Tractor beam physics: Carbon nanotubes experience pulling force from light

A recent study has demonstrated that a beam of light can exert a pulling force on carbon nanotubes, contrary to the usual pushing effect of light. Published in iScience by Tomer Berghaus, Touvia Miloh, and Gregory Ya. Slepyan, the research highlights that this optical pulling force arises from the non-local conductivity of the nanotube’s electrons. This phenomenon, which cannot be explained by traditional local models, represents a new understanding of light-matter interactions at the nanoscale.

The concept of optical forces dates back to the early 20th century, with significant advancements made by Arthur Ashkin in the 1970s and 1980s. These developments led to the creation of optical tweezers, which manipulate microscopic particles using focused laser beams. In typical scenarios, light exerts forces that push particles along its propagation direction. However, nanoscale phenomena, including chirality and plasmonic resonances, can result in negative optical forces, drawing objects toward the light source.

The unique aspect of this study is the mechanism behind the pulling force on carbon nanotubes. Unlike previous research that relied on specific geometries or materials, this effect is due to the spatial dispersion of the nanotube’s conductivity. The study utilized the Kubo formalism to account for the non-local conductivity, which considers the influence of the electric field’s curvature along the nanotube’s axis.

The geometry of the carbon nanotube plays a crucial role. Previous models treated nanotubes as infinitely long, but this study considers finite-length tubes, where electron reflections at the ends create effective multipoles. These multipoles are absent in local models and are key to reversing the optical force direction. The research also highlights the importance of non-local conductivity in ensuring mathematically consistent calculations of optomechanical forces.

The study provides quantitative predictions, showing that pulling occurs in short nanotubes, approximately 100 to 200 nanometers long, within a specific frequency range. The force's magnitude, in the tens of femtonewtons, aligns with existing optical trapping experiments. The research suggests potential applications in measuring nanotube doping levels, sorting by length or radius, and developing mechanical sensors.

The theoretical framework is intentionally simple, focusing on a single metallic nanotube in vacuum. However, the authors propose extensions to more complex environments, such as fluids or optical cavities, which could enhance the pulling force. Despite limitations, the study underscores the non-local nature of light-matter interactions at the nanoscale, inviting experimental validation of these findings.