Sub-50 fs temporal resolution in an FEL-optical laser pump-probe experiment at FLASH2.

High temporal resolution is essential for ultra-fast pump-probe experiments. Arrival time jitter and drift measurements, as well as their control, become critical especially when combining XUV or X-ray free-electron lasers (FELs) with optical lasers due to the large scale of such facilities and their distinct pulse generation processes. This paper presents the application of a laser pulse arrival time monitor that actively corrects the arrival time of an optical laser relative to the FEL's main optical clock. Combined with post-analysis single pulse jitter correction this new approach improves the temporal resolution for pump-probe experiments significantly. Benchmark measurements on photo-ionization of xenon atoms performed at FLASH beamline FL26, demonstrate a sub-50 fs FWHM overall temporal resolution.

Spatio-spectral couplings in saturated collinear OPCPA.

Ultrafast laser pulses featuring both high spatio-temporal beam quality and excellent energy stability are crucial for many applications. Here, we present a seed laser with high beam quality and energy stability, based on a collinear optical parametric chirped pulse amplification (OPCPA) stage, delivering 46 µJ pulses with a 25 fs Fourier limit at 1 kHz repetition rate. While saturation of the OPCPA stage is necessary for achieving the highest possible energy stability, it also leads to a degradation of the beam quality. Using simulations, we show that spectrally dependent, rotationally symmetric aberrations dominate the collinear OPCPA in saturation. We experimentally characterize these aberrations and then remove distinct spatial frequencies to greatly improve the spectral homogeneity of the beam quality, while keeping an excellent energy stability of 0.2 % rms measured over 70 hours.

Saturation control of an optical parametric chirped-pulse amplifier.

Optical parametric chirped-pulse amplification (OPCPA) is a light amplification technique that provides the combination of broad spectral gain bandwidth and large energy, directly supporting few-cycle pulses with multi-terawatt (TW) peak powers. Saturation in an OPCPA increases the stability and conversion efficiency of the system. However, distinct spectral components experience different gain and do not saturate under the same conditions, which reduces performance. Here, we describe a simple and robust approach to control the saturation for all spectral components. The demonstrated optimal saturation increases the overall gain, conversion efficiency and spectral bandwidth. We experimentally obtain an improvement of the pulse energy by more than 18%. This technique is easily implemented in any existing OPCPA system with a pulse shaper to maximize its output.

Fast-tunable femtosecond visible radiation via sum-frequency generation from a high power NIR NOPO.

We report on a high power ultra-broadband, quickly tunable non-collinear parametric oscillator with highly efficient intra-cavity sum-frequency generation. It simultaneously delivers femtosecond pulses in two synchronized output beams: up to 4.9 W tunable from 650 to 1050 nm in the near infrared and up to 1.9 W from 380 to 500 nm in the visible spectral range. The (to our knowledge) novel source is ideally suited for spectroscopy or multi-color imaging. First results of two-color functional microscopy are presented.

Compact Ho:YLF-pumped ZnGeP2-based optical parametric amplifiers tunable in the molecular fingerprint regime.

We report on a compact mid-infrared laser architecture, comprising a chain of $ {\rm ZnGeP}_2 $ZnGeP2-based optical parametric amplifiers (OPAs), which afford a higher energy yield ($ \mathbin{\lower.3ex\hbox{$\buildrel \lt \over{\smash{\scriptstyle\sim}\vphantom{_x}}$}} 60\;\unicode{x00B5} {\rm J} $∼x<60µJ at 1 kHz) compared to most conventional OPA gain media transparent in the 2-8-µm wavelength range. Specifically, our OPA scheme allows ready tunability in the molecular fingerprint regime and is tailored for strong-field excitation and coherent control of both stretch and bend (or torsional) vibrational modes in molecules. The OPAs are pumped and directly seeded (via supercontinuum generation) by a 2-µm, 3-ps Ho:YLF regenerative amplifier. The compressibility of the OPA output is demonstrated by a representative measurement of the near-Gaussian temporal profile of a dispersion-compensated 105-fs idler pulse at a central wavelength of 5.1 µm, corresponding to ${\sim}6 $∼6 optical cycles. Detailed numerical simulations closely corroborate the experimental measurements, providing a benchmark and a platform to further explore the parameter space for future design, optimization, and implementation of high-energy, ultrafast, mid-infrared laser schemes.

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. The employed scheme represents a route toward compact few-cycle sources driven by industrial-grade Yb:YAG lasers at high average power.

Multimicrojoule GaSe-based midinfrared optical parametric amplifier with an ultrabroad idler spectrum covering 4.2-16 µm.

We report a multimicrojoule, ultrabroadband midinfrared optical parametric amplifier based on a GaSe nonlinear crystal pumped at ∼2 µm. The generated idler pulse has a flat spectrum spanning from 4.5 to 13.3 µm at -3 dB and 4.2 to 16 µm in the full spectral range, with a central wavelength of 8.8 µm. The proposed scheme supports a subcycle Fourier-transform-limited pulse width. A (2+1)-dimensional numerical simulation is employed to reproduce the obtained idler spectrum. To our best knowledge, this is the broadest -3 dB spectrum ever obtained by optical parametric amplifiers in this spectral region. The idler pulse energy is ∼3.4 µJ with a conversion efficiency of ∼2% from the ∼2 µm pump to the idler pulse.

Temporal and spatial effects inside a compact and CEP stabilized, few-cycle OPCPA system at high repetition rates.

We present a compact and ultra-stable few-cycle OPCPA system. In two non-collinear parametric amplification stages pulse energies up to 17 µJ at 200 kHz repetition rate are obtained. Recompression of the broadband pulses down to 6.3 fs is performed with chirped mirrors leading to peak powers above 800 MW. The parametric amplification processes were studied in detail employing (2 + 1) dimensional numerical simulations and compared to experimental observations in terms of spectral shapes, pulse energy, spatial effects as well as delay dependent nonlinear mixing products. This gives new insights into the parametric process and design guidelines for high repetition rate OPCPA systems.

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. Here, we combine a broadband XUV probe pulse from high-order harmonic generation with an FEL pump pulse to observe dissociation pathways leading to fragments in different quantum states. We temporally resolve the dissociation of a specific O2+ state into two competing channels by measuring the resonances of ionic and neutral fragments. This scheme can be applied to investigate convoluted dynamics in larger molecules relevant to diverse science fields.

Two-color pumped OPCPA system emitting spectra spanning 1.5 octaves from VIS to NIR.

We present a two-color pumped OPCPA system which delivers an ultra-broadband spectrum spanning from 430 nm to 1.3 µm with a Fourier limited pulse duration of sub-3 fs and 1 µJ of pulse energy at a repetition rate of 200 kHz. All frequency components propagate on a common path, thus the spectral phase along the whole spectrum is well-defined. The inner part of the spectrum has been compressed to sub-5 fs pulses.

Multi-µJ, CEP-stabilized, two-cycle pulses from an OPCPA system with up to 500 kHz repetition rate.

We present a two-stage OPCPA system based on a Ti:sapphire seed and a thin-disk regenerative amplifier producing compressed pulse energies of more than 3 µJ and durations of less than 6 fs at a high repetition rate of 143 kHz. In combination with the obtained CEP stability and the repetition rate scalability between 100 and 500 kHz the system forms an ideal tool for high field and phase sensitive spectroscopic experiments.

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. Here we show first results from the commissioning of a high-harmonic beamline that is fully synchronized with the free-electron laser FLASH, installed at beamline FL26 with permanent end-station including a reaction microscope (REMI). An optical parametric amplifier synchronized with the FEL burst mode drives the HHG process. First commissioning tests including electron momentum measurements using REMI, demonstrate long-term stability of the HHG source over more than 14 hours. This realization of the combination of these light sources will open new opportunities for time-resolved studies targeting different science cases including core-level ionization dynamics or the electron dynamics during the transformation of a molecule within a chemical reaction probed on femtosecond timescales in the ultraviolet to soft X-ray spectral region.