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.