44-fs, 1-MHz, 70-µJ Yb-doped fiber laser system for high harmonic generation.

We report the development of a robust Yb-doped fiber laser system based on chirped-pulse amplification (CPA), generating 44-fs laser pulses with up to 70-µJ pulse energy at a 1-MHz repetition rate. It consists of a Yb-doped nonlinear polarization evolution (NPE) mode-locked fiber oscillator, a chirped fiber Bragg grating (CFBG) stretcher, a wave-shaper for manipulating the spectrum of the signal, cascaded fiber amplifiers, and two compression units. The output pulse duration of 44 fs for efficient high harmonic generation (HHG) was achieved by a multi-pass multi-plate Herriott-type non-linear compression unit.

UV 30 fs laser pulse generation using a multi-pass cell.

Ultrashort ultraviolet (UV) pulses are pivotal for resolving ultrafast electron dynamics. However, their efficient generation is strongly impeded by material dispersion and two-photon absorption, in particular, if pulse durations around a few tens of femtoseconds or below are targeted. Here, we present a new (to our knowledge) approach to ultrashort UV pulse generation: using the fourth-harmonic generation output of a commercial ytterbium laser system delivering 220 fs UV pulses, we implement a multi-pass cell (MPC) providing 5.

Time-resolving state-specific molecular dissociation with XUV broadband absorption spectroscopy.

The electronic and nuclear dynamics inside molecules are essential for chemical reactions, where different pathways typically unfold on ultrafast timescales. Extreme ultraviolet (XUV) light pulses generated by free-electron lasers (FELs) allow atomic-site and electronic-state selectivity, triggering specific molecular dynamics while providing femtosecond resolution. Yet, time-resolved experiments are either blind to neutral fragments or limited by the spectral bandwidth of FEL pulses.

Post-compression of multi-millijoule picosecond pulses to few-cycles approaching the terawatt regime.

Advancing ultrafast high-repetition-rate lasers to shortest pulse durations comprising only a few optical cycles while pushing their energy into the multi-millijoule regime opens a route toward terawatt-class peak powers at unprecedented average power. We explore this route via efficient post-compression of high-energy 1.2 ps pulses from an ytterbium InnoSlab laser to 9.

Dispersion-engineered multi-pass cell for single-stage post-compression of an ytterbium laser.

Post-compression methods for ultrafast laser pulses typically face challenging limitations, including saturation effects and temporal pulse breakup, when large compression factors and broad bandwidths are targeted. To overcome these limitations, we exploit direct dispersion control in a gas-filled multi-pass cell, enabling, for the first time to the best of our knowledge, single-stage post-compression of 150 fs pulses and up to 250 µJ pulse energy from an ytterbium (Yb) fiber laser down to sub-20 fs. Dispersion-engineered dielectric cavity mirrors are used to achieve nonlinear spectral broadening dominated by self-phase modulation over large compression factors and bandwidths at 98% throughput.

Below-threshold harmonic generation in gas-jets for Th-229 nuclear spectroscopy.

The generation of below-threshold harmonics in gas-jets constitutes a promising path towards optical frequency combs in the vacuum ultra-violet (VUV) spectral range. Of particular interest is the 150 nm range, which can be exploited to probe the nuclear isomeric transition of the Thorium-229 isotope. Using widely available high-power, high-repetition-rate Ytterbium-based laser sources, VUV frequency combs can be generated through the process of below-threshold harmonic generation, in particular 7 harmonic generation of 1030 nm.

Few-cycle pulse generation by double-stage hybrid multi-pass multi-plate nonlinear pulse compression.

Few-cycle pulses present an essential tool to track ultrafast dynamics in matter and drive strong field effects. To address photon-hungry applications, high average power lasers are used which, however, cannot directly provide sub-100-fs pulse durations. Post-compression of laser pulses by spectral broadening and dispersion compensation is the most efficient method to overcome this limitation.

Long-term stable, synchronizable, low-noise picosecond Ho:fiber NALM oscillator for Ho:YLF amplifier seeding.

We demonstrate a 41.6 MHz, 1.3 ps, 140 pJ Ho:fiber oscillator using a nonlinear amplifying loop mirror (NALM) as saturable absorber.

Synchronized beamline at FLASH2 based on high-order harmonic generation for two-color dynamics studies.

We present the design, integration, and operation of the novel vacuum ultraviolet (VUV) beamline installed at the free-electron laser (FEL) FLASH. The VUV source is based on high-order harmonic generation (HHG) in gas and is driven by an optical laser system synchronized with the timing structure of the FEL. Ultrashort pulses in the spectral range from 10 to 40 eV are coupled with the FEL in the beamline FL26, which features a reaction microscope (REMI) permanent endstation for time-resolved studies of ultrafast dynamics in atomic and molecular targets.

Temporal pulse quality of a Yb:YAG burst-mode laser post-compressed in a multi-pass cell.

Nonlinear pulse post-compression represents an efficient method for ultrashort, high-quality laser pulse production. The temporal pulse quality is, however, limited by amplitude and phase modulations intrinsic to post-compression. We here characterize in frequency and time domain with high dynamic range individual post-compressed pulses within laser bursts comprising 100-kHz-rate pulse trains.

60 fs, 1030 nm FEL pump-probe laser based on a multi-pass post-compressed Yb:YAG source.

This paper reports on nonlinear spectral broadening of 1.1 ps pulses in a gas-filled multi-pass cell to generate sub-100 fs optical pulses at 1030 nm and 515 nm at pulse energies of 0.8 mJ and 225 µJ, respectively, for pump-probe experiments at the free-electron laser FLASH.

Noncollinear Enhancement Cavity for Record-High Out-Coupling Efficiency of an Extreme-UV Frequency Comb.

We demonstrate a femtosecond enhancement cavity with a crossed-beam geometry for efficient generation and extraction of extreme-ultraviolet (XUV) frequency combs at a 154 MHz repetition rate. We achieve a record-high out-coupled power of 600  μW, directly usable for spectroscopy, at a wavelength of 97 nm. This corresponds to a >60% out-coupling efficiency.

Flexible all-PM NALM Yb:fiber laser design for frequency comb applications: operation regimes and their noise properties.

We present a flexible all-polarization-maintaining (PM) mode-locked ytterbium (Yb):fiber laser based on a nonlinear amplifying loop mirror (NALM). In addition to providing detailed design considerations, we discuss the different operation regimes accessible by this versatile laser architecture and experimentally analyze five representative mode-locking states. These five states were obtained in a 78-MHz configuration at different intracavity group delay dispersion (GDD) values ranging from anomalous (-0.

Postcompression of picosecond pulses into the few-cycle regime.

In this work, we demonstrate postcompression of 1.2 ps laser pulses to 13 fs via gas-based multipass spectral broadening. Our results yield a single-stage compression factor of about 40 at 200 W in-burst average power and a total compression factor >90 at reduced power.

A synchronized VUV light source based on high-order harmonic generation at FLASH.

Ultrafast measurements in the extreme ultraviolet (XUV) spectral region targeting femtosecond timescales rely until today on two complementary XUV laser sources: free electron lasers (FELs) and high-harmonic generation (HHG) based sources. The combination of these two source types was until recently not realized. The complementary properties of both sources including broad bandwidth, high pulse energy, narrowband tunability and femtosecond timing, open new opportunities for two-color pump-probe studies.

Tunable dual-comb from an all-polarization-maintaining single-cavity dual-color Yb:fiber laser.

We demonstrate dual-comb generation from an all-polarization-maintaining dual-color ytterbium (Yb) fiber laser. Two pulse trains with center wavelengths at 1030 nm and 1060 nm respectively are generated within the same laser cavity with a repetition rate around 77 MHz. Dual-color operation is induced using a tunable mechanical spectral filter, which cuts the gain spectrum into two spectral regions that can be independently mode-locked.

A nozzle for high-density supersonic gas jets at elevated temperatures.

We present the development of a gas nozzle providing high-density gas at elevated temperatures inside a vacuum environment. Fused silica is used as the nozzle material to allow the placement of the nozzle tip in close proximity to an intense, high-power laser beam, while minimizing the risk of sputtering nozzle tip material into the vacuum chamber. Elevating the gas temperature increases the gas-jet forward velocity, allowing us to replenish the gas volume in the laser-gas interaction region between consecutive laser shots.

Sub-cycle ionization dynamics revealed by trajectory resolved, elliptically-driven high-order harmonic generation.

The sub-cycle dynamics of electrons driven by strong laser fields is central to the emerging field of attosecond science. We demonstrate how the dynamics can be probed through high-order harmonic generation, where different trajectories leading to the same harmonic order are initiated at different times, thereby probing different field strengths. We find large differences between the trajectories with respect to both their sensitivity to driving field ellipticity and resonant enhancement.

High-order harmonic generation using a high-repetition-rate turnkey laser.

We generate high-order harmonics at high pulse repetition rates using a turnkey laser. High-order harmonics at 400 kHz are observed when argon is used as target gas. In neon, we achieve generation of photons with energies exceeding 90 eV (∼13 nm) at 20 kHz.

Beta-aminoketones as prodrugs for selective irreversible inhibitors of type-1 methionine aminopeptidases.

We identified and characterized β-aminoketones as prodrugs for irreversible MetAP inhibitors that are selective for the MetAP-1 subtype. β-Aminoketones with certain structural features form α,β-unsaturated ketones under physiological conditions, which bind covalently and selectively to cysteines in the S1 pocket of MetAP-1. The binding mode was confirmed by X-ray crystallography and assays with the MetAPs from Escherichia coli, Staphylococcus aureus and both human isoforms.

Macroscopic effects in noncollinear high-order harmonic generation.

We study two-color high-order harmonic generation using an intense driving field and its weak second harmonic, crossed under a small angle in the focus. Employing sum- and difference-frequency generation processes, such a noncollinear scheme can be used to measure and control macroscopic phase matching effects by utilizing a geometrical phase mismatch component, which depends on the noncollinear angle. We further show how spatial phase effects in the generation volume are mapped out into the far field allowing a direct analogy with temporal carrier envelope effects in attosecond pulse generation.

Attosecond pulse walk-off in high-order harmonic generation.

We study the influence of the generation conditions on the group delay of attosecond pulses in high-order harmonic generation in gases. The group delay relative to the fundamental field is found to decrease with increasing gas pressure in the generation cell, reflecting a temporal walk-off due to the dispersive properties of the nonlinear medium. This effect is well reproduced using an on-axis phase-matching model of high-order harmonic generation in an absorbing gas.

Characterization of the p53 cistrome--DNA binding cooperativity dissects p53's tumor suppressor functions.

p53 protects us from cancer by transcriptionally regulating tumor suppressive programs designed to either prevent the development or clonal expansion of malignant cells. How p53 selects target genes in the genome in a context- and tissue-specific manner remains largely obscure. There is growing evidence that the ability of p53 to bind DNA in a cooperative manner prominently influences target gene selection with activation of the apoptosis program being completely dependent on DNA binding cooperativity.