Abstract: Bright, coherent soft X-ray radiation is essential to a variety of applications in fundamental research and life sciences. To date, a high photon flux in this spectral region can only be delivered by synchrotrons, free-electron lasers or high-order harmonic generation sources, which are driven by kHz-class repetition rate lasers with very high peak powers. Here, we establish a novel route toward powerful and easy-to-use SXR sources by presenting a compact experiment in which nonlinear pulse self-compression to the few-cycle regime is combined with phase-matched high-order harmonic generation in a single, helium-filled antiresonant hollow-core fibre. This enables the first 100 kHz-class repetition rate, table-top soft X-ray source that delivers an application-relevant flux of 2.8 x 10(6) photon s(-1) eV(-1) around 300 eV. The fibre integration of temporal pulse self-compression (leading to the formation of the necessary strong-field waveforms) and pressure-controlled phase matching will allow compact, high-repetition-rate laser technology, including commercially available systems, to drive simple and cost-effective, coherent high-flux soft X-ray sources.
Abstract: High harmonic sources can provide ultrashort pulses of coherent radiation in the XUV and X-ray spectral region. In this paper we utilize a sub-two-cycle femtosecond fiber laser to efficiently generate a broadband continuum of high-order harmonics between 70 eV and 120 eV. The average power delivered by this source ranges from > 0.2 µW/eV at 80 eV to >0.03 µW/eV at 120 eV. At 92 eV (13.5 nm wavelength), we measured a coherent record-high average power of 0.1 µW/eV, which corresponds to 7 · 109 ph/s/eV, with a long-term stability of 0.8% rms deviation over a 20 min time period. The presented approach is average power scalable and promises up to 1011 ph/s/eV in the near future. With additional carrier-envelop phase control even isolated attosecond pulses can be expected from such sources. The combination of high flux, high photon energy and ultrashort (sub-) fs duration will enable photon-hungry time-resolved and multidimensional studies.
Abstract: Intense, ultrafast laser sources with an emission wavelength beyond the well-established near-IR are important tools for exploiting the wavelength scaling laws of strong-field, light-matter interactions. In particular, such laser systems enable high photon energy cut-off HHG up to, and even beyond, the water window thus enabling a plethora of subsequent experiments. Ultrafast thulium-doped fiber laser systems (providing a broad amplification bandwidth in the 2 μm wavelength region) represent a promising, average-power scalable laser concept in this regard. These lasers already deliver ∼100 fs pulses with multi-GW peak power at hundreds of kHz repetition rate. In this work, we show that combining ultrafast thulium-doped fiber CPA systems with hollow-core fiber based nonlinear pulse compression is a promising approach to realize high photon energy cut-off HHG drivers. Herein, we show that thulium-doped, fiber-laser-driven HHG in argon can access the highly interesting spectral region around 90 eV. Additionally, we show the first water window high-order harmonic generation experiment driven by a high repetition rate, thulium-doped fiber laser system. In this proof of principle demonstration, a photon energy cut-off of approximately 400 eV has been achieved, together with a photon flux <105 ph/s/eV at 300 eV. These results emphasize the great potential of exploiting the HHG wavelength scaling laws with 2 μm fiber laser technology. Improvements of the HHG efficiency, the overall HHG yield and further laser performance enhancements will be the subject of our future work.
Abstract: We present HHG results obtained with thulium-doped fiber lasers. It is the first time that a photon energy cut-off close to 400 eV has been demonstrated using this highly scalable laser technology.
Abstract: A HHG source generating a broadband continuum from 70 eV to 120 eV with an average power of 2 µW is presented. At 92 eV (13.5 nm) 7 10^9 ph/s/eV are generated with an rms deviation of 0.8% over 20 minutes.
Abstract: We report on soft x-ray HHG driven by a thulium-doped fiber laser. It is the first time that a photon energy cut-off ~400 eV has been demonstrated using this highly scalable laser technology.
Abstract: A nonlinear compression of 515 nm pulses resulting in 17.8 fs-, 50 µJ-pulses at 1 MHz, 50 W average power and near diffraction limited beam quality is presented.
Abstract: Separation of the high average power driving laser beam from the generated XUV to soft-X-ray radiation poses great challenges in collinear HHG setups due to the losses and the limited power handling capabilities of the typically used separating optics. This paper demonstrates the potential of driving HHG with annular beams, which allow for a straightforward and power scalable separation via a simple pinhole, resulting in a measured driving laser suppression of 5⋅10−3. The approach is characterized by an enormous flexibility as it can be applied to a broad range of input parameters and generated photon energies. Phase matching aspects are analyzed in detail and an HHG conversion efficiency that is only 27% lower than using a Gaussian beam under identical conditions is demonstrated, revealing the viability of the annular beam approach for high flux coherent short-wavelength sources and high