The microprocessor inside the common computer is a small programmable device comprised of smaller devices called transistors. These transistors are circuits that switch electric signals from ‘on’ to ‘off’ and vice versa, incorporating a technology called the complementary metal-oxide semiconductor (CMOS).
In modern microprocessors, the switching speed of transistors has been steadily increasing. However, overall microprocessor speeds have been held back by the quality of connections between transistors, limited by extant technology.
To improve this, optical solutions have been suggested, where signals between transistors are not transferred through a circuit but by shining a laser between them. If implemented, this technique would increase microprocessor speeds.
To make this happen, silicon, the main material in the CMOS industry, must work as a laser. When light of a particular frequency (colour) is shined on it, it must amplify the light’s power and then transmit it to nearby circuits.
By 2006, hybrid silicon lasers were developed to do just this, but the technology has not become ubiquitous due to integration challenges and high costs.
A recent paper published in Nature Photonics proposes a way out. Instead of silicon, researchers from the Paul Scherrer Institute (PSI) and ETH, both Swiss, and Politecnico di Milano, Italy, have demonstrated that the element germanium (Gr) can be made to lase cost-effectively and with fewer resources by stretchingit a little.
A slight tension is created in germanium by the way it is evaporated on silicon and the silicon is straightened. The tension induces a strain in germanium, and the researchers exploited this to create thin, strained layers of germanium on silicon strips.
Next, they chipped away the sides of the germanium layer along the length, rendering the layer ‘I’-shaped, whose middle section was a narrow constricted bridge, called a microbridge. Because strain is inversely proportional to the cross-sectional area, it is higher in the microbridge than at the ends.
The electrons in Gr atoms in the microbridge must jump from a higher energy to a lower energy. When this jump happens, energy is released in the form of photons. This results in lasing.
There is a compulsion to keep the microbridge in a strained state to allow even the energy-deficient electrons to jump.
By stretching the microbridge by 3.1 per cent, the team found that the material deforms enough to allow even the electrons that fall short by 3.2 x 10 joules to jump. In this stretched state, the microbridge emits around 25 times more photons than when in a relaxed state, which is “enough to build lasers,” says Richard Geiger, a doctoral student at PSI and part of the team.