Abstract
Cavity-enhanced high-order harmonic generation (HHG) affords broadband, coherent extreme-ultraviolet (XUV) pulse trains with repetition rates of several tens of MHz. Geometrically coupling out the intracavity generated XUV beam through a small on-axis hole in the cavity mirror following the HHG focus has enabled scaling the photon energies attainable with this technology to 100 eV and more, promising new applications of XUV frequency-comb spectroscopy and attosecond-temporal-resolution, multidimensional photoelectron spectroscopy and nanoscopy. So far, in this approach the features of the macroscopic response of the gas target are neither accessible directly nor indirectly via the out-coupled XUV beam due to the loss of spatial information caused by the truncation at the hole. Here, we derive a simple analytical model for the divergence of the intracavity harmonic beam as a function of experimental design parameters such as gas target position, cavity geometry and driving pulse intensity, thereby establishing a connection between the measured XUV spectra and the macroscopic response of the intracavity nonlinear medium. We verify this model by comparison to numerical simulations as well as to systematic measurements, and apply it to elucidate a trade-off between the efficiency of geometric output coupling and that of the HHG process, and the underlying physical mechanisms. These findings illuminate the share of the output coupling efficiency to the overall HHG conversion efficiency and provide—together with previously studied plasma-related enhancement limitations—a holistic means of optimizing the overall efficiency with this architecture that uniquely combines high repetition rates with high photon energies. Furthermore, quantitatively connecting the output coupled, observable XUV radiation to the nonlinear conversion at the cavity focus allows for a better insight into the dynamics of intracavity HHG and might benefit other applications of femtosecond enhancement cavities, such as high-repetition-rate HHG spectroscopy.
Highlights
IntroductionIn modern high-order harmonic generation (HHG) systems, the nonlinear conversion is driven by amplified femtosecond pulses, reaching energies of several 100 μJ [5,6,7,8,9]
Where space-charge effects limit the useful number of particles per pulse, and frequency-comb spectroscopy [13, 14] at high photon energies, where the power per comb line scales with the repetition rate and the comb spacing should be larger than the line width of the studied transition
A full scan of the gas target position z relative to the focal plane (z > 0 signifies that the gas target is placed behind the laser focus) and the backing pressure p applied to the nozzle was performed and the circulating infrared (IR) power and, using an XUV spectrometer, the output-coupled XUV photon counts per harmonic order were recorded at each point, resulting in the p–z-maps shown in figure 2(a)
Summary
In modern HHG systems, the nonlinear conversion is driven by amplified femtosecond pulses, reaching energies of several 100 μJ [5,6,7,8,9]. This results in repetition rates significantly lower than 1 MHz. some applications require operation in the multi-10 MHz repetition-rate regime. Examples include experiments involving the detection of charged particles such as photoelectron spectroscopy and microscopy [10,11,12] and coincidence measurements [7], where space-charge effects limit the useful number of particles per pulse, and frequency-comb spectroscopy [13, 14] at high photon energies, where the power per comb line scales with the repetition rate and the comb spacing should be larger than the line width of the studied transition
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
