Resonant Terahertz Generation by Optical Mixing of Two Laser Pulses in Rippled Density Plasma

Two collinear laser pulses of finite spot size propagating in a plasma at an angle to density ripple wave vector are shown to produce terahertz (THz) radiation at the beat frequency. The laser pulses exert a ponderomotive force on the electrons and impart them oscillatory velocity at the difference frequency. The latter beats with the density ripple and produces a nonlinear current, whose curl is nonzero
$(\nabla\times\vec{J}^{NL}\ne 0)$
. The current drives the coherent THz radiation. The ripple helps THz generation process in two ways: first by making the nonlinear current irrotational, and second by providing the phase matching when ripple wave number
$\vec{q}$
equals
$\vec{k}_{1}-\vec{k}_{2}-\vec{k}_{THz}$
where
$\vec{k}_{1}$
and
$\vec{k}_{2}$
are the laser wave vectors and
$\vec{k}_{THz}$
is the THz wave vector.