Abstract
Modern ultrafast laser architectures enable high-order harmonic generation (HHG) in gases at (multi-) MHz repetition rates, where each atom interacts with multiple pulses before leaving the HHG volume. This raises the question of cumulative plasma effects on the nonlinear conversion. Utilizing a femtosecond enhancement cavity with HHG in argon and on-axis geometric extreme-ultraviolet (XUV) output coupling, we experimentally compare the single-pulse case with a double-pulse HHG regime in which each gas atom is hit by two pulses while traversing the interaction volume. By varying the pulse repetition rate (18.4 and 36.8 MHz) in an 18.4-MHz roundtrip-frequency cavity with a finesse of 187, and leaving all other pulse parameters identical (35-fs, 0.6-μJ input pulses), we observe a dramatic decrease in the overall conversion efficiency (output-coupled power divided by the input power) in the double-pulse regime. The plateau harmonics (25–50 eV) exhibit very similar flux despite the twofold difference in repetition rate and average power. We attribute this to a spatially inhomogeneous plasma distribution that reduces the HHG volume, decreasing the generated XUV flux and/or affecting the spatial XUV beam profile, which reduces the efficiency of output coupling through the pierced mirror. These findings demonstrate the importance of cumulative plasma effects for power scaling of high-repetition-rate HHG in general and for applications in XUV frequency comb spectroscopy and in attosecond metrology in particular.
Highlights
We present an experimental comparison of high-order harmonic generation (HHG) in the single-pulse (SP) regime with the case in which each atom interacts with two pulses of the driving laser
We observed a dramatic dependence of the conversion efficiency on the repetition rate, which can be attributed to cumulative plasma effects in the HHG target
The SP configuration is preferable in terms of the total output coupled flux [Fig. 3(c)] and conversion efficiency
Summary
Precision spectroscopy with XUV frequency combs[5,6,17] and high-speed multi-dimensional laser-dressed XUV photoemission spectroscopy[18,19] are prominent examples For these repetition rates, the period between two pulses becomes comparable to—or shorter than—the time atoms take to travel through the volume where they can interact with the laser pulses. We present an experimental comparison of HHG in the single-pulse (SP) regime (each atom is hit only once) with the case in which each atom interacts with two pulses of the driving laser (double-pulse, DP) To this end, we set up an 18.4-MHz-repetition-rate femtosecond enhancement cavity with either one circulating pulse or two circulating pulses and systematically evaluated the spectra and flux of the XUV radiation coupled out through a pierced mirror following the HHG focus. We observed a dramatic dependence of the conversion efficiency on the repetition rate, which can be attributed to cumulative plasma effects in the HHG target
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