Enhancement of two-color high harmonic by using two compound strong ionic transitions in double-target scheme

The authors generated high-order harmonics by using the two-component laser-ablation plumes, which were irradiated by a femtosecond laser pulse. The harmonic spectra from double-target schemes comprised those obtained from separate targets. For indium/chromium plumes, the enhancement of two harmonics (13th and 29th orders) was obtained at the wavelengths of 61.15 and 27.41nm, respectively. The conversion efficiencies of these two harmonics were estimated to be 10−5. The authors attribute the enhancement of 13th and 29th harmonics to the multiphoton resonances with the strong radiative transitions of ions in indium and chromium plumes, respectively.