  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 moir\'e 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.
