
Highlights
Along with optical manipulation arising in electromagnetic fields with a gradient [1,2,3], tractor beams [4,5,6,7,8,9,10,11] have been showing unprecedented phenomena that once only existed in science fiction
Unlike conventional optical tweezers which rely on adjusting the positions of intensity gradients to move objects, tractor beams can exert nonconservative pulling forces on particles by continuously dragging them towards the beam sources
As found from many theoretical and experimental studies, non-diffracting beams [17,18,19] are promising candidates for realizing tractor beams due to their unique properties of maintaining both intensity and spatial extent in the direction of propagation. Such tractor beams can exert pulling forces on a particle, which can be explained by using the law of momentum conservation, i.e. when the forward scattering exceeds the backward one, the particle experiences a backward-directed momentum [20]
Summary
Along with optical manipulation arising in electromagnetic fields with a gradient [1,2,3], tractor beams [4,5,6,7,8,9,10,11] have been showing unprecedented phenomena that once only existed in science fiction. As found from many theoretical and experimental studies, non-diffracting beams [17,18,19] are promising candidates for realizing tractor beams due to their unique properties of maintaining both intensity and spatial extent in the direction of propagation. Such tractor beams can exert pulling forces on a particle, which can be explained by using the law of momentum conservation, i.e. when the forward scattering exceeds the backward one, the particle experiences a backward-directed momentum [20]. We discuss below the correlation between near-field (within one wavelength from the particle’s surface) and far-field Poynting vectors and we elaborate the origin of the pulling force as a consequence of the specific energy flow
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