The generation of coherent extreme ultraviolet radiation by phase-matc
hed high-order difference frequency mixing in plasmas is reported. The
plasmas are produced by optical field ionization of atomic and molecu
lar gases in an intense Ti:sapphire laser field. Phase matching for th
e high-order two-color (omega,2 omega) mixing process, 9 omega = 6(2 o
mega) - 3 omega, is demonstrated for the first time.