Attosecond light pulses in the extreme ultraviolet have drawn a great deal of attention due to their ability to interrogate electronic dynamics in real time. Nevertheless, to follow charge dynamics and excitations in materials, element selectivity is a prerequisite, which demands such pulses in the soft X-ray region, above 200 eV, to simultaneously cover several fundamental absorption edges of the constituents of the materials. Here, we experimentally demonstrate the exploitation of a transient phase matching regime to generate carrier envelope controlled soft X-ray supercontinua with pulse energies up to 2.9±0.1 pJ and a flux of (7.3±0.1) × 107 photons per second across the entire water window and attosecond pulses with 13 as transform limit. Our results herald attosecond science at the fundamental absorption edges of matter by bridging the gap between ultrafast temporal resolution and element specific probing.
Attosecond pulses at photon energies that cover the principal absorption edges of the building blocks of materials are a prerequisite for time-resolved probing of the triggering events leading to electronic dynamics such as exciton formation and annihilation. We demonstrate experimentally the isolation of individual attosecond pulses at the carbon K-shell edge (284 eV) in the soft X-ray water window with pulse duration below 400 as and with a bandwidth supporting a 30-as pulse duration. Our approach is based on spatiotemporal isolation of long-wavelength-driven harmonics and validates a straightforward and scalable approach for robust and reproducible attosecond pulse isolation.
We report on the first X-ray absorption fine structure (XAFS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy are well established methods for retrieving structural information about the composition of solid state materials and soft matter. The water window spectral range between 284 eV and 543 eV is of special interest as it contains the K-shell absorption edges of the biological building blocks: carbon (284 eV), nitrogen (410 eV) and oxygen (543 eV). Up until recently only facility scale light sources have been capable of generating coherent water window radiation: synchrotrons with a high degree of spatial coherence and hundreds of femtoseconds pulse durations, and X-ray free electron lasers with a high degree of spatial coherence and femtosecond temporal resolution [1]. High harmonic generation (HHG) [2, 3] offers an attractive alternative approach since it is realizable on a small table-top scale and is capable of generating fully coherent radiation, i.e. femto-to atto-second and possibly even zeptosecond pulse durations. The ability to generate coherent water window radiation from HHG is extremely exciting as it would bring ultra-short time resolution to structural probing with a table top method. HHG is most commonly driven by Ti:sapphire sources at 800 nm with the highest achievable photon energy, the so called cutoff, scaling linearly with the laser intensity and quadratically with the driving wavelength [4]. While the water window range is reachable with such sources via nonphase-matched HHG [5], the contradicting requirements of increasing the cutoff with higher laser intensity while avoiding excessive ionisation, severely limits the achievable flux in the water window. A solution to this dilemma is to use a source with a similar peak intensity and pulse duration, but at much longer emission wavelengths in order to exploit the quadratic wavelength scaling of the HHG cutoff. A drawback of such an approach is the unfavourable single atom response scaling of harmonic yield with λ −9 [6] which can however be mitigated, to a large extent, through high gaspressure phase matching [7]. This concept was demonstrated by reaching a 1.6 keV cutoff when driving with a mid-IR laser system [8]. Despite this cutting-edge result, the 20 Hz repetition rate and stability of the system have thus far proved insufficient for applications, thereby underlining the need for significant improvements of the laser parameters.We find that while high X-ray flux can be achieved through phase-matched HHG driven by kHz or higher repetition rate long-wavelength sources, achieving sufficient intensity and carrier to envelope phase (CEP) stability of the driver laser is an essential key both for producing attosecond pulses and for generating reproducible X-ray spectra from each laser pulse and throughhout an X-ray measurement.Currently at the kHz level and with long wavelength drivers, the lower end of the water window at 300 eV was reached using a Ti:sapphire pumped optical parametric amplifier (OPA) at 1.5 μm [9]...
We present a novel all-fiber pumped OPCPA architecture to generate self-CEP stable, sub-8 optical cycle duration pulses at 7-micron wavelength approaching millijoulelevel pulse energy at 100 Hz repetition rate. The system yields a peak power of 1.1 GW and, if focused to the diffraction limit, would reach a peak intensity of 7x10 14 W/cm 2. The OPCPA is pumped by a 2-micron Ho:YLF chirped pulse amplifier to leverage the highly efficient and broadband response of the nonlinear crystal ZGP. The 7-micron seed at 100 MHz is generated via DFG from an Er:Tm:Ho multi-arm fiber frequency comb and a fraction of its output optically injects the Ho:YLF amplifier. While the pulse bandwidth at 7 micron is perfectly suited for nonlinear and spectroscopic applications, current parameters offer, for the first time, the possibility to explore strong field physics in an entirely new wavelength range with a ponderomotive force 77 times larger than from an 800 nm source. The overall OPCPA system is very compact and provides a new tool for investigations directly in the molecular finger print region of the electromagnetic spectrum or to drive high harmonic generation to produce fully coherent X-rays in the multi-keV range and possibly zeptosecond temporal waveforms.
We report on the first streaking measurement of water-window attosecond pulses generated via high harmonic generation, driven by sub-2-cycle, CEP-stable, 1850 nm laser pulses. Both the central photon energy and the energy bandwidth far exceed what has been demonstrated thus far, warranting the investigation of the attosecond streaking technique for the soft X-ray regime and the limits of the FROGCRAB retrieval algorithm under such conditions. We also discuss the problem of attochirp compensation and issues regarding much lower photo-ionization cross sections compared with the XUV in addition to the fact that several shells of target gases are accessed simultaneously. Based on our investigation, we caution that the vastly different conditions in the soft X-ray regime warrant a diligent examination of the fidelity of the measurement and the retrieval procedure.
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