Abstract
Recently, increasing attention has been devoted to mastering a new technique of optical delivery of micro-objects tractor-beam’1, 2, 3, 4, 5, 6, 7, 8, 9. Such beams have uniform intensity profiles along their propagation direction and can exert a negative force that, in contrast to the familiar pushing force associated with radiation pressure, pulls the scatterer toward the light source. It was experimentally observed that under certain circumstances, the pulling force can be significantly enhanced6 if a non-spherical scatterer, for example, a linear chain of optically bound objects10, 11, 12, is optically transported. Here we demonstrate that motion of two optically bound objects in a tractor beam strongly depends on theirs mutual distance and spatial orientation. Such configuration-dependent optical forces add extra flexibility to our ability to control matter with light. Understanding these interactions opens the door to new applications involving the formation, sorting or delivery of colloidal self-organized structures.
Highlights
Increasing attention has been devoted to mastering a new technique of optical delivery of micro-objects tractorbeam’[1,2,3,4,5,6,7,8,9]. Such beams have uniform intensity profiles along their propagation direction and can exert a negative force that, in contrast to the familiar pushing force associated with radiation pressure, pulls the scatterer toward the light source
It was experimentally observed that under certain circumstances, the pulling force can be significantly enhanced[6] if a non-spherical scatterer, for example, a linear chain of optically bound objects[10,11,12], is optically transported
We demonstrate that motion of two optically bound objects in a tractor beam strongly depends on theirs mutual distance and spatial orientation
Summary
By the laser beam (indicated by region i–ii in Figure 1c) and subsequently pulled against the beam propagation. Here, the stable particle position jumps from the second to the third lobe with increasing inter-particle distance and, in contrast to the previous case, the interaction force changes its sign and remains the leading force (|Fint2,z|4|Fisol2,z|) in the investigated region. This change from pushing to pulling occurs because the particles in the second and third lobes are on opposite sides with respect to the lobe intensity maximum and are attracted in opposite directions (Figure 2c and d, and Supplementary Media 2). Even the rigorous calculations support the intuitive conclusion that the particles ‘surf’ along the
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