Systematic investigation of resonance-induced single-harmonic enhancement in the extreme-ultraviolet range

We demonstrate the intensity enhancement of single harmonics in high-order harmonic generation from laser plasma. We identified several targets (In, Sn, Sb, Cr, and Mn) that demonstrate resonance-induced enhancement of single harmonic, that are spectrally close to ionic transitions with strong oscillator strengths. We optimized and obtained enhancements of the 13th, 17th, 21st, 29th, and 33rd harmonics from the above targets, by varying the chirp of the $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength femtosecond laser. We also observe harmonic enhancement by using frequency-doubled pump laser ($400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength). For Mn plasma pumped by the $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength laser, the maximum order of the enhanced harmonic observed was the 17th order $(\ensuremath{\lambda}=23.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$, which corresponds to the highest photon energy $(52.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ reported for an enhanced single harmonic.

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