Morphology-independent general-purpose optical surface tractor beam

Optical tractor beams capable of pulling particles backward have garnered significant and increasing interest. Traditional optical tractor beams are limited to free space beams with small forward wavevectors, enabling them to pull selected particles. Here, we present a comprehensive theory for the optical force exerted by a surface wave using analytical and numerical calculations, revealing the relationship between the canonical momentum and optical forces. Based on this theory, we demonstrate a general purpose optical surface tractor beam that can pull any passive particle, regardless of size, composition, or geometry. The tractor beam utilizes a surface wave with negative canonical momentum characterized by a single well-defined negative Bloch k vector. The tractor beam relies on a mechanism where the negative incident force always surpasses the recoil force. As such, the tractor beam, when excited on the surface of a double-negative index metamaterial, can pull particles with different morphologies. Unlike traditional approaches, which typically involve the adjustment of incident and scattering angles to reduce the incident force and increase the recoil force, the optical pulling effect observed here is uniquely driven by the negative incident force associated with a negative k value. This characteristic endows it with independence from scattering feature control, resulting in exceptional robustness. Next, we present a demonstration illustrating the generation of such an OSTB by employing a double-negative metamaterial with ε, μ < 0.

Neng Wang, Jack Ng, Guo Ping Wang