Nonlinear photoresponse of type-II Dirac semimetal NiTe₂

Topological semimetals, including Weyl and Dirac semimetals, represent an unusual state of quantum matter that possesses massless relativistic quasiparticles with linearly dispersing bands around nodes. Among them, the type-II Dirac semimetal NiTe₂ features unique Fermi surface configurations, such as chiral spin textures and strongly tilted Dirac cones close to the Fermi surface. Here we use terahertz (THz) emission spectroscopy to explore the nonlinear photoresponse in type-II Dirac semimetal NiTe₂ and its heterostructure as a function of polarization, helicity, and azimuthal angle. With the breaking of the inversion symmetry, the Dirac node splits into two Weyl nodes of opposite chirality and is seen by the helicity-dependent photocurrent generated by the circular photogalvanic effect. Reversing the helicity of the pump light can switch the THz polarity. The THz photocurrent response under linear polarization pump is also explored in detail.