T2B: Carbon nanotubes and opportunities for wireless on-chip interconnect

With the growing sophistication of systems-on-chip and networks-on-chip (NoCs), communication among the various parts of such systems is becoming increasingly difficult and interconnect is presenting one of the main bottlenecks to further scaling and increased integration densities. Indeed, materials innovation and improvements to traditional interconnect schemes may not be sufficient for continuing the scaling trend for much longer, and new paradigms might be required. One such paradigm is that of wireless NoCs, where at least a portion of the interconnect lines are replaced by wireless links. However, to implement this scheme using even the highest available electronic frequencies requires antenna devices with dimensions at least on the millimeter scale, meaning that unacceptably large areas of the chip would be taken by these devices and limiting the number of such wireless links. Going to substantially higher frequencies, that is optical frequencies, where the corresponding wavelengths are in the micro/nano scale and the antennas would thus be much smaller, may represent a viable approach to tackle this challenge. The optical properties of carbon nanotubes and, indeed, other nanoscale wires, represent an interesting and rich regime of operation. On one hand, given their electronic band structure and typical band gap values, the interband transitions in these devices, like in many other solid-state device, could lead to the generation or absorption of visible or infrared light. On the other hand, these nanoscale wires can be made to lengths on the order of several hundreds of nanometers or micrometers, comparable to the wavelength of the radiation being emitted or absorbed. Therefore, their geometry could play a significant role in the distribution of the electromagnetic wave and how the emitted or absorbed light interacts with them, similar to the situation in radio or microwave antennas (albeit at much shorter wavelengths). In other words, both the quantum nature of light and its wave nature are manifest at the same time, combining the worlds of light-emitting diodes and photodetectors with that of electromagnetic antennas. This leads to a rich set of effects and possibilities. In this tutorial, we will review the optical absorption and emission properties of carbon nanotubes and describe how these properties could potentially enable us to establish a wireless network at the small scale for interconnect applications. Devices could be implemented where the signal generation/detection and antenna functionalities are combined. Other devices may only benefit from a subset of these functionalities enabled by nanotubes. We will also discuss some of the performance benefits that may be achieved through such an approach.