Enhanced nonlinear optical response of nodal-loop materials

Toy model of a Hopf-link Fermi surface. Figure adapted from https://link.aps.org/doi/10.1103/PhysRevB.103.195125

Example of real material with multiple nodal linkages in reciprocal space. Figure adapted from https://link.aps.org/doi/10.1103/PhysRevB.102.035138