Phase-Matched High-Order Difference-Frequency Mixing in Plasmas

The generation of coherent extreme ultraviolet radiation by phase-matched high-order difference-frequency mixing in plasmas is reported. The plasmas are produced by optical field ionization of atomic and molecular gases in an intense Ti:sapphire laser field. Phase matching for the high-order two-color $(\omega ,2\mathrm{}\omega )$ mixing process, $9\omega =6\mathrm{}\left(2\omega \right)-3\mathrm{}\omega$, is demonstrated for the first time.