Nanolasers Grown “On-Chip” For Faster Data Transfer

Researchers have found a way to grow lasers directly on the semiconductor chips present in your computer. The lasers are supposed to accelerate the process of sending and receiving large size files inside a computer or between two different machines. The idea is not something very new but the implementation of this idea is a milestone which was unconquered till now. Why was it not practical till now? Well, the reason lies in the fact that the best laser materials are incompatible with the silicon chips.

However, the scientists at the University of California have overcome this hurdle by culturing nanolasers on silicon substrate. These nanolasers are made from materials generally termed as exotic semiconductors. This accomplishment will help in faster computing and faster downloading in supercomputers. The process of data transfer within a laptop has become a bottleneck which is hindering the overall processing speed. The maximum speed that can be attained using the conventional copper wire cables is around 10 giga-bits per second. About a decade ago, this much data rate was more than sufficient. However, as of today it is regarded as very slow and the data transfer between the various components of the PC like RAM, hard disk, cache memory is proving to be a major obstruction putting constraints in the design of machines. As a result, the product engineers have to put these components as close as possible for fast transmission and low heat dissipation losses.

Photonics is the branch which deals with the opto-electronic hybrid devices which can send and collect data in a quicker manner. Data which is converted into light energy signal can travel faster as the light pulses travel at the speed of light. One more advantage of optical communications is that the attenuation is negligible and hence low power loss. But the difficulty is that silicon cannot be used as a laser material because most of the energy is wasted and a small fraction is successfully converted to laser beams. Also, the materials which are used to produce laser, do not have the same lattice structure as that of the silicon molecules so they cannot be easily grown silicon substrate layer.

The elements which are commonly used for making efficient laser sources belong to the third and fifth columns of the periodic tables and are hence called “III-V” semiconductors. These materials also possess a crystalline body structure but the size of individual atoms in these semiconductors and silicon are not equal. As a result, their crystal lattices do not coincide exactly when placed one over the other. So when the “III-V” materials are grown over a silicon substratum, the crystals of the laser semiconductors try to align themselves in a way so that they match the silicon lattice. This produces a strain on the original lattice structure causing problems in laser performance.

However, this difficulty has been solved by Connie Chang-Hasnain, professor of electrical engineering and computer science at Berkeley. She performed this feat by exploiting the properties of nanostructures and by using a process to continuously monitor and check the growth of the nanopillers. In this method, the scientists begin with keeping the silicon base in a chemical growth chamber. The temperature in this room is raised to about 400 °C and gases like arsenide, indium and gallium are introduced. Chang-Hasnain discovered that by controlling the ratios and flow of these gases, the lasers can be grown in a controlled manner. In this process, the growth starts from a point called the seed. This seed then germinates into an indium-gallium “nanopiller” to form a perfect crystal. Thus the seed prevents any deformities in crystal due to silicon layer. Finally a shell of same material is formed around the pillar. This method of growing nanostructures is cheaper and better than the previous methods and can be used to grow high quality nano lasers at lower temperatures.

When a nanopiller is pumped with light from some other laser source, the light is trapped in the pillar due to difference in the nature of silicon and pillar crystals. The light moves in a spiral manner in the nanostructure exciting photons to an energy level so that they are emitted. The nanolasers can also be pumped using electrical excitation. This technology is surely going to boost the data transfer rates to mind boggling speeds with estimated average speed reaching a whopping 50 GB per seconds!