Generation of enhanced even harmonics of fundamental radiation in temporally separated two-color laser fields
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2019 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 ( 0 ) and its second harmonic (2 0 ) in argon (Ar), molecular nitrogen (N 2 ) and carbon dioxide (CO 2 ) gases having close ionization potentials. The special feature of this study is that the 0 and 2 0 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 ( 0 + 2 0 ) field more than an order of magnitude enhancement of the HHs signal was observed compared to the 0 or the 2 0 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.