Abstract
We studied high-harmonic generation (HHG) in Ar, Ne and He gas jets using a broadly tunable, high-energy optical parametric amplifier (OPA) in the visible wavelength range. We optimized the noncollinear OPA to deliver tunable, femtosecond pulses with 200-500 μJ energy at 1-kHz repetition rate with excellent spatiotemporal properties, suitable for HHG experiments. By tuning the central wavelength of the OPA while keeping energy, duration and beam size constant, we experimentally studied the scaling law of conversion efficiency and cut-off energy with the driver wavelength in argon and helium respectively. Our measurements show a λ−5.9±0.9 wavelength dependence of the conversion efficiency and a λ1.7±0.2 dependence of the HHG cut-off photon energy over the full visible range in agreement with previous experiments of near- and mid-IR wavelengths. By tuning the central wavelength of the driver source and changing the gas, the high order harmonic spectra in the extreme ultraviolet cover the full range of photon energy between ~25 eV and ~100 eV. Due to the high coherence intrinsic in HHG, as well as the broad and continuous tunability in the extreme UV range, a high energy, high repetition rate version of this source might be an ideal seed for free electron lasers.
Highlights
Free electron lasers (FELs) are revolutionary extreme UV (EUV) sources due to their very high photon flux allowing for pump-probe and imaging experiments
FELs typically operate by selfamplified spontaneous emission, which inevitably contributes to the poor coherence, pulse-to-pulse timing jitter and poor electric field reproducibility of the output pulses, limiting the time resolution in pump-probe experiments to around 100 fs
Pulse energies in the nJ range [1] are required to effectively seed FELs in the tens of eV photon energy range to overcome spontaneous emission and transfer the beneficial high harmonic generation (HHG) coherence properties to the FEL output light
Summary
Free electron lasers (FELs) are revolutionary extreme UV (EUV) sources due to their very high photon flux allowing for pump-probe and imaging experiments. Pulse energies in the nJ range [1] are required to effectively seed FELs in the tens of eV photon energy range to overcome spontaneous emission and transfer the beneficial HHG coherence properties to the FEL output light. We show full tunability of the generated EUV pulses, between 25 eV and 100 eV
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