Collection and spectral control of high-order harmonics generated with a 50 W high-repetition rate Ytterbium femtosecond laser system

Abstract

We generate high-order harmonics with a 50 W, Yb femtosecond fiber laser system operating at 100 kHz in a tight focusing configuration. We achieve a high photon flux even with pulses longer than 500 fs. We collect the diverging XUV harmonic beam in a 35 mrad wide solid angle by using a spectrometer designed to handle the high thermal load under vacuum and refocus the XUV beam onto a detector where the beam is characterised or can alternatively be used for experiments. This setup is designed for a 50 eV XUV bandwidth and offers the possibility to perform XUV-IR pump probe experiments with both temporal and spectral resolution. The high-order harmonics were generated and optimized at 100 kHz by using several gas target geometry (a gas jet and a semi-infinite gas cell) and several gases (Argon, Krypton, Xenon) that provide XUV beams with different characteristics. After the spectrometer and for HHG in Xenon, we detect more than 4x10 10 photons per second over 4 harmonics, that is a useful XUV power on target of 0.1 µW. This corresponds to the emission of more than 1µW per harmonic at the source and we achieved similar flux with both the semi-infinite cell and the jet. In addition, we observe a strong spectral selectivity when generating harmonics in a semi-infinite gas cell as few harmonics clearly dominate the neighbouring harmonics. We attribute this spectral selectivity to phase matching effects.

Efficient high-harmonic generation from a stable and compact ultrafast Yb-fiber laser producing 100 μJ, 350 fs pulses based on bendable photonic crystal fiber

“…Using our metric for conversion efficiency with their stated average input laser power, this corresponds to a maximum of 4000 nW of XUV power for 50-W input power, or 80 nW/W, which is lower than achieved here. This is expected because the authors report the same pulse energy as here but a longer (>500 fs) pulse duration [30].…”

“…Cabasse et al [30] detect 10 12 photons/s with a driving laser power of 50 W. This measurement is made over harmonics 15,17,19, 21 of a 1030-nm driving laser, corresponding to the following photon energies in each harmonic: 2.89 × 10 −18 J (15th), 3.28 × 10 −18 J (17th), 3.67 × 10 −18 J (19th), and 4.05 × 10 −18 J (21st). At 10 12 ph/s, we estimate their average XUV power to be approximately 3500-4000 nW (an average of 1 µW per harmonic, which agrees with their abstract).…”

Absolute Gas Density Profiling In High Order Harmonic Generation

“…With these considerations, several works reported on the generation of XUV light with a high conversion efficiency when driving HHG using µJ-level pulses [9][10][11][12][13][14][15][16][17]. The quadratic dependence of the medium length and density on the scaling factor underlines the sensitivity of the HHG yield to these parameters.…”

Time-resolved and spectrally resolved ionization with a single ultrashort XUV-IR beamline

“…This plate is coated with an antireflection coating at 800 nm having a thin Nb2O5 uppermost layer to reflect the S polarized XUV light with an estimated efficiency of ~50% [11,12] and to transmit the main part of the IR laser beam. The XUV light is then reflected by a gold coated toroidal mirror with 30 cm focal length used at 70° incidence angle and designed to image the XUV source in the sensitive zone of a velocity map imaging (VMI) spectrometer with a magnification factor of ~1.…”