Photonic tractor beams: a review

Abstract

Usually, an unfocused light beam, such as a paraxial Gaussian beam, can exert a force on an object along the direction of light propagation, which is known as light pressure. Recently, however, it was found that an unfocused light beam can also exert an optical pulling force (OPF) on an object toward the source direction; the beam is accordingly named an optical tractor beam. In recent years, this intriguing force has attracted much attention and a huge amount of progress has been made both in theory and experiment. We briefly review recent progress achieved on this topic. We classify the mechanisms to achieve an OPF into four different kinds according to the dominant factors. The first one is tailoring the incident beam. The second one is engineering the object's optical parameters. The third one is designing the structured material background, in which the light-matter interaction occurs, and the fourth one is utilizing the indirect photophoretic force, which is related to the thermal effect of light absorption. For all the methods, we analyze the basic principles and review the recent achievements. Finally, we also give a brief conclusion and an outlook on the future development of this field.

Optical trapping with structured light: a review

“…In contrast to the above discussed optical tweezers, where the optical force exerted on the micro-and nanoparticles produces an acceleration along the same direction of the photon flow in an optical tractor beam, the optical forces drag small microparticles against this flow, i.e., toward the photons source. 28,[176][177][178][179][180] In contrast to a conveyor belt that requires gradient forces to pull microparticles toward the source of the photons, 118 the pulling forces of a tractor beam originate from a momentum conservation law. In essence, when the forward scattered light is more collimated than the incident light, and if the backward scattering is weak enough, the momentum conservation law predicts the existence of a pulling force.…”

Plasmonic tweezers: for nanoscale optical trapping and beyond

“…In recent years, laser [187][188][189][190][191] and plasmonic 64,66,192 traps have demonstrated unexpected phenomena that have drawn considerable interest. These unique properties are interesting and important for developments in assembly, crystallization and organization of micro-and nanostructures.…”

“…The pulling force acts in the direction opposite to the optical wave propagation 191 by either structuring the incident field [193][194][195] or modifying the surroundings of the manipulated object 196,197 . For specific plasmonic cases, Shalin et al verified that directional excitation of SPPs can enhance the linear momentum of scattered light and induce a considerable negative force on a particle, as shown in Fig.…”

Optical Forces: From Fundamental to Biological Applications

“…The counterintuitive optical pulling force draws increasing attention because of its attractive physical mechanism and potential applications. [ 74 ] In 2011, Chen et al theoretically demonstrated that microparticles can be accelerated against the photon flow and named this phenomenon as “optical pulling.” [ 29 ] To verify the optical pulling concept experimentally, Brzobohaty et al designed an optical setup using two Gaussian laser beams focused through objective lenses or using a single beam that was retro‐reflected from a mirror ( Figure a‐I). [ 28 ] Because of the action‐and‐reaction mechanism, the light‐matter momentum transfer leads to backward motion of the objects when the photons mainly propagate against the direction of wave vector k = k 1 + k 2 (Figure 3a‐II).…”