Selective enhancement (>10³) of harmonics extending to the water window (~4 nm) generated in an argon gas filled straight bore capillary waveguide is demonstrated. This enhancement is in good agreement with modeling which indicates that multimode quasi-phase-matching is achieved by rapid axial intensity modulations caused by beating between the fundamental and higher-order capillary modes. Substantial pulse energies (>10 nJ per pulse per harmonic order) at wavelengths beyond the carbon K edge (~4.37 nm, ~284 eV) up to ~360 eV are observed from argon ions for the first time.
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Zepf et al. (2007) studied this question.
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