Nonlinear optical responses in nodal line semimetals

Nonlinear optical responses, such as shift current and second-harmonic generation (SHG), of topological semimetals have been subjects of great interest because the momentum-space topological structures near Weyl or Dirac points lead to unusually large responses. This is especially important for technological applications as the shift current and SHG are often used for solar cell and frequency-changing applications for lasers, respectively. We demonstrate that nodal line semimetals can offer even larger responses at low frequencies. For example, we show that a large SHG response arises at finite doping when an external dc electric field is applied to break the inversion symmetry of an otherwise inversion-symmetric model of a nodal line semimetal. Furthermore, we introduce a model with intrinsically broken inversion symmetry, where both the shift current and SHG are singular in a range of parameters.

Automated summary

Nonlinear optical responses in topological semimetals, such as shift current and second-harmonic generation (SHG), are of great interest due to the large responses arising from the momentum-space topological structures. Nodal line semimetals can offer even larger responses at low frequencies, with the SHG response being significantly enhanced by breaking the inversion symmetry. Additionally, a model with intrinsically broken inversion symmetry exhibits singular shift current and SHG responses within a range of parameters.