  We report strong terahertz (~10^12 Hz) high harmonic generation in thin films
of Cd3As2, a three-dimensional Dirac semimetal at room temperature. The third
harmonics is detectable with tabletop light source and can be as strong as 100
V/cm by applying the fundamental field of 6.5 kV/cm inside the film, showing an
unprecedented efficiency for terahertz frequency conversion. Our time-resolved
terahertz spectroscopy and calculations also clarify the microscopic mechanism
of the nonlinearity originating in the coherent acceleration of Dirac electrons
in momentum space. Our results provide clear insights for nonlinear current of
Dirac electrons driven by terahertz field under an influence of scattering,
paving the way toward novel devices for high-speed electronics and photonics
based on topological semimetals.
