A new study conducted by U of T professor Dr. Sajeev John and PhD student Xu Ma sheds light on the potential for future developments in optical information processing, which could eliminate major bottlenecks experienced with current electronic devices.
By researching the interaction of laser beams with quantum dots (more commonly known as artificial atoms) within a periodic optical nanostructure called a photonic crystal, John and Ma identified unique characteristics of the quantum dots that make it easy to excite, or unexcite, the dots with short optical pulses.
In order for the light to propagate through the crystal and reach the quantum dots, a circuit path must be established. By removing the periodic dielectric units from the photonic crystal, a waveguide was created within the structure. Despite its simplistic application, this waveguide is unlike conventional fibres. Such fibres “confine light through total internal reflection,” explains John. “A different mechanism occurs within a photonic crystal [since] light is not being guided through air, [but] through a high dielectric material.”
“In ordinary materials and space, atoms would decay quickly via spontaneous emission. [However] in these materials, it is possible to control how these atoms can be excited, remain excited, or be de-excited using the laser pulse,” says John.
Once the quantum dots have been excited, they can affect the propagation of another pulse. “This process of controlling the passage of a signal pulse with another pulse creates an optical switch, controlling light with light,” says Ma. A similar phenomenon occurs in electronic transistors where one current controls another by using a logical operation processing the information. The difference is that by using optical logic, the process can occur at low power and one hundred times faster than the electronic method.
A previous project attempted to control the quantum dot’s state using a continuous laser beam. While using a similar mechanism, these short laser pulses can perform at a faster speed, and at lower power consumption than the continuous laser beam. They can also produce a larger switching contrast making them more capable of switching the optical switch on and off. Optimally, the laser pulses would be made in terms of solitans (a self-reinforcing wave pulse), which preserve their geometry during propagation; the geometric property of solitans would enhance the efficiency of the laser pulses in changing the medium from active to passive.
The problem of heat buildup experienced with electronic devices can be avoided, since the optical materials do not have a resistance to the flow of light (as semi-conductors and electronic processors have to the flow of electrons). The potential heat produced limits the size of electric circuits because the denser they are packed, the more heat is generated, which could damage the device. “In photonic devices there is no issue of heat dissipation. “[In contrast to electrons], photons have low interaction among themselves. You can send in many channels of information along one waveguide; in electronics on one metal wire you can only send one electric current,” says Ma.
The present development consists of prototype materials. New tools are required to lower the cost and increase the production level to a commercially viable output. “[Just as when] Edison was making the light bulb, [he thought it] would be a niche market for a few rich people, he said that he’d make the light bulb so cheap that only the rich could afford to burn candles. The same sort of challenge applies now,” says John.
In addition to optical information-processing, this discovery could vastly improve solar cell efficiency. “The biggest bottleneck in the efficiency of solar cells right now is the amount of light that gets absorbed. If we can improve the light trapping and absorption efficiency, we might improve the efficiency of these cells by a factor of 30 or more,” says John.
The study was published in the journal Physical Review Letters in 2009. The research is noteworthy not only because it could revolutionize optical information-processing, but also due to its application in renewable energy advances. “Given that technology advances over time, there is a good chance for breakthrough mass-production techniques,” says Ma.
