The generation of coherent extreme ultraviolet radiation by phase-matched high-order differencefrequency 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 highorder two-color ͑v, 2v͒ mixing process, 9v 6͑2v͒ 2 3v, is demonstrated for the first time.[S0031-9007(96)00084-1]
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