Jens Biegert

Carrier‐envelope phase (CEP)‐stable ultrashort pulse sources in the short‐wave infrared (SWIR) and mid‐infrared (MWIR) are central to modern ultrafast laser science. By combining robust CEP control with high average power and near‐single‐cycle pulse durations, these sources provide compact and versatile platforms for applications ranging from high‐resolution ultrafast spectroscopy and coherent imaging to strong‐field physics. This review surveys recent advances that are driving the broader adoption of coherent broadband few‐cycle infrared ultrafast systems, and we discuss frequency down‐conversion schemes based on difference‐frequency generation and intra‐pulse difference‐frequency generation, optical parametric generation and amplification, passive CEP‐stable architectures, and coherent multi‐octave infrared sources. Such ultrafast CEP‐stable sources provide unprecedented control over spectral tunability, phase stability, and pulse characteristics, fostering exciting opportunities across spectroscopy, imaging, and attosecond science.

We recently reported a refine analysis of a previously conducted soft X-ray (SXR) attosecond streaking measurement [1], employing the Variational Phase Gradient Temporal Analysis (VPGTA) retreival algorithm [2]. This re-evaluation, prompted by new metrological insights, revealed a 19.2 attosecond pulse - consistent with the expectations and estimates of our earlier work [3]. Shortly thereafter, a comment by M. Han [4] challenged our findings, citing "physical and technical issues concerning the original experiment and the new characterisation" inclusing purported contributions from Auger electrons, unfiltered low-energy harmonics, and uncompensated intrinsic chirp. These concerns, however, stem from misinterpretation of the experimental regime, selective citation of context, and disregard for well-established results in the peer-reviewed literature. In this reply, we address and clarify these points, demonstrating that the issues raised are not relevant under our experimental conditions. Given the highly specialized nature of attosecond genertaion and metrology in the soft X-ray water-window regime, we consider it essential to clarify these aspects of different regimes and reaffirm teh validity of our conclusions.

Generating high-energy pulses in the short- to mid-wave infrared typically relies on parametric amplification, for which producing a suitable high-energy seed remains key. We introduce a compact and efficient seed-generation scheme based on a picosecond Yb thin-disk laser and anti-resonant-reflection photonic crystal fibers. This setup directly delivers 9-fs seed pulses with 40% conversion efficiency. Using intra-pulse difference-frequency mixing, we generate self-CEP-stable pulses with an energy of 100 nJ and an efficiency of up to 1%. A single-stage parametric amplifier then scales the output to 14 µJ at 2200 nm. The system demonstrates remarkable performance, with 1.15% energy stability over 30 minutes and 217 mrad CEP stability over 10 minutes. Our architecture is compact, energy- and wavelength-scalable, and uniquely provides high-energy, self-CEP-stable seeds. This makes it an ideal platform for advanced spectroscopy, nonlinear optics, and high-field applications in attoscience and plasma physics.

Electronic correlations occur on attosecond timescales, dictating how chemical bonds form, energy flows, and materials respond to light. Capturing such many-body processes requires light pulses of similar duration. The soft X-ray water window is vital because it encompasses the principle absorption edges of carbon, nitrogen, and oxygen that underpin chemistry, biology, and materials science. However, generating and characterising isolated attosecond pulses that reach into the soft X-ray water window has remained a challenge. We addressed this need and report an isolated attosecond soft X-ray pulse with a duration of 19.2 attoseconds. This pulse reaches into the water window and is shorter than the atomic unit of time (24.2 as). The pulse is supported by a spectrum centred at 243 eV, extending up to 390 eV, and crossing the carbon K-edge with record photon flux: 4.8x10$^10$ photons per second overall and 4.1x10$^9$ photons per second in a 10% bandwidth at the carbon K-shell edge (284 eV). Such an extremely short soft X-ray pulse combines extreme temporal resolution with a coherent ultrabroadband soft X-ray spectrum, opening new opportunities to study electron dynamics in atoms, molecules, and solids, disentangle many-body interactions in correlated systems, and follow non-adiabatic energy flow in molecular complexes. Our results establish a new benchmark for table-top attosecond technology and lay the foundation for its widespread application in science and technology.

Charge density waves (CDW) appear as periodic lattice deformations which arise from electron-phonon and excitonic correlations and provide a path towards the study of condensate phases at high temperatures. While characterization of this correlated phase is well established via real or reciprocal space techniques, for systems where the mechanisms interplay, a macroscopic approach becomes necessary. Here, we demonstrate the application of polarization-resolved high-harmonic generation (HHG) spectroscopy to investigate the correlated CDW phase and transitions in TiSe₂. Unlike previous studies focusing on static crystallographic properties, the research examines the dynamic reordering that occurs within the CDW as the material is cooled from room temperature to 14 K. By linking ultrafast field-driven dynamics to the material’s potential landscape, the study demonstrates HHG’s unique sensitivity to highly correlated phases and their strength. The findings reveal an anisotropic component below the CDW transition temperature, providing insights into the nature of this phase. The investigation highlights the interplay between linear and nonlinear optical responses and their departure from simple perturbative dynamics, offering a fresh perspective on correlated quantum phases in condensed matter systems.

Chemical reactivity is fundamentally governed by the coupled dynamics of electrons and nuclei, which extend beyond the scope of the Born-Oppenheimer approximation. Directly resolving these coupled motions on their intrinsic timescales has remained a significant challenge. We employ a two-color mid-infrared electron-nuclear streaking technique to image electron-nuclear dynamics in real-time. This approach enables the control and tracking of transient electron localization within the hydrogen molecular ion (H₂^⁺) during its transition from a neutral molecule into an atomic state. We capture the dissociation process of H₂^⁺ into two nuclei, revealing the evolution of the quantum tunneling barrier. These findings constitute the first complete electron-nuclear visualization of a photochemical reaction, offering unprecedented insights into molecular dissociation dynamics. This methodology provides a foundation for extending such ultrafast imaging techniques to more complex molecular systems, advancing the understanding of chemical processes at their most fundamental level.

Electronic correlations occur on attosecond timescales, dictating how chemical bonds form, energy flows, and materials respond to light. Capturing such many-body processes requires light pulses of similar duration. The soft x-ray water window is vital because it encompasses the principal absorption edges of carbon, nitrogen, and oxygen that underpin chemistry, biology, and materials science. However, generating and characterizing isolated attosecond pulses that reach into the soft x-ray water window has remained a challenge. We addressed this need and report an isolated attosecond soft x-ray pulse with a duration of 19.2 as. This pulse reaches into the water window and is shorter than the atomic unit of time of 24.2 as. The pulse is supported by a spectrum centered at 243 eV, extending up to 390 eV, and crossing the carbon K-edge with record photon flux: 4.8 × 10 10 photons per second overall and 4.1 × 10 9 photons per second in a 10% bandwidth at the carbon K-shell edge (284 eV). Such an extremely short, soft x-ray pulse combines extreme temporal resolution with a coherent, ultrabroadband soft x-ray spectrum, opening new opportunities to study electron dynamics in atoms, molecules, and solids, to disentangle many-body interactions in correlated systems, and to follow nonadiabatic energy flow in molecular complexes. Our results establish a new benchmark for table-top attosecond technology and lay the foundation for its widespread application in science and technology.

Mid-infrared microscopy is an important tool for biological analyses, allowing a direct probe of molecular bonds in their low energy landscape. In addition to the label-free extraction of spectroscopic information, the application of broadband sources can provide a third dimension of chemical specificity. However, to enable widespread deployment, mid-infrared microscopy platforms need to be compact and robust while offering high speed, broad bandwidth, and high signal-to-noise ratio. In this study, we experimentally showcase the integration of a broadband, high-repetition-rate dual-comb spectrometer (DCS) in the mid-infrared range with a scanning microscope. We employ a set of 1-GHz mid-infrared frequency combs, demonstrating their capability for high-speed and broadband hyperspectral imaging of polymers and ovarian tissue. The system covers 1000 cm−1 at νc = 2941 cm−1 with 12.86 kHz spectra acquisition rate and 5 µm spatial resolution. Taken together, our experiments and analysis elucidate the trade-off between bandwidth and speed in DCS as it relates to microscopy. This provides a roadmap for the future advancement and application of high-repetition-rate DCS hyperspectral imaging.

This work explores the use of polarization-resolved high harmonic generation (HHG) spectroscopy to investigate the quantum phases and transitions in the correlated charge density wave (CDW) phase of TiSe$_2$. Unlike previous studies focusing on crystallographic changes, the research examines the reordering that occurs within the CDW phase as the material is cooled from room temperature to 14 K. By linking ultrafast field-driven dynamics to the material's potential landscape, the study demonstrates how HHG is sensitive to quantum phase transitions. The findings reveal an anisotropic component below the CDW transition temperature, providing new insights into the nature of this phase. The investigation highlights the interplay between linear and nonlinear optical responses and their departure from simple perturbative dynamics, offering a fresh perspective on correlated quantum phases in condensed matter systems.