The elusive goal of integrating lasers and electronics has come a big step closer with the first growth of nanoscale lasers directly on silicon. The tiny lasers are made from compound semiconductors that can emit light far more efficiently than silicon itself can.
Integrating optical processing into electronic chips holds great promise for high-performance computing. Electronics are very good at processing information because electrons interact strongly with each other. However, as electrons are moved to transfer information, those interactions also cause background noise and weaken the signal, and so cutting-edge chips are pushing the limits of what electrons can do carrying signals on circuit boards and in the chips themselves.
In contrast, photons have little effect on each other, so they can transfer information much more efficiently than electrons. That's why fibre-optic cables have replaced wires in the main circuit boards of high-performance computers as well as in cables running kilometres or more.
Computers suffer a crucial limitation when it comes to working with light, however: although silicon can transmit and detect light signals, it can't generate light efficiently. Compound semiconductors such as gallium arsenide and indium phosphide are needed to make good lasers. Chip maker Intel and the University of California, Santa Barbara, have succeeded in bonding indium-phosphide lasers to silicon so tightly that light generated in the indium-phosphide layer is transferred into silicon light guides. However, such bonding is costly and cannot be integrated into standard chip manufacture, and it hasn't been possible to "grow" lasers made of those materials on silicon.
Now Connie Chang-Hasnain's group at the University of California, Berkeley, has overcome the crystalline mismatches between silicon and gallium-arsenide compounds that had blocked laser growth. That let them grow tapered hexagonal pillars of indium-gallium arsenide with bases only about half a micrometre across onto silicon chips. The semiconductors were grown using chemical vapour deposition in the same way that LEDs are created. These nanopillars act as lasers when an external laser shines on their top: the laser light bounces around inside the pillar, following a helical path from top to bottom, where some of the light leaks out.
In practical applications, the researchers expect that the nanolasers will be able to produce their own light, without the need for an external laser.
This demonstration is a crucial step on the path to integrating optics and electronics, but many remain to be made before we reach the goal, says lead author Roger Chen, also at the University of California, Berkeley.
Other key challenges that remain include transferring light from the laser to a light guide in the chip, and modulating the light.NewScientist