Abstract
The enhancement of high-order harmonics of ultrashort pulses is crucial for application of coherent extreme ultraviolet (EUV) pulses in optoelectronics. Strong resonances of ionic species are useful for enhancement of the yield of single harmonic in EUV region. Another attractive concept is the quasi-phase matching (QPM) allowing the growth of conversion efficiency of the groups of harmonics in different ranges of EUV. Here we demonstrate simultaneous enhancement of 9th, 13th and 17th harmonics of 800 nm, 30 fs pulses in laser-produced plasmas (LPP) created on the surfaces of Zn, In and Sn targets. The enhancement of harmonics is attributed to the resonance transitions of singly charged particles of those materials. Strong 9th harmonic is generated in Zn plasma in a broad range of the delays between heating and probe laser pulses. We analyze strong even harmonics enhanced in Zn + In and Sn + In LPP at the conditions of two-color pump. We also show the concurrence of micro- and macroscopic processes in indium plasma by analyzing the resonance conditions for 13th harmonic and QPM for 21st and 31st harmonic using different configurations of LPP. We find the phase differences of harmonics generated in indium and zinc plasma, and show the pronounced effect of the resonances on the attosecond timing of the harmonic emission.
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