Japanese scientists have revealed that lasers can produce complex and stable electron spin patterns in semiconductor films

For decades, scientists have been exploring spintronics, which manipulates the spin of electrons to theoretically run faster and use less energy than conventional electronic devices. Now researchers have found that lasers can produce stable electron spin patterns in thin layers of semiconductor materials, a finding that could help enable advanced spin-based memory and computing, a new study finds.

One can imagine the spin of an electron whose axis of rotation points up or down, similar to a compass needle pointing north or south. Just as the presence or absence of an electric charge can represent bits equal to 1 or 0, the same can be done for spins up or down.

Flipping the spin to change a point takes less time and effort than moving the charge. This means that spintronics theoretically use less power than conventional electronics. In addition, they can retain data even when the device is turned off. Spin is already used in storage devices, such as reluctance RAM (MRAM).

The spins of electrons in a material can be arranged by magnetic fields or light. Now, scientists in Japan have revealed that lasers can produce complex stable electron spin patterns, called "spin textures," in semiconductor films. These spin textures could help achieve the holy grail of spintronics -- an ultra-efficient spin transistor.

The new discovery is based on how light has momentum, like a physical object moving through space, even though it has no mass. This means that light shining on an object can generate force. The linear momentum of light provides thrust in the direction the light is moving, while the angular momentum of light exerts torque.

A ray can have two different angular momenta. A beam of light's spin angular momentum can cause the object it hits to rotate in place, and its orbital angular momentum can cause the object to rotate around the center of the light.

Light beams carrying spin angular momentum alone are circularly polarized. This means that the way its electric and magnetic fields fluctuate in space rotates along the axis of the light, like threads on a screw.

In contrast, a beam carrying only orbital angular momentum resembles a vortex, moving in a spiral pattern through space, like a corkscrew. While a conventional beam is brightest at its center, a vortex beam has a ring shape but is dark at the center, due to how some of the waves that make up a vortex beam can interfere with each other.

In the new study, researchers experimented with laser beams carrying both spin and orbital angular momentum. In these "vector vortex beams," electric and magnetic fields rotate around the dark center of each beam.

The scientists found that vector vortex beams can leave a persistent spiral spin texture inside a gallium arsenide quantum well 20 nanometers deep. Vector vortex beams can generate spin textures in about 10 picoseconds, about 10 times longer than conventional lasers.

One potentially very useful property of vortex beams is that they do not interfere with each other if they all have different distortion patterns. In telecommunications, this fact could allow a theoretically infinite number of vortex beams to stack on top of each other while carrying an infinite number of data streams. This multiplexing capability could also be useful in spintronics, said Jun Ishihara, lead author of the study and a physicist at Tohoku University in Japan.

Ishihara cautions that these preliminary experiments were conducted at -266.15 °C. Looking ahead, "the main hurdle for foresight is how to achieve room temperature operation," he said.

Ishihara and his colleagues detailed their findings March 24 in the journal Physical Review Letters.

Source: Laser Net