Researchers have found that lasers can generate stable patterns of electron spins in a thin layer of semiconductor material, a discovery that may help lead to advanced spin-based memory and computing. The scientists have revealed that lasers could generate complex stable patterns of electron spins called “spin textures” in thin films of semiconductors. These spin textures could help lead to what may be the holy grail of spintronics, a superefficient spin-based transistor.
The new findings are based on how light has momentum, just as a physical object moving through space does, even though light does not have mass. This means that light shining on an object can exert a force. Whereas the linear momentum of light supplies a push in the direction that light is moving, the angular momentum of light applies torque.
A ray of light can possess two different kinds of angular momentum. The spin angular momentum of a beam of light can make objects it shines on rotate in place, whereas its orbital angular momentum can make objects rotate around the center of the ray. A beam of light that carries just spin angular momentum is circularly polarized. This means the way in which its electric and magnetic fields ripple through space rotates along the axis of the ray much like threads on a screw.
In contrast, a beam of light that carries just orbital angular momentum resembles a vortex, moving through space with a spiraling pattern like a corkscrew. Whereas a conventional light beam is brightest at its center, vortex beams have ringlike shapes that are dark in the center, due to how some of the waves making up vortex beams can interfere with one another.
In the new study, the researchers experimented with laser beams that simultaneously carry both spin and orbital angular momentum. In these “vector vortex beams,” the electric and magnetic fields vary in a rotating manner around each beam’s dark center.
The scientists found that vector vortex beams could imprint a persistent helix-shaped spin texture within gallium arsenide quantum wells 20 nanometers deep. The vector vortex beams could generate spin textures in roughly 10 picoseconds, about 10 times as fast as conventional lasers.
A potentially extraordinarily useful property of vortex beams is that they do not interfere with each other if they all possess different twisting patterns. In telecommunications, this fact may let a theoretically infinite number of vortex beams get overlaid on top of each other to carry an unlimited number of data streams at the same time. This multiplexing capability may also prove useful when it comes to spintronics, says study lead author Jun Ishihara, a physicist at Tohoku University in Japan.
These preliminary experiments were conducted at -266.15 °C, Ishihara iterates. Moving forward, “the key obstacle foreseen is how to achieve room-temperature operation,” he says.