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.