Nanostructured Silicon Success

A reduction in the size of integrated optical devices and components while maintaining a high level of performance is a key challenge in photonics. The footprint of devices becomes even more critical for applications with limited physical space such as on-chip, silicon-based photonic devices. Optical interconnects are recently emerging as a promising approach for onand off-chip communications owing to their ability to transfer data with higher bandwidths, faster speeds and less power dissipation compared with their electronic counterparts. However, photonic components currently used in optical interconnects are relatively large and not ideally suited to on-chip, high-density integration. Although plasmonics1 can provide substantial size reduction for optical components, it comes with the cost of optical losses that are often undesired. An all-dielectric platform for controlling the flow of light at subwavelength dimensions could enable highly attractive ultra-compact, low-loss and high-performance photonic devices. Now, reporting in Nature Photonics, two independent research groups experimentally demonstrate small-footprint, all-dielectric, on-chip components — a wavelength demultiplexer2 and a polarization beamsplitter3 — that are inverse-designed using optimization algorithms. Both sets of devices are based on complex nanophotonic structures, consisting of patterns of specially shaped air holes etched into silicon (Figs 1a,b). Importantly, the actual shape and pattern of the etched holes comes from an algorithm that has the desired functionality and optical performance of the device as its inputs. By engineering the dielectric permittivity of silicon locally at the subwavelength scale by introducing tiny air holes, it is possible to manipulate the flow of light inside the silicon through mode conversion (Figs 1c,d). These two papers2,3 are the first experimental demonstration of on-chip, silicon photonic components based on complex all-dielectric nanophotonic structures. Wavelength-division multiplexers (WDMs) and polarization beamsplitters (PBS) are both important components for optical interconnects and are used to filter either the wavelength or polarization of light, respectively4. Conventional WDMs such as arrayed waveguide gratings, echelle diffraction gratings, and microring resonators are relatively large and their dimensions can be anywhere from tens of micrometres to hundreds of micrometres depending on the number of WDM channels4,5. The silicon-on-insulator platform is widely used for creating such WDMs and the devices can be designed using parameters such as the periodicity of the gratings, waveguide width, silicon thickness and microresonator radius, for example. Due to the diffraction limit of light, it is rather difficult to further reduce the size of conventional WDMs. Recently Lu and Vučković6 proposed an inverse-design approach, opening the full parameter space for designing various linear, INTEGRATED OPTICS