Low-symmetry nanophotonics

Abstract

Photonics and optoelectronics are at the foundations of widespread technologies, from high-speed Internet to systems for artificial intelligence, automotive LiDAR, and optical quantum computing. Light enables ultrafast speeds and low energy for all-optical information processing and transport, especially when confined at the nanoscale level, at which the interactions of light with matter unveil new phenomena, and the role of local symmetries becomes crucial. In this Perspective, we discuss how symmetry violations provide unique opportunities for nanophotonics, tailoring wave interactions in nanostructures for a wide range of functionalities. We discuss geometrical broken symmetries for localized surface polaritons, the physics of moiré photonics, in-plane inversion symmetry breaking for valleytronics and nonradiative state control, time-reversal symmetry breaking for optical nonreciprocity, and parity-time symmetry breaking. Overall, our Perspective aims at presenting under a unified umbrella the role of symmetry breaking in controlling nanoscale light, and its widespread applications for optical technology. The invention of fiber optics, in which light is trapped in the lateral direction through total internal reflection, has enabled high-speed and efficient long-distance optical communications. Highly efficient light propagation and the availability of advanced repeaters, amplifiers and remarkable bandwidth have made this technology ideal for broadband connections well above 1 Gbps. Fig. 1a shows a map of intercontinental submarine fiber-optic connections, through which the vast majority of information is transmitted today. It is difficult to overestimate the value of this technology, which emerged in the middle of the last century but found its use in vital applications much later. However, this technology faces a significant bottleneck: when information is received in the form of optical pulses at one of the data centers, it must be converted into an electrical form to be processed electronically. Electronic components run much slower than light and are characterized by large energy consumption. According to the U.S. Department of Energy, some of the world's largest data centers contain tens of thousands of electronic devices and use over 100 megawatts (MW) of power enough energy to power about 80,000 U.S. households. Most of this energy goes into heat and it is eventually lost. All-optical signal processing and transport holds the promise for large improvements in terms of speed and efficiency of today’s data centers. Indeed, the idea of all-optical computing has been gaining traction in recent years. Unlike electrons, photons are bosons and, as such, they do not interact with each other in linear media, which offers challenges in the context of data processing and computing, but also various new opportunities. Indeed, the bosonic nature of photons allows them to maintain a quantum state for a long time, much longer than the typical quantum coherence time in solid-state quantum systems, including quantum dots (QDs), defect centers (for example, NV centers) and quantum circuits with Josephson junctions. The robustness of the quantum state of light has enabled applications in quantum cryptography and emerging optical quantum computing. In addition,