Attosecond spectroscopy in the soft X-ray range provides access to charge, spin, and orbital dynamics in transition metals on their natural timescales. However, until now, phase-controlled attosecond sources have been limited to energies below about 450 eV, and the only demonstration of phase matching above 1 keV used multi-cycle pulses without CEP control. This work sits alongside studies on sub-cycle pulse generation and attosecond spectroscopy in the "water window," but it stands out because it is the first to combine phase matching in a neutral medium, CEP control, and energy scalability in the kiloelectronvolt range.
A CEP-stabilized laser based on dual-chirped optical parametric amplification (DC-OPA) was used, delivering up to 48 mJ per pulse with a tunable wavelength of 2.26–2.68 μm. The pulses were focused by a concave mirror with a 3 m focal length into a 50 mm long gas cell filled with helium at about 1 atm. Phase matching was ensured by weak ionization (η = ηc/2) and a large beam radius at the focus (~200 μm), which reduced the required pressure by an order of magnitude compared to tight focusing. Spectra were recorded with a diffraction spectrometer equipped with a CCD detector.
High harmonics were generated with a cutoff up to 1.2 keV at a pump wavelength of 2.68 μm. The signal scaled quadratically with pressure, confirming coherent buildup under phase matching. A pronounced modulation of the spectrum was observed as the CEP was varied: the cutoff shifted by up to 100 eV, and a continuous region as wide as 300 eV corresponded to a Fourier-limited duration of 12 as. In the range of the L-edges of transition metals, a continuum was obtained with an estimated duration of 32 as. Broadband XANES spectroscopy was demonstrated on the L-edges of Ti, Fe, Co, Ni, and the K-edge of oxygen in a single measurement.
This work fills a long-standing gap between kiloelectronvolt photon energies and phase-controlled attosecond radiation. Unlike earlier approaches that relied on strong ionization or high pressures, the proposed scheme uses meter-scale loose focusing and low pressure, simplifying engineering and paving the way for energy scaling. This is the first experimental demonstration of CEP-dependent emission with phase matching in neutral helium up to 950 eV, which is twice the previous record for such sources.
Increasing the pump wavelength to 3 μm and the pulse energy will push the cutoff to 1.5 keV, while moving to half-cycle pulses will extend the continuous spectrum to about 750 eV. Higher laser energy will allow a larger transverse beam size and, consequently, a higher harmonic yield without losing phase matching. This will lay the foundation for attosecond spectroscopy of magnetic, quantum, and strongly correlated materials.
The source opens the possibility of studying ultrafast demagnetization, spin transfer, and the dynamics of electronic correlations in transition metals with sub-femtosecond resolution. It is also applicable to element-selective spectroscopy in the "water window" for biological and chemical systems.
Increase the laser pulse energy and beam size to boost the harmonic yield. Move to half-cycle pulses to obtain broader continuous spectra and conduct the first time-resolved attosecond experiments at the L-edges of transition metals.
This work directly addresses the unresolved problem of observing charge, spin, and orbital dynamics in strongly correlated materials on attosecond timescales. It provides the missing tool for testing theoretical predictions about ultrafast spin dynamics and energy transfer in magnetic systems.
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What the work does
This work demonstrates high-harmonic generation in low-pressure helium using a CEP-stabilized mid-wave infrared laser, reaching all the way into the sub-cycle regime. Free focusing over a meter provided phase matching at ~1 atm, which made it possible to produce harmonics up to 1.2 keV and to observe CEP dependence up to 950 eV, indicating isolated attosecond pulses. The source was applied to broadband XANES spectroscopy of the L-edges of Ti, Fe, Co, and Ni together with the oxygen K-edge in a single measurement.
Where its strength is
The strength of this work lies in combining phase matching in a neutral medium, CEP control, and energy scalability into the kiloelectronvolt range, which is twice the previous record for phase-controlled sources. This opens the door to attosecond spectroscopy of transition-metal L-edges with time resolution that was previously out of reach.
Where it could go next
Worth trying
Why this may matter
Progress along these lines could make kiloelectronvolt attosecond spectroscopy more accessible and more versatile, paving the way to studies of charge and spin dynamics in functional materials, catalysts, and biological systems with unprecedented time and elemental resolution.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.57, against a usual 0.64. It looks like few have drilled nearby.
The estimated pulse duration of 12 as is shorter than one atomic unit of time (24 as), which is considered the characteristic timescale of electron motion in an atom.