Enhancement of the 'tractor-beam' pulling force on an optically bound structure

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 objects^{10, 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.