Abstract
High harmonic generation (HHG) produces coherent light in the extreme ultraviolet (XUV) spectral region. We present results on generation of high harmonics (HHs) with a temporally separated two laser pulses of fundamental (ω0) and its second harmonic (2ω0) in argon (Ar), molecular nitrogen (N2) and carbon dioxide (CO2) gases having close ionization potentials. The special feature of this study is that the ω0 and 2ω0 fields are temporary separated, so that the fundamental radiation comes first and the second harmonic radiation arrives to the interaction region with a delay. High harmonic spectra were measured for parallel and perpendicular relative polarizations of the fundamental and the second harmonic fields. With the combined (ω0 + 2ω0) field more than an order of magnitude enhancement of the HHs signal was observed compared to the ω0 or the 2ω0 field alone. An increased conversion efficiency into HHs, notably of the even orders of 2(2m + 1) (m = 1 − 3) was found for the latter case. We also discuss similarities and differences of the HH generation in the three gas media with single and two-color fields.
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