Gate-Tunable Floquet Weyl Photon Emission from Topological Dirac Semimetal Cd₃As₂

Controlling the electronic structure of Dirac materials with light and electric fields has reconfigured two-dimensional systems such as graphene, but achieving comparable control in 3D Dirac semimetals has remained a major challenge. Here, we demonstrate all-optical and electrical control of the band topology of the 3D Dirac semimetal Cd₃As₂. Circularly polarized femtosecond light dynamically breaks time-reversal symmetry, splitting the Dirac nodes into Floquet chiral Weyl nodes. The resulting non zero Berry curvature generates helicity-dependent transverse anomalous photocurrents, directly detected through terahertz (THz) emission at normal incidence. An applied positive electric field further shifts the Fermi level away from the Dirac point, enlarging the Dirac cone projection. As a result, we achieve precise electrical control over Floquet band engineering, resulting in a 60% modulation of THz emission. Moreover, at oblique incidence, the circular photon-drag effect induces helicity-dependent longitudinal photocurrents. By independently controlling the transverse and longitudinal photocurrents, we achieve electrical tuning of the helicity of emitted THz radiation. Our results establish a route to electrically programmable Floquet band engineering and tunable chiral THz sources, opening new opportunities for ultrafast quantum photonics, optoelectronics and topological quantum technologies.