Optical signal transmission and processing is the hardware basis for internet communication, which enables the modern Information Age. Silicon photonics promises to deliver high performance integrated photonic systems on semiconductor chips for this and other applications such as computing and sensing, fabricated at a low cost in CMOS foundries. These applications require a wealth of integrated optical devices, such as couplers, filters and mode converters, all of which benefit from specific material properties for optimal performance. Introducing new materials into CMOS process is often impractical, but the high resolution of modern lithography techniques enables an altogether different solution: creating metamaterials with nano-patterned silicon waveguides – so-called subwavelength gratings. When we started to work on this topic in the mid-2000’s the properties of subwavelength gratings on surfaces for applications in free-space optics were already quite well known in the decades-old field of subwavelength diffractive optics. It was this work that inspired us to pioneer the use of subwavelength gratings in integrated photonic waveguide devices at a time when the term “metamaterials” was used predominantly for complex metallic structures designed to demonstrate negative index materials. Our paper outlines the story of how these tiny blocks of silicon have become the ultimate tool to design tomorrow’s integrated photonic devices.
A silicon photonic waveguide, shown on the left-hand side of the title figure, is only about 500 nm across – that’s around twenty times thinner than the core of an optical fiber. Efficiently coupling light from a silicon photonic chip to a fiber is thus very challenging, but, at the same time, indispensable for information to flow between the chip and the rest of our connected world. It was precisely in trying to solve this light coupling conundrum that we first proposed the use of subwavelength gratings in the year 2006. As illustrated in the title figure, a lightwave that couples from a silicon waveguide into a subwavelength grating gradually expands. This is because in the latter it experiences an overall lower index, and, consequently is less confined than in the silicon waveguide. With a judicious design, this property can be exploited to substantially expand the mode size. Indeed, placing this type of mode converter at the chip facets allows efficient and broadband coupling to an optical fiber.
It soon became obvious that the ability of subwavelength gratings to engineer effective materials with a wide range of refractive indices could be leveraged to enhance the performance of many other integrated photonic devices. Around the year 2010, these structures were already being successfully used for apodized grating couplers, low loss waveguide crossings, and wavelength multiplexers, to name a few. With subwavelength gratings being relatively easy to design, and amenable to simple, single-etch fabrication processes, the idea of refractive index engineering has since become widely adopted, and many new integrated devices with unprecedented performance are being reported from groups all around the world.
While silicon is an optically isotropic material, a subwavelength grating presents a strong birefringence arising from the arrangement of the silicon blocks, making it akin to uniaxial optical crystals. Researchers have only very recently begun to exploit this unique property in device design, with surprising results: waveguide couplers with record-breaking bandwidths (shown in the figure below), and densely packed waveguides with minimal crosstalk have been demonstrated.
We anticipate that these properties, in combination with techniques of transformation optics, will open new venues to design ultra-compact silicon devices with unmatched performance. Indeed, we believe that these tiny blocks of silicon will become the preferred way to create materials with bespoke properties in future integrated photonic systems-on-a-chip.
If you want to learn more about this exciting topic head over to Nature for our full review paper.