Silicon forms the basis of everything from solar cells to the integrated circuits at the heart of our modern electronic gadgets. However the laser, one of the most ubiquitous of all electronic devices today, has long been one component unable to be successfully replicated in this material. Now researchers have found a way to create microscopically-small lasers directly from silicon, unlocking the possibilities of direct integration of photonics on silicon and taking a significant step towards light-based computers.
Whilst there has been a range of microminiature lasers incorporated directly into silicon over the years, including melding germanium-tin lasers with a silicon substrate and using gallium-arsenide (GaAs) to grow laser nanowires, these methods have involved compromise. With the new method, though, an international team of researchers has integrated sub-wavelength cavities, the basic components of their minuscule lasers, directly onto the silicon itself.
To help achieve this, a team of collaborating scientists from Hong Kong University of Science and Technology, the University of California, Santa Barbara, Sandia National Laboratories and Harvard University, first had to find a way to refine silicon crystal lattices so that their inherent defects were reduced significantly enough to match the smooth properties found in GaAs substrate lasers. They did this by etching nano-patterns directly onto the silicon to confine the defects and ensure the necessary quantum confinement of electrons within quantum dots grown on this template.
The researchers were then able to use optical pumping, which is a process in which light is used to raise or "pump" electrons from a lower energy level to a higher one, to demonstrate that the devices they created were able to operate as lasers.
"Putting lasers on microprocessors boosts their capabilities and allows them to run at much lower powers, which is a big step toward photonics and electronics integration on the silicon platform," says professor Kei May Lau, Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology. "Our lasers have a very low threshold and match the sizes needed to integrate them onto a microprocessor, and these tiny high-performance lasers can be grown directly on silicon wafers, which is what most integrated circuits (semiconductor chips) are fabricated with."
Ordinarily, lasers used in consumer electronics devices are reasonably large for an electronic component, at around 1 x 1 mm. This is because lasers any smaller than this suffer what is known as large mirror loss due to mismatching at the waveguide/mirror junction. However, with the new silicon lasers being some 1,000 times shorter in length, and 1 million times smaller in area at around 1 micron in diameter, the researchers were able to avoid this issue by using whispering gallery mode lasers. A whispering gallery is a circular or hemispherical structure where whispered acoustic communication is possible from any part of the internal circumference to any other, and the same effect can be replicated with light.
The researchers expect that an immediate use for the technology, when brought to market, would be in the realm of high-speed data communications.
"Photonics is the most energy-efficient and cost-effective method to transmit large volumes of data over long distances. Until now, laser light sources for such applications were 'off chip' – missing – from the component," said Lau. "Our work enables on-chip integration of lasers, an [indispensable] component, with other silicon photonics and microprocessors."
The next step for this research, according to Lau, is to see if it is possible to create electrically-pumped lasers using standard microelectronics technology.
The results of this research were recently published in the journal Applied Physics Letters.
Source: AIP Publishing