Cumulative plasma effects in cavity-enhanced high-order harmonic generation in gases

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.

Cavity-enhanced noncollinear high-harmonic generation

“…In conclusion, we have demonstrated a femtosecond enhancement cavity with an intracavity wave-front rotation, enabled by the technique of transverse mode gating, and showed first HHG spectra of wave-form stable XUV pulse trains at photon energies beyond 100 eV generated with this method, in a focusing/repetition-rate regime avoiding cumulative plasma effects [13,14] (see also Appendix G). Our experimental findings agree well with theoretical expectations.…”

“…This led to the first space-charge-free PES experiments at multi-MHz repetition rates [8,9], in particular also with attosecond temporal resolution [8]. Significant efforts have addressed the understanding of cavity-enhanced HHG conversion efficiency limitations related to plasma nonlinearity [10][11][12] and plasma cumulative effects [13,14]. Accelerating the gas to provide (nearly) single-pass conditions even at several tens of MHz has been shown to strongly mitigate the latter limitation, resulting in mW-level VUV frequency combs [13].…”

“…Yet, despite of the growing attention dedicated to this laser architecture uniquely combining high XUV photon energies with high repetition rates, two limitations remain without solution to this day. Firstly, geometric output coupling (OC) employing the fundamental transverse mode of the EC [6][7][8]14] -the most broadband OC technique demonstrated so far -suffers from poor efficiency [16]. Noncollinear methods or methods employing spatially tailored resonator modes have been recognized early on as promising alternatives [17][18][19][20][21].…”

Attosecond intra-valence band dynamics and resonant-photoemission delays in W(110)

“…The laser system consists of a Ti:Sa oscillator seeding a Yb-doped fiber amplification system with actively stabilized carrier-envelope phase 37 . The repetition rate of 18.4 MHz rate guarantees space-charge free experimental conditions 21 , avoids cumulative effects in the HHG gas target 38 , and ensures a temporal detection duty cycle of photoelectrons close to 100% 21 . After amplification, the 1030 nm pulses are spectrally broadened in a multi-pass cell and temporally compressed 39 to a final pulse duration of 39 fs at 83 W of average power.…”

High-flux ultrafast extreme-ultraviolet photoemission spectroscopy at 18.4 MHz pulse repetition rate

“…In general, for PES applications a pulse repetition rate high enough so that space charge does not impair the application is desirable. However, an upper bound is set by cumulative effects in the HHG process that limit the XUV yield 65,66 . Furthermore, for ToF-PES there is a trade-off between repetition rate, energy resolution and observable energy window, caused by the ToF dispersion of the electrons.…”