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

Abstract

Laser-dressed photoelectron spectroscopy, employing extreme-ultraviolet attosecond pulses obtained by femtosecond-laser-driven high-order harmonic generation, grants access to atomic-scale electron dynamics. Limited by space charge effects determining the admissible number of photoelectrons ejected during each laser pulse, multidimensional (i.e. spatially or angle-resolved) attosecond photoelectron spectroscopy of solids and nanostructures requires high-photon-energy, broadband high harmonic sources operating at high repetition rates. Here, we present a high-conversion-efficiency, 18.4-MHz-repetition-rate cavity-enhanced high harmonic source emitting 5 × 105 photons per pulse in the 25-to-60-eV range, releasing 1 × 1010 photoelectrons per second from a 10-µm-diameter spot on tungsten, at space charge distortions of only a few tens of meV. Broadband, time-of-flight photoelectron detection with nearly 100% temporal duty cycle evidences a count rate improvement between two and three orders of magnitude over state-of-the-art attosecond photoelectron spectroscopy experiments under identical space charge conditions. The measurement time reduction and the photon energy scalability render this technology viable for next-generation, high-repetition-rate, multidimensional attosecond metrology.

Temporal solitons in free-space femtosecond enhancement cavities

“…Compared with previous CS platforms, free-space femtosecond ECs offer unparalleled flexibility regarding dispersion control, access to the beam, geometric scalability of the nonlinearity, and peak powers significantly exceeding the catastrophic self-focusing limit in optical materials. In this implementation, CS promise new opportunities for established applications of femtosecond ECs, such as high-repetition-rate high-order harmonic generation 16 for (extreme-) ultraviolet frequency-comb 19,20 and photoemission [31][32][33] spectroscopies, or path-length enhancement for vibrational spectroscopy 17,18,21 , and are likely to enable new applications. In particular, the temporal, spectral and spatial filtering, and the low-frequency RIN suppression afforded by CS are highly attractive properties for the compression of ultrashort laser pulses at the full repetition rate of their primary source.…”

Cavity-enhanced noncollinear high-harmonic generation

“…We compare the method to OC using a symmetric fundamental Gaussian mode focused down to the same focal spot area, using an OC mirror with a hole, with an angular diameter chosen for the same round-trip loss (2.30 mrad). Because the divergence of the XUV beamlets depends on the harmonic order, intensity and target gas [16], we consider two cases: OC of the 33 th harmonic produced in argon (39.9 eV, compare [8]), with a peak intensity of 1.5 × 10 14 W/cm 2 in the target plane, and of the 79 th harmonic produced in neon (95.6 eV, compare [26]), with a peak intensity of 3.0 × 10 14 W/cm 2 . The results are plotted in Fig.…”

“…Direct evidence of the temporal coherence of the emerging harmonic spectrum [3,4] has demonstrated the viability of transferring NIR frequency combs to the VUV/XUV with ECs, thus paving the way towards precision frequency metrology of electronic transitions [5]. Very recently, geometrically coupling out the harmonic radiation through an on-axis opening in the mirror following the HHG focus [6][7][8] enabled MHz-HHG with photon energies high enough to liberate core electrons from metals via single-photon photoelectron spectroscopy (PES). 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].…”

“…Very recently, geometrically coupling out the harmonic radiation through an on-axis opening in the mirror following the HHG focus [6][7][8] enabled MHz-HHG with photon energies high enough to liberate core electrons from metals via single-photon photoelectron spectroscopy (PES). 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].…”

“…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].…”