Italian physicists have taken a crucial step towards the creation of a silicon laser -- the holy grail of 'optoelectronics', the marriage of electronic and light-based information technology.
Miniaturized solid-state lasers made from semiconducting materials liberated information technology from its reliance on electronics and magnetism. We can now use light to store and transmit information.
But electronic integrated circuits are carved in silicon and silicon refuses to emit light efficiently. The lasers that read CDs and send light pulses down optical cables are made from different, incompatible semiconductors such as gallium arsenide.
And gallium arsenide doesn't stick to silicon. So semiconductor lasers can't easily be fabricated on silicon chips. This makes current optoelectronic technology cumbersome, a shotgun marriage of two unlikely partners.
If only silicon itself would emit light. An electrical current injected into gallium arsenide stimulates the release of energy as photons, particles of light. But in silicon the energy gets squandered in other ways.
Silicon's ability to emit light can be enhanced by cutting it into very small pieces. If the material is fashioned into wires, sheets or lumps measuring just a few nanometres (millionths of a millimetre) across, it begins to glow when electrically stimulated. Quantum mechanics, which takes over at very small sizes, relieves the factors that normally suppress the formation of photons.
Lorenzo Pavesi of the University of Trento in Italy and colleagues have taken advantage of these 'quantum size effects' to wring something close to laser-like emission from silicon, as they report in
Says Leigh Canham, a British physicist who was the first to see light being emitted from silicon as a result of quantum size effects, the work is "a major milestone in our attempts to develop silicon-based optoelectronics".
Pavesi's team made 'nanoparticles' of pure silicon, just three nanometres across, by firing high-energy ions into silicon dioxide, in its mineral form, quartz. They gave the same treatment to a thin layer of silicon dioxide grown on a silicon chip, to show that the nanoparticles could be made on a chip.
Many researchers have shown light emission from silicon nanoparticles. But making laser light is something else.
In a laser, a whole horde of photons is conjured up at once by 'stimulated emission'. The photons bounce back and forth between mirrors, liberating others on each pass, thereby amplifying the light pulse. Eventually all the photons escape in a very bright, focussed beam, all vibrating in step.
This 'light-amplified stimulated emission of radiation' (from which the acronym 'laser' derives) can also be induced by illuminating the 'lasing' material. Pavesi's group found that when they directed a conventional ultraviolet laser onto their silicon nanoparticles, the particles emitted red and infrared light.
If the stimulated-emission process characteristic of laser action is taking place, a 'probe' laser beam of the same wavelength should gain in brightness when passed through the material. The researchers detected this telltale sign in their samples.
This does not in itself amount to true laser action, but it demonstrates that laser action could be possible from these specks of silicon.