Researchers demonstrated a fibre-based “optical tractor beam” that can pull microscopic droplets toward a light source over record 40-cm distances using direct photon momentum. By engineering the waveguide’s diameter and wavelength, they switch between optical pushing and pulling with high speed and stability. The findings were published in a recent article published in the journal Nature Communications.
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Background
At the heart of the study is the fundamental physics of photon momentum transfer within optical waveguides and the associated optical forces. Traditionally, optical pushing is explained by the transfer of photon momentum through absorption or reflection, leading to a force in the light’s propagation direction. When dealing with nanofibers, the guided modes, especially the fundamental HE11 mode, exhibit evanescent fields extending outside the fiber core, which can interact with objects in proximity. The momentum carried by these modes can be characterized using Minkowski’s formulation, relevant in inhomogeneous dielectric media, which effectively accounts for the photon’s momentum within a waveguide system. The prior understanding of optical propulsion and their limitations motivated this study to explore whether the waveguided modes within ultra-thin fibers could serve as an effective platform for long-distance optical pulling, circumventing traditional constraints.
The Current Study
The experimental setup hinges on fabricating and utilizing sub-wavelength diameter silica nanofibers with lengths extending up to 40 centimeters. The nanofibers are produced through a controlled tapering process, ensuring a diameter of approximately 325 nm, a value less than one-third of the wavelength of the employed laser light at 1552 nm, a condition critical for manipulating the photon momentum within the waveguide. The fibers are integrated with standard optical components to form a continuous, low-loss guide for the laser light, ensuring that the guided modes are maintained over extensive distances.
The optical modes within these fibers, primarily the HE11 mode, are meticulously characterized to understand their modal profile, effective index, and evanescent field distribution. The laser source delivers continuous-wave light at the specified wavelength, with the input power adjustable up to 1 W. Precise control of the input power allows modulation of the optical forces acting on micro-droplets attached to the fiber surface.
The core of the methodology involves launching the laser light into the nanofiber and observing the resultant forces on the attached droplets. For qualitative and quantitative analysis, the experiments include measuring droplet velocities, displacements, and pulling distances over time. A key aspect involves varying the fiber diameter in taper regions and the input power to assess the relationship between optical power, modal properties, and the resulting optical forces.
Theoretical models underpin the experimental observations, employing electromagnetic simulations like finite-difference time-domain (FDTD) and mode analysis to calculate the electromagnetic field distribution, the photon momentum in the waveguide, and the expected optical forces.
Results and Discussion
The experiments successfully demonstrate that optical pulling of micro-droplets over distances of up to approximately 40 centimeters is feasible with the nanofiber-based system. When a continuous-wave laser beam propagates through the fiber, the evanescent field exerts a force on the attached droplets. Notably, as the fiber diameter decreases below roughly one-third of the wavelength, the photon momentum associated with the guided mode can be engineered to support pulling forces, contrary to the conventional expectation of a push.
Quantitative measurements reveal that the pulling force is directly proportional to the input optical power, aligning well with electromagnetic simulations. The droplet velocities, recorded at up to several micrometers per second, corroborate theoretical predictions based on the light's electromagnetic momentum transfer within the nanofiber's guided mode. For a 65-micrometer droplet in a vertical configuration, the authors report an optical pulling force of about 1.15 nanonewtons at 0.5 W, sufficient to balance gravity and pull the droplet upward. In a separate experiment, a 45-micrometer droplet is pulled horizontally along a 40-centimeter nanofiber using 1 W of 1552 nm light, achieving an average speed of about −75 μm/s over the full distance.
This capability hinges on the unique wavevector and photon momentum properties of the nanofiber modes. When the fiber diameter is sufficiently small, the guided mode becomes loosely confined and its effective index approaches unity. In this regime, the droplet focuses a substantial fraction of the light back into the high-index fiber, so the outgoing photon momentum exceeds the incoming momentum, leading to a net backward (pulling) force. In taper regions, the gradual change in fiber diameter modulates the pulling force: as the diameter increases, the droplet’s velocity decreases and it eventually stops near the critical D/λ ≈ 1/3 where the force changes sign.
Conclusion
This research marks a significant milestone in the field of optical manipulation, demonstrating that nanofiber-guided optical modes can be harnessed for ultra-long-range optical pulling of microdroplets. By exploiting the wavevector and photon momentum properties intrinsic to sub-wavelength diameter fibers, the authors achieved the first experimental realization of a device capable of pulling objects over tens of centimeters.
The implications of this technique are far-reaching within the optics domain. It offers a new route for remote, contactless actuation and transportation of microscopic objects with unprecedented range and precision. Beyond fundamental physics, these insights could translate into practical applications such as long-distance delivery of biological samples, microfluidic flow control, or nonlinear optical systems where long-range interactions are essential.
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Journal Reference
Zhang J., Liu K., et al. (2025). Ultra-long-range optical pulling with an optical nanofibre. Nature Communications 16, 7424. DOI: 10.1038/s41467-025-62536-w, https://www.nature.com/articles/s41467-025-62536-w