Abstract
Fully coherent, soft X-ray attosecond pulses are now available through high-order harmonic generation (HHG); however, the output energy is insufficient for various applications, such as attosecond-scale soft X-ray nonlinear experiments, the seeding of soft X-ray free-electron lasers, attosecond-pump-attosecond-probe spectroscopies, and single-shot imaging. In this paper, we combine a newly developed TW class mid-infrared femtosecond laser and a loose focusing geometry for HHG. A soft X-ray harmonic beam up to the water window region is demonstrated, which is more than 100 times intense compared to previous works. We achieve a high conversion efficiency, low beam divergence and a significantly reduced medium gas pressure. As the first application, we demonstrate near edge X-ray absorption fine structure experiments with clear fine absorption spectra near the K- and L-edges observed. The robust energy scaling method on HHG opens the door for demonstrating single-shot absorption spectrum and live-cell imaging with a femtosecond time resolution.
Highlights
Coherent, soft X-ray attosecond pulses are available through high-order harmonic generation (HHG); the output energy is insufficient for various applications, such as attosecond-scale soft X-ray nonlinear experiments, the seeding of soft X-ray free-electron lasers, attosecond-pump-attosecond-probe spectroscopies, and single-shot imaging
We demonstrate a robust approach to generate over nJ per pulse energy HHs in the water window soft X-ray, using our recently developed TW-class MIR femtosecond laser system with a maximum pulse energy of 100 mJ in the wavelength tuning range of 1.2–2.4 μm[30,31] using a dual-chirped optical parametric amplification (DC-OPA) scheme[32,33,34]
The driving laser for HHG is a TW MIR laser with a central wavelength tuned to the air transmission window of 1.55 μm with a duration of ~45 fs, which is provided by a DC-OPA system operated at a 10 Hz repetition rate[30]
Summary
Soft X-ray attosecond pulses are available through high-order harmonic generation (HHG); the output energy is insufficient for various applications, such as attosecond-scale soft X-ray nonlinear experiments, the seeding of soft X-ray free-electron lasers, attosecond-pump-attosecond-probe spectroscopies, and single-shot imaging. In 2011, Togashi, Takahashi, and coworkers demonstrated a seeded XUV-FEL using a XUV HHs seeder pulse[9] They achieved 650 times the intensity compared to the unseeded condition and dramatically reduced the shot-toshot fluctuation. A nano-joule class coherent soft X-ray source has been needed as a XFEL seeded pulse Another important application is attosecond-pump-attosecondprobe experiments in the soft X-ray region, for example, timeresolved X-ray absorption fine structure (XAFS) research of the two-photon nonlinear process. Rapidly developed highquality X-ray sources based on HHG have opened the door for applications on an attosecond time scale[13,14,15]; the insufficient energy has restricted a variety of applications
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