Nanostructured silicon delivers unprecedented optical devices

On-chip optical and photonic devices are key to major advances in fields as diverse as optical communications, sensing, and quantum physics. These integrated devices enable complex optical functionalities on a single chip (i.e., within a few square millimeters) that might otherwise occupy an entire optical table when implemented with bulk optical components. Currently, many commercial photonic chips are made from group III–V materials (i.e., containing elements in groups 13 and 15 of the periodic table), such as indium phosphide. Over the past decade, integrated photonic systems based on group IV materials—elements in group 14, particularly silicon and germanium—have drawn a lot of attention and are being developed by research groups around the world as well as industrial players, such as IBM and Intel. The main advantage of silicon photonics is that the CMOS infrastructure of the micro-electronics industry can be leveraged, potentially leading to high-volume and low-cost fabrication. However, in terms of performance and optical bandwidth—the range of optical wavelengths (colors) that a device can process accurately—many integrated photonic devices cannot yet compete with their bulkoptics counterparts. Here, we present a new silicon optical waveguide device that offers high performance and ultra-broad bandwidth operation with a very compact footprint. In photonic devices, the flow of light is governed by variations in refractive index, which engineers exploit in a range of materials to enable optical functionalities (e.g., for optical waveguides). In silicon photonics, the choice of materials is limited to silicon (with a refractive index n 3.5), silicon dioxide (n 1.4), and several polymers (n 1.6), which hinders the fabrication of high-performance, high-bandwidth devices. This limitation can be overcome using layers of materials with different thicknesses, which produce different Figure 1. A schematic representation of a new on-chip optical beamsplitter based on a nanostructured silicon multimode interference coupler showing the input (left) and output (right) light waves.