It is shown that three‐dimensional Dirac semimetals are promising candidates for highly
efficient optical‐to‐terahertz conversion due to their extreme optical nonlinearities. In
particular, it is predicted that the conversion efficiency of Cd 3 As 2 Cd_3As_2 exceeds
typical materials like LiNbO 3 LiNbO_3 by> 5000 times over nanoscale propagation
distances. Studies show that even when no restrictions are placed on propagation distance,
Cd 3 As 2 Cd_3As_2 still outperforms LiNbO 3 LiNbO_3 in efficiency by> 10 times. The …

In this section, we show how the linear and nonlinear conductivities can be computed from
the density matrix equations of motion. We begin by considering the time-dependent
Schrödinger equation (TDSE): ih∂ t= ˆH0+ ˆHint,(S1) where ˆ H0=∑ j vjσjpj, σj is the Pauli
matrix with j∈ x, y, z. The Fermi velocity and the momentum operator along direction j in
Cartesian coordinates, are represented by vj and pj respectively. The interaction part can be
defined either in the velocity gauge or in the length gauge as