Observation of Highly Efficient Electric-Quadrupole Second Harmonic Generation in a Dirac Semimetal

Topological semimetals hold great promise for nonlinear optics (NLO). It is challenging to probe tilted Dirac fermion-driven intrinsic second harmonic generation (SHG) and topological band structure modulation of second-order nonlinearity in spatial inversion and time-reversal-symmetric nonmagnetic Dirac semimetals. Here, we identify NiTe2, featuring a type-II Dirac cone, as an ideal platform for investigating SHG in centrosymmetric crystals. Through comprehensive NLO measurements, we demonstrate that the highly efficient SHG emission in NiTe2 arises predominantly from bulk electric-quadrupole contribution. Under interband optical transition conditions, we quantify a giant effective nonlinear susceptibility |χEQ(3)q| ≈ 7.07 pm/V at 475 nm, which is an order of magnitude higher than that of commonly used NLO crystals. Furthermore, the striking spectral consistency between SHG and four-wave mixing (FWM) unambiguously identifies the shared interband excitation and radiation channels as the unified driving mechanism. Collectively, these findings pave the way for harnessing centrosymmetric topological semimetals in next-generation nonlinear optoelectronic devices.