Unlocking and Controlling Efficient Second-Order Nonlinear Terahertz Photocurrents in Centrosymmetric Dirac Semimetals

Topological three-dimensional (3D) Dirac semimetals are promising for advanced optoelectronics due to their nontrivial band topology and ultrahigh carrier mobility. Their intrinsic centrosymmetry, however, should forbid second-order nonlinear optical processes, severely limiting their nonlinear photonics potential. Contrary to this expectation, we observe pronounced coherent terahertz (THz) emission arising from second-order nonlinear effects in both pristine 3D Dirac semimetal Cd3As2 thin films and its noncentrosymmetric alloy counterpart. This highly efficient, broadband THz radiation can be controlled by varying the pump polarization and incident angle, with the resulting transient photocurrents aligning with symmetry analysis. Crucially, we demonstrate two effective strategies to activate these nonlinear photocurrents: (i) using oblique excitation to create an asymmetric carrier distribution in momentum space, and (ii) deliberately breaking inversion symmetry via alloy engineering. Both strategies are generalizable approaches for unlocking second-order transient photocurrents in centrosymmetric Dirac semimetals through symmetry breaking. These findings establish new approaches for controlling nonlinear responses in 3D topological materials, advancing their application in next-generation, on-chip THz photonic devices.