High-order harmonic generation is a well-established process for producing coherent pulses in the soft-x-ray range by direct frequency conversion of femtosecond near-infrared pulses. Here, we show that a metallic bowtie-shaped gold nanoelement generates the extreme-ultraviolet pulses by means of plasmonic field enhancement. The intensity enhancement factor reaches a peak of ∼8100, allowing generation up to the 85th harmonic of a femtosecond Ti:sapphire laser in hydrogen gas, with a modest incident intensity of ∼1011 Wcm–2.
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