The ability to resolve the complete electric field of laser pulses from terahertz to mid-infrared spectral ranges has enriched time-domain spectroscopy for decades. Field-resolved measurements in this range have been performed routinely in ambient air by various techniques like electro-optic sampling, photoconductive switching, field-induced second harmonic generation, and time stretch photonics. On the contrary, resolving the electric field of light at the near-infrared spectral range has been limited to attosecond streaking and other techniques that require operation in vacuum. Recent advances are circumventing these shortcomings and extending the direct, ambient air field detection of light to petahertz frequencies. In the first part of this letter, recent field-resolved techniques are reviewed. In the second part, different approaches for temporal scanning are discussed, as the temporal resolution of the time-domain methods is prone to temporal jitter. The review concludes by discussing technological obstacles and emerging applications of such advancements.

A beamline for temporal diagnostics of extreme ultraviolet (XUV) femtosecond pulses at the free-electron laser in Hamburg (FLASH) at DESY was designed, built and put into operation. The intense ultra-short XUV pulses of FLASH fluctuate from pulse to pulse due to the underlying FEL operating principle and demand single-shot diagnostics. To cope with this, the new beamline is equipped with a terahertz field-driven streaking setup that enables the determination of single pulse duration and arrival time. The parameters of the beamline and the diagnostic setup as well as some first experimental results will be presented. In addition, concepts for parasitic operation are investigated.

Self-amplified spontaneous-emission (SASE) free-electron lasers (FELs) deliver ultrashort pulses with femtosecond durations. Due to the fluctuating nature of the radiation properties of SASE FELs, characterizing FEL pulses on a single-shot basis is necessary. Therefore, we use terahertz streaking to characterize the temporal properties of ultrashort extreme ultraviolet pulses from the free-electron laser in Hamburg (FLASH). In this study, pulse duration as well as pulse energy are measured in a wavelength range from 8 to 34 nm as functions of undulators contributing to the lasing process. The results are compared to one-dimensional and three-dimensional, time-dependent FEL simulations.

The Free-electron LASer in Hamburg - FLASH - generates intense ultrashort pulses with femtosecond duration in the range of 3.3 − 90 nm in the fundamental. In addition, higher harmonic contributions are always present in the spectral distribution, extending this range to much shorter wavelengths. This study presents a detailed exploration of the evolution of the radiation properties of the third harmonic contribution in the self-amplified spontaneous emission (SASE) amplification process. Utilizing a terahertz streaking setup, we measured and analyzed the pulse durations and energies of the fundamental and third harmonic during the amplification process under various operational configurations and compared these with results from the 3D time-dependent free electron laser (FEL) simulation code FAST.

Free-electron lasers (FELs) operating in the extreme ultraviolet (XUV) and X-ray regions deliver ultrashort pulses with unprecedented intensity, enabling groundbreaking research across various scientific disciplines. A potential chirp (frequency change within the pulse) of these pulses influences their spectral properties, directly impacting the experimental outcomes and FEL performance. The accurate characterization of the chirp is, therefore, important for optimizing FEL operation and interpreting experimental results. This study presents a comprehensive comparison of two techniques determining the chirp of the XUV pulses at FLASH by directly measuring the XUV pulses with THz streaking and by detecting the chirp of the electron bunches by a Transverse Deflection Structure (PolariX TDS) to infer the XUV chirp. We conducted simultaneous measurements using both techniques at FLASH2 while tuning the FEL to produce various energy chirps on the electron bunch.