Laser quest
A breakthrough in silicon-based lasers could pave the way for lightning fast communications devices and innovative medical applications. European Semiconductor Magazine takes a look.
Intel has developed the worlds first continuous wave silicon laser using standard silicon manufacturing processes. This technology could help bring low-cost lasers and optical devices to mainstream use in computing, communications and medical applications. Researchers from the worlds largest chip maker achieved the breakthrough by exploiting the so-called Raman effect and silicons crystalline structure to amplify light.
Raman lasers and amplifiers are already used in the telecommunications industry but they rely on literally miles of fibre to amplify light. By using silicon, Intel was able to generate a continuous laser beam using a chip just a few centimetres in size.
While still far from becoming a commercial product, the chip paves the way for lasers to be built inexpensively from standard silicon. “Fundamentally, we have demonstrated for the first time that standard silicon can be used to build devices that amplify light,” said Dr Mario Paniccia, director, Intels Photonics Technology Lab. “The use of high-quality photonic devices has been limited because they are expensive to manufacture, assemble and package. This research is a major step towards bringing the benefits of low-cost, high-bandwidth silicon based optical devices to the mass market.” Possible applications for the new silicon-based lasers could include inexpensive optical devices for transferring data inside and between computers at the speed of light.
The lasers could also be used in medical applications. There are special wavelengths of light that are optimal for interactions with human tissue. For example, one type of laser wavelength is useful for working on gums and another one for excavating cavities in teeth. At present, these lasers cost tens of thousands of euros each, limiting their use.
Building a Raman laser in silicon begins with etching a waveguide – a conduit for light on a chip. Silicon is transparent to infrared light so when light is directed into a waveguide it can be contained and channelled across a chip. As with the first laser developed in 1960, Intel researchers used an external light source to “pump” light into their chip.
As light is pumped in, the natural atomic vibrations in silicon amplify the light as it passes through the chip. This amplification – the Raman effect – is more than 10,000 times stronger in silicon than in glass fibres. By coating the sides of the chip with a reflective thin-film material, similar to coatings used on high-quality sunglasses, the team was able to contain and amplify the light as it bounced back and forth inside the chip. As they increased the pump energy, a critical threshold point was reached where instantaneously a very precise beam of coherent light – a laser beam – exited the chip.
Initially, the Intel researchers discovered that increasing the light pump power beyond a certain point no longer increased amplification and eventually even decreased it. The reason was a physical process called two-photon absorption, which occurs when two photons from the pump beam hit an atom at the same time and knock an electron away. These excess electrons build up over time and collect in the waveguide until they absorb so much light that amplification stops.
Intels breakthrough solution was to integrate a semiconductor structure, technically called a PIN (P-type-Intrinsic-N-type) device, into the waveguide. When a voltage is applied to the PIN, it acts like a vacuum and removes most of the excess electrons from the lights path. The PIN device combined with the Raman effect produces a continuous laser beam.
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