Data centers—dedicated spaces for storing, processing and disseminating data—enable everything from cloud computing to video streaming. In the process, they consume a large amount of energy transferring data back and forth inside the center. With demand for data growing exponentially, there is increasing pressure for data centers to become more energy efficient.
In a paper published online July 4 in Nature Nanotechnology, a team led by University of Washington scientists with researchers from the University of Maryland, Stanford and MIT, reported the design of an energy-efficient, silicon-based non-volatile switch that manipulates light through the use of a phase-change material and graphene heater.
Silicon photonic switches are widely used in part because they can be made using well-established semiconductor fabrication techniques.Traditionally, these switches have been tuned through thermal effect, a process where heat is applied, often by passing a current through a metal or semiconductor, to change the optical properties of a material in the switch, thus changing the path of the light. However, not only is this process not energy-efficient, but the changes it induces are not permanent. As soon as the current is removed, the material reverts to its previous state and the connection – and flow of information – is broken.
One approach would be to make a thinner silicon film, but silicon doesn’t propagate light well if it is thinner than 200 nm. So instead, the team used an un-doped 220 nm silicon layer to propagate light and introduced a layer of graphene between the silicon and phase-change material to conduct electricity. Like metal, graphene is an excellent conductor of electricity, but unlike metal, it is atomically thin – it consists of just a single layer of carbon atoms arranged in a two-dimensional honeycomb lattice. This design eliminates wasted energy by directing all heat generated by the graphene to go towards changing the phase-change material. In fact, the switching energy density (which is calculated by taking the switching energy divided by the volume of the material being switched) of this setup is only 8.7 attojoules (aJ)/nm3, a 70-fold reduction compared to the widely used doped silicon heaters, the current state-of-the-art. This is also within one order of magnitude of the fundamental limit of switching energy density (1.2 aJ/nm3).
Even though using graphene to conduct electricity induces some optical losses, meaning some light is absorbed, graphene is so thin that not only are the losses minimal, but the phase-change material can still interact with the light propagating in the silicon layer. The team established that a graphene-based heater can reliably switch the state of the phase-change material more than 1,000 cycles. This is a notable improvement over the doped silicon heaters, which have only been shown to have an endurance of around 500 cycles.
"We have demonstrated the lowest energy approach--close to the theoretical limit--to switch phase change materials on photonic circuits. This would enable low-energy on-chip photonic technologies for both classic and quantum telecommunications and computing," said UMD Assistant Professor in Materials Science, Carlos Rios Ocampo.