Tractor beams, a staple of science fiction, may be moving closer to science fact. In a paper published earlier this spring, physicists have proposed a structure that may enable light to pull objects.
Normally, light pushes on objects, albeit weakly. In the field of optical manipulation optical tweezers employ this pushing force to move microscopic objects from atoms to bacteria. The ability to pull as well would increase the precision and scope of optical manipulation. For spaceflight, engineers have proposed sails to capture the force exerted by light.
Rather than towing space vessels, the newly proposed tractor beam might be more useful in biology or medicine. "If you want to pull something towards you, you just reduce the pressure," says Mordechai Segev, a physicist at Technion–Israel Institute of Technology, who describes his team's idea in an April Optics Express paper. "You make a little bit of vacuum," he adds. The problem is that in sensitive medical applications, such as lung surgery, it is important not to change the pressure or introduce any new gases. "Here, the light will be the suction device," he says, "so the pressure would not change at all. It is just the light."
Previous ideas for a "tractor beam" have often focused on creating new gravitational fields to drag objects, heating air to create pressure differences or inducing electric and magnetic charges in objects so that they move against the direction of an incoming laser beam.
The latest proposal takes advantage of a phenomenon called negative radiation pressure. Russian physicist Victor Veselago first theorized its existence in his 1967 paper about materials with an unusual property called negative refraction index. An index of refraction is a number that describes the way light is bent when it goes into a glass lens or other medium, and at the time of the paper nobody knew if this number could be negative in any material. But in the past couple of decades several teams of researchers proved that negative refraction can occur in specially made substances called metamaterials, which have led to limited invisibility cloaks and distortion-free "super" lenses.
The mechanism of negative radiation pressure depends on two aspects of a light wave: its group and phase velocities. A light wave consists groups of smaller waves; the group velocity is the speed and direction of the overall wave group, the phase velocity refers to the speed and direction of a point on one of the smaller constituent waves. The electromagnetic energy of the light wave goes in the direction of the group velocity whereas the wave's effect on a particle goes in the direction of the phase velocity. If these two velocities point in different directions, then negative radiation pressure can result.
The use of metamaterials to move particles via negative radiation pressure has been hindered by the fact that most of these materials are solid, and introducing a gap for particles would eliminate the negative radiation pressure. Additionally, all current metamaterials contain metals, which absorb electromagnetic energy, rendering the pulling effect on particles negligible.
Instead of using metamaterials, the Technion team proposes a waveguide made of materials with a property called birefringence to create the necessary optical effects. Birefringence, which occurs naturally in crystals such as quartz and calcite, describes materials that have multiple indices of refraction, depending on what direction light enters the material. Place a calcite crystal on a newspaper, and suddenly the image is doubled.
Segev and his group's design uses layers of materials with different types of birefringence, along with specially designed mirrors, to make a practical model for how negative radiation pressure might be achieved. In this waveguide, the group velocity would move in one direction and the phase velocity in the opposite. Most important, it includes a large gap between the layers. This gap, which does not interfere with the optical properties of the material, allows the introduction of particles to be pulled into the waveguide. "It's like a sandwich," Segev says.
The proposed design can use a variety of birefringent materials, which are widely available and contain no metal, so they don't rob light of much energy. Additionally, although the birefringent materials used would be only micrometers thick, the gap could be millimeters wide, enabling fairly large particles to be manipulated by light.
Viktor Podolskiy, a physicist at the University of Massachusetts Lowell who was not part of this research, says that both the metamaterial approach and the birefringence approach address different issues in the creation of negative radiation pressure and have different advantages and drawbacks. "Metamaterials are addressing a set of issues where you try to confine the light to smaller, special spaces," Podolskiy explains. In contrast, the birefringence approach "does the opposite. It brings negative refraction to the level of larger-scale objects." Both approaches could someday find applications.
Jack Ng, a research assistant professor of physics at the Hong Kong University of Science and Technology who worked on the tractor beam proposal involving charge induction, says that the study may have some interesting ideas but also some flaws. In particular, he says that although the group showed the energy transfer can be negative, they "did not show that the force can be negative." In other words, the particles may not move.
In any case, the idea of generating negative radiation pressure by any means exists largely on paper; Segev's lab does not even have the resources required to create its proposed waveguide. Segev says, however, that several companies can make the necessary materials, and the researchers hope to find a firm soon so they can test their design experimentally. Until then, particles will just have to wait to experience the feeling of being drawn toward the light.
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