  Shift current and second harmonic generation (SHG) are nonlinear optical
responses that are often used for photovoltaic effect and the detection of
subtle symmetry breaking patterns in materials. It has been recently shown that
topological semimetals such as Weyl semimetals with broken inversion symmetry
can generate large nonlinear optical responses. In this paper, we investigate
both shift current and SHG in a class of topological nodal line semimetal
(NLSM) systems with broken inversion symmetry, which may arise intrinsically or
via an applied electric field. It is shown that certain classes of NLSM systems
offer more singular shift current and SHG in comparison to the well-known Weyl
semimetal systems.
