First germanium laser could pave way for optical computers
One of the issues of current chip design is the excessive power needed to transport and store ever increasing amounts of data. A possible solution is to use optics not just for sending data, but also to store information and perform calculations, which would reduce heat dissipation and increase operating speeds. Disproving previous beliefs in the matter, MIT researchers have demonstrated the first laser built from germanium which can perform optical communications... and it's also cheap to manufacture.
As Moore's law keeps giving us faster and faster computers, chip builders also need higher-bandwidth data connections. But excessive heat dissipation and power requirements make conventional wires impractical at higher frequencies, which has lead researchers to develop new ways to store, transmit and elaborate optically-encoded information.
If optical-based data elaboration is to have a future, researchers will need to find a cheap and effective way to integrate optical and electronic components onto silicon chips.
The solution found by the MIT team and detailed in a paper published in the journal Optics Letters is notable not only because it achieves these objectives, but also because it changes the way physicists have been looking at a class of materials that were previously thought to be unsuitable for manufacturing lasers.
In a semiconductor, electrons that receive a certain amount of energy enter a "conduction band" and are free to conduct electrical charge. Once they fall out of this excited state, the electrons can either release their energy as heat or as photons. Materials such as the expensive gallium arsenide are thought to be the best for manufacturing lasers, because their excited electrons tend to go fall back into the photon-emitting state.
However, the MIT team demonstrated that materials such as germanium, whose electrons would normally tend to go in the heat-emitting state, can be manipulated to emit photons and used to produce lasers that are cheap not only because of the cost of the materials, but also because the processes used to build them are already very familiar to chip manufacturers.
The researchers found two ways to make germanium "optics-friendly". The first is a technique called "doping," which involves implanting very low concentrations of a material such as phosphorous to force more electrons in the conduction band and modify the electrical properties of the material.
The second strategy was to "strain" the germanium, pulling its atoms slightly farther apart than they would be naturally by growing it directly on top of a layer of silicon. This makes it easier for electrons to jump into the photon-emitting state.
The team now needs to find a way to increase the concentration of phosphorus atoms in the doped germanium to increase the power efficiency of the lasers, making them more attractive as sources of light for optical data connections and, one day, for computing as well.
The work is part of the Si-Based-Laser Initiative of the Multidisciplinary University Research Initiative (MURI), and was sponsored by the Air Force Office of Scientific Research (AFOSR).