Physicist Richard Feynman once gave a lecture titled “There is plenty of room at the bottom.” This lecture is often quoted to highlight the successes of modern micro- and nano-fabrication techniques, and the value of available space that comes with advances in miniaturization. In this respect, silicon, the bedrock of modern computers, mobile communications, and photonic devices, has proven to be extremely capable. These advances are usually described in terms of Moore’s law. However, modern processors are essentially stacks of planar structures. In this sense, silicon microelectronics and photonics are still 2-D.