Data centers are spaces that store, process, and disseminate data. They enable everything from video streaming to cloud computing. They also consume large amounts of energy to transfer data from one location to another. Data centers must be more efficient with increasing data demand.
High-powered servers are located in data centers. They communicate with each other via interconnects. These physical connections allow for the exchange and transfer of data. To decrease energy consumption within data centers, light can be used to connect information using electrically controlled optic switches that control the stream of light and communication between them. To support data center expansion, these optical switches must be multifunctional and efficient. A team of scientists from the University of Washington published a paper online on July 4th in Nature Nanotechnology. They described the creation of an efficient, silicon-based, non-volatile switch that manipulates sunlight through a phase change material and graphene heater.
Arka Majumdar is a UW professor in physics, electrical, and computer engineering. She is also a faculty member of the Institute for Molecular & Engineering Sciences and the UW Institute for and Nano-Engineered Systems. This technology will significantly reduce energy consumption in data centers for controlling photonic circuits compared to what is currently used. It will also make them more sustainable and eco-friendly. Because they are easy to make, silicon photonic switches are very popular. These switches were previously tuned by thermal effect. This is a process in which heat is applied to a material, often a semiconductor or metal, to change its optical properties and alter the path of light. This process is not efficient, and the effects it causes are not permanent. The current is removed, and the material returns to its original state. This means that the information flow and connection are broken.
Researchers have previously used doped silicon to heat phase-change materials. Although silicon alone cannot conduct electricity, it can be selectively doped with elements such as boron or phosphorus so that it can transmit electricity and light without excess absorption. A current can be pumped through doped silicon to change the phase-change material. This is not an energy-efficient process. The energy required to switch the phase-change materials is comparable to traditional thermo-optic switches. The doped silicon layer of 220 nanometers (nm thick) must be heated to convert only 10 nm phase-change material. Switching a smaller volume of phase-change material takes a lot of energy to heat such a large amount of silicon.
A thinner silicon film could be an option, but silicon won’t spread light well if it’s thinner than 200 nanometers. Instead, they used a 220 nm un-doped silicon layer to propagate light. They also added a layer of graphene between silicon and the phase-change material to conduct electric power. Graphene, which is similar to metal, is an excellent conductor. However, unlike metal, it’s atomically thin. It consists of a single layer made up of carbon atoms that are arranged in a honeycomb lattice. This design reduces waste energy as all graphene heat is directed to the change of the phase-change material. This setup’s switching energy density, which is calculated as the switching energy divided by the volume of the material to be switched, is only 8.7 attojoules, a 70-fold decrease compared with the widely-used doped silicon heaters. This is also within one-tenth of the fundamental limit for switching energy density (1.2 J/nm).
Although graphene conducts electricity and can cause some optical losses (meaning some light is absorbed), graphene is thin enough that the phase-change material and light propagating through the silicon layer can interact. A graphene-based heater was found to reliably change the state of the phase-change material for more than 1,000 cycles. This is an improvement on the doped silicon heaters, which only have a lifespan of 500 cycles.
