[en] When two beams of electromagnetic radiation having finite diameters intersect within a plasma, they generate, in the interaction volume, a second-order charge density which is proportional to the amplitudes of both beams. Provided that a resonance condition is satisfied, this charge density acts as directional antenna emitting longitudinal waves. In this process power is transferred from the beam of higher frequency to the beam of lower frequency with the excess going into the longitudinal waves. The power emitted is P=P₁P₂/P*, where P/subi/ are the powers of the electromagnetic beams. P* is computed for both Langmuir and sound waves. In the first case this process can probably be used as a means to measure the plasma density locally