Plasma-Driven Secondary Radiations for Nonharmonic Two-Color Mixing

Four-wave mixing (FWM) of ultrashort laser pulses in gases is known [1] to provide an important source of various coherent radiations. A mixing of two laser electric fields, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$E_{\omega_{1}}(t)$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$E_{{\omega}_2} (t)$</tex>, operating at different frequencies <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\omega_{1}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\omega_{2}$</tex> with phase difference <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\phi$</tex> produces emissions centered at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2\omega_{1}-\omega_{2}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2 \omega_2-\omega_1$</tex> through FWM, besides third harmonics and high-order components [2]. When the laser beam is intense enough to create a plasma, ionization also contributes to the same radiations [3]. An outstanding question is thus to know which player, among the plasma or the Kerr-induced FWM, is the most active in the conversion process.