Can crystal symmetry reshape ENZ photonics?: Opinion

Over the last decade, epsilon-near-zero (ENZ) photonics has been driven by the search for lower losses and stronger nonlinear responses. Here, I ask a different question: can crystal symmetry also be used to shape the ENZ response? I focus on low-symmetry conductors, where the geometry of the electronic states can produce electric currents that are not present in ordinary Drude materials. These currents may enable polarization-dependent gain, nonreciprocal effects, and ultrafast nonlinearities controlled by symmetry and an external bias. I suggest that such materials may be useful for active and time-varying nanophotonics.