Meng Han

Generating ever-shorter and brighter light pulses is a central goal of ultrafast science, enabling coherent control and observation of electron dynamics on their natural timescale. State-of-the-art isolated attosecond pulse generation currently achieves pulse durations of 40-50 attoseconds. Here we demonstrate isolated attosecond light pulses with durations of 18 attoseconds, via high-order harmonic generation driven by a post-compressed industrial-grade Yb laser system. The high-harmonic spectrum spans photon energies from 50 to 320 eV, covering the carbon K-edge, with a calibrated photon flux exceeding 10¹² photons per second. Pulse durations were characterized by angle-resolved photoelectron streaking in helium and optimized using a series of filters with different thicknesses to compensate the attochirp. We further developed a robust, fast-converging pulse-retrieval algorithm capable of reconstructing broadband isolated attosecond pulses and double pulses. These results establish Yb-laser-driven high-order harmonic generation as a powerful platform for bright isolated attosecond sources and next-generation ultrafast spectroscopy.

The capability of generating attosecond (10-18s) pulses of light in the extreme-ultraviolet domain through high-harmonic generation (HHG) has opened a broad range of possibilities in studying the fastest dynamics in matter. Notably, the creation of light pulses with tailored, time-dependent polarization states has opened a new window into the most fundamental structural and electronic dynamics underlying molecular chirality. This article reviews the work of our group on three forms of chiroptical spectroscopy: (i) high-harmonic spectroscopy with tailored light fields, which has achieved a ~13% dichroism effect in discrimination of enantiomers and the chirality-sensitive observation of a dissociative reaction, (ii) femtosecond time-resolved photoelectron circular dichroism (PECD), applied to resolve the photodissociation dynamics of two chiral molecules and (iii) the development of circularly polarized attosecond pulse trains, applied to coherently control PECD on the attosecond timescale and measure chirality-sensitive attosecond photoionization delays. These methods advance chiroptical spectroscopy to the attosecond timescale and open new perspectives for probing and controlling molecular chirality on electronic timescales.

Time-resolved measurement of Auger-Meitner decay [Drescher et al., Nature (London) 419, 803 (2002)NATUAS0028-083610.1038/nature01143] marked a milestone in the development of attosecond science. To date, the time constants for the Auger-Meitner decay processes obtained from the time-domain experiments were found to be consistent with the values deduced from conventional energy-domain measurements. One of the main factors limiting the temporal resolution of these studies is the unlocked carrier-envelope-phase (CEP) of the laser pulses used to probe the electronic dynamics triggered by inner-shell photoabsorption. In this Letter, we report time-resolved inner-shell electron spectroscopy of xenon and krypton using attosecond soft x-ray (atto-SXR) pulses centered at 130 eV in combination with CEP-stabilized few-cycle Yb laser pulses. We observed that the N_{4,5}OO Auger electrons from xenon exhibit a clear streaking pattern, but with an unexpected time shift of ∼1.32 fs relative to the 4d photoelectrons. Furthermore, the energy-integrated yield of streaked Auger electrons from xenon exhibits a pronounced minimum at a pump-probe time delay of 4 fs. Neither of these observations can be explained by current streaking theories and both are inconsistent with lifetimes inferred from energy-domain measurements. The M_{4,5}NN Auger electrons from krypton partly overlap in energy with the 3d inner-shell photoelectrons and do not show these anomalous features. This Letter offers new insights into the inner-shell electron dynamics of heavy atoms in the giant dipole resonance region, laying the groundwork for attosecond soft x-ray spectroscopy of molecular systems containing iodine or bromine atoms.

We introduce circular under-threshold RABBITT (cuRABBITT) as a new interferometric method to probe discrete electronic excitations in atoms with attosecond resolution. By combining circularly polarized attosecond pulses with broadband ("rainbow") spectral analysis, we directly access two-photon ionization amplitudes and their relative phases. Time-dependent Schrödinger simulations, supported by Green's function theory, reveal strong resonances in helium and argon and a Cooper-like minimum in xenon. These results demonstrate that cuRABBITT provides continuous spectral mapping of bound-state resonances and extends Fano's propensity rule into the under-threshold regime. Our Letter establishes cuRABBITT as a powerful attosecond metrology technique, opening the way to polarization-resolved studies of resonant dynamics in atoms and molecules.

Ardana-Lamas et al. recently reported a reanalysis of an attosecond streaking dataset originally published in Physical Review X 7 , 041030 (2017), claiming a pulse duration of 19.2 attoseconds (as). This claim represents the third substantially different pulse-duration estimate extracted from the same experimental data: approximately 332 as (upper limit) in the original publication, 165 as in a later reanalysis, and now 19.2 as—reported nearly 8 years after the experiment was published. The title of the work of Ardana-Lamas et al., first appearing in the preprint posted on 2025 October 5, is notably similar to that of the preprint, “Bright 25-attosecond light pulses reach the one atomic unit of time”, posted on arXiv on 2025 August 20. After the appearance of the preprint, we exchanged detailed comments with the authors regarding their analysis and the reported 19.2-as value, but fundamental disagreements remain unresolved. Because claims of world-record pulse durations carry broad implications for attosecond science, they must be supported by transparent and reproducible analysis. We therefore propose that the original streaking spectra be made publicly available and that independent groups be invited to analyze the data. Only through such open scrutiny can the validity of the claimed pulse duration be reliably established.

The concept of using photoelectron interferometry in short laser fields to probe electron dynamics and target structures was introduced more than two decades ago. However, the quality of experimental data has remained insufficient for quantitative analysis, largely due to the instability of few-cycle Ti:sapphire laser pulses-the current workhorse of short pulses. Here, we report the first systematic strong-field ionization experiments performed with industrial-grade, carrier-envelope-phase stabilized, near-single-cycle Yb lasers. By measuring photoelectron momentum distributions in the direct-ionization regime, we show that single-cycle cosine-shaped pulses can separate and enhance both spider-leg and fishbone holographic structures. The spider-leg structure enables extraction of the electron scattering phase from the Ar atomic potential-information typically accessible through attosecond metrology-while the fishbone structure reveals the orbital-parity contrast between Ar atoms and nitrogen molecules. Our measurements are quantitatively reproduced by both semiclassical Herman-Kluk-propagator and ab initio simulations, paving the way for precision studies of electron-molecule scattering with widely accessible industrial-grade lasers.

A coherent broadband ultraviolet light source is essential for both fundamental research and industrial applications, yet its generation remains challenging. Here, we demonstrate microjoule-level ultraviolet emission spanning 300-450 nm from carbon dioxide ions (CO_{2}^{+}), driven by carrier-envelope-phase (CEP)-stabilized, near-single-cycle light pulses. The radiation arises from transitions between the first excited and ground cationic states, created by strong-field ionization with near-infrared pulses, and self-seeded by either self-phase modulation (SPM) or third-harmonic (TH) generation in the ultraviolet range. Interestingly, while TH-seeded stimulated emission is unaffected by CEP, SPM-seeded ultraviolet radiation shows strong CEP dependence. This arises from a Ramsey-type interference, where the relative phase between the SPM and the strong-field-prepared ionic polarization controls the constructive or destructive interference of the superposition between two ionic states, directly modulating the ionic dipole strength. Leveraging this CEP-sensitive lasing, we demonstrate a single-shot CEP tagging technique with remarkable resolution and simplicity. Our Letter establishes a powerful method for generating and controlling broadband ultraviolet stimulated emission in molecular ions using strong, waveform-stabilized ultrashort pulses, highlighting new possibilities in strong-field ionic quantum optics.