Nearly every gadget you might usefrom a hair dryer to a Gameboyexploits the electron's negative charge by design. But apart from charge, electrons possess another fundamental traitnamely, spinthat could give rise to a whole new class of electronic devices. Researchers have managed to make use of spin in metals, specifically in the circuits within computer hard disk drives (see image). But they have been unable to manipulate spin well enough to harness it within semiconductor-based devices. Finding a way to use spin, however, could lead to super-speedy compact computers, and might also bring quantum computers within closer reach.
Findings published in this week's issue of Physical Review Letters (PRL 86, 4358) reveal one way in which scientists can create so-called "spin polarized" electric currents in semiconductor layers. Sergey Ganichev of the University of Regensburg in Germany and colleagues shined pulses of circularly polarized laser light on the surfaces of quantum wellssandwiches of different semiconductors that trap electrons in exceptionally thin middle layers. They knew that circularly polarized light could generate more electrons with either up or down spin. And they hoped the imbalance would create current, based on a rarely used property of quantum well materials: electron transport theory says that asymmetries in the crystal lattice will ensure that spin up and spin down electrons have opposite, non-zero average velocities.
They found that, as expected, spin polarized current flowed automatically in a direction perpendicular to the laser beam. And the direction of the current indicated that the laser had imparted angular, but not linear, momentum to the electrons. The researchers also discovered that they could readily reverse the direction of the current by simply reversing the direction of the circular polarization. In short, the system works "like a wheel on a road," Ganichev says. "If you change the rotation direction, then you change the linear direction too." The team has applied for a patent on a high-speed detector for circularly polarized light and hopes that the technique will lead to other advances.