Giant enhancement of terahertz high-harmonic generation by cavity engineering of Dirac semimetal

Engineered micro- or nano-structures based on nonlinear optical materials offer versatile opportunities for optoelectronic applications. While extensive efforts have been devoted to design tailored microcavities to promote and increase the optical nonlinearities of graphene, the potential of engineering its three-dimensional counterparts -- three-dimensional Dirac semimetals -- remains largely unexplored. Here we report on exceptionally strong terahertz nonlinearities in a cavity-engineered Dirac semimetal microstructure, and demonstrate a giant enhancement of terahertz third- and fifth-order harmonic yields by more than three orders of magnitude. By fabricating a designed structure of metallic metasurface microcavities on a nanometer thin film of the threedimensional Dirac semimetal Cd3As2, we significantly enhance the near-field intensity of a picosecond terahertz excitation pulse in resonance with the microcavity eigenmode. This transiently modifies the nonlinearities of the thin film and drives the nonlinear responses of the Dirac fermions from a weakly to a deeply nonperturbative regime where the observed high-harmonic generation essentially saturates.