Li Laboratory

Tractor beam – pulling tiny particles and smut spores over a meter-long distance and lifting large objects by lasers

A tractor beam is a theoretical or fictional device that can attract or pull objects toward the source of the beam without any physical contact. If it can be achieved over longer distances, particles from space could be pulled back and captured for analysis. It might even be possible to lift particles from the surface of a planet without a landing craft. In ECU Physics department, Dr. Yong-Qing Li and his students have successfully demonstrated that micron-sized absorbing particles or biological-originated smut spores can be pulled and manipulated over a meter-scale distance in a low-pressure gaseous environment using a collimated laser beam based on optically induced “negative” force.

Physics student, Joshua Manumg, works on 1-meter pulling experiment in the lab (left). The experiment for laser pulling over a 10-meter distance was set up in the hallway (right).

 
The experiment is based on photophoresis, a physical phenomenon where small particles move when exposed to a light beam. When light shines on a tiny particle (like dust or a smut spore), the particle absorbs energy from the light and becomes hotter than the surrounding gas molecules. Collisions between the gas molecules and the hotted particle result in a change in the particle’s momentum. According to Newton’s laws, this momentum change produces a photophoretic force, which can push or pull the particle either forward or backward depending on its size, shape, and material properties. Interestingly, a “negative” photophoretic force that pulls the particle toward the light source can occur due to differences in heat exchange between the gas molecules and the particle’s surface, even if the particle is uniformly heated. This force can be several orders of magnitude stronger than both the radiation pressure and gravitational force, making it effective for moving particles over long distances, like how a tractor beam works.

The lifting of a gold cylindrical vane by a laser. Time-lapse images of a cylindrical gold leaf (7 × 7 mm, 0.2 μm thickness) illuminated by a red laser under a low-pressure environment.

 
Although laser has become a powerful tool to manipulate micro-particles by radiation pressure or photophoretic force, but its effectiveness for large objects is less noticeable because the magnitude of these forces is generally much weaker than the gravitational force. We have successfully observed sufficiently strong light-induced attractive forces that allow manipulating centimeter-sized curved objects. The experiment is based on the radiometric effect caused by the curvature of object’s surface and photo-heating effect, in which the photon energy of the incident light is first converted into the thermal energy of the object and then asymmetrical momentum transfer between the hot curved surface and the surrounding gas molecules produces a “negative” radiometric force, which allows pulling the object towards the light source by adjusting the orientation of the concave surface of the object. This large force is sufficient to rotate a motor with four curved vanes at speeds up to 600 rpm and even lifting a large vane off the ground. Pulling macroscopic objects with light may find significant applications in solar radiation–powered near-space propulsion systems, in which sunlight acts as a tractor beam.

Optical Pulling of an Airborne Particle Over 1 Meter