  We theoretically propose that the Dirac fermion in two-dimensions shows the
giant nonlinear responses to electromagnetic fields in terahertz region. A
scaling form is obtained for the current and magnetization as functions of the
normalized electromagnetic fields $E/E_\omega$ and $B/B_\omega$, where the
characteristic electric (magnetic) field $E_\omega$ ($B_\omega$) depends on the
frequency $\omega$ as $\hbar\omega^2/ev_F$ ($\hbar\omega^2/ev_F^2$), and is
typically of the order of 80 V/cm ( 8 mT) in the terahertz region. Applications
of the present theory to graphene and surface state of a topological insulator
are discussed.
