Giant Enhancement of Terahertz High-Harmonic Generation by Cavity Engineering of a Three-Dimensional Dirac Semimetal

We report on time-resolved ultrafast terahertz high-harmonic generation of strong field-driven dynamics of many-body Dirac fermions. We demonstrate an experimental realization of near-saturation regime of the high-order nonlinear responses with a giant enhancement of terahertz third- and fifth-order harmonic yields by cavity-engineering a three-dimensional Dirac semimetal, Cd3As2. By fabricating a designed structure of metasurface microcavities on a nanometer Cd3As2 thin film, we significantly enhance the near-field intensity of a picosecond terahertz excitation pulse in resonance with the microcavity eigenmode. The strong terahertz field drives the far-from-equilibrium Dirac fermions deeply into a nonperturbative regime, leading to the observation of near-saturation high-harmonic emission. Our experimental results confirm the predictions by Boltzmann transport theory and substantiate a field-driven kinetic description of the strong nonthermal nonlinearity at terahertz frequencies.