Miltcho Bojanov Danailov

Externally seeded free-electron lasers (FELs) provide radiation with full longitudinal and transverse coherence in the extreme ultraviolet and soft x-ray spectral ranges, inaccessible to conventional laser sources. Until now, externally seeded operation has been limited to the extreme ultraviolet and soft x-ray regime, while shorter wavelengths have been accessed primarily through stochastic self-amplified spontaneous emission processes (or derivatives thereof, one based on self-seeding) or through nonlinear harmonic generation at strongly reduced pulse energy. At the FERMI FEL facility, we have demonstrated stable operation in high-gain harmonic generation (HGHG) covering the nitrogen K-edge, most of the water-window spectral range, and approaching the oxygen K-edge by combining short-wavelength ultraviolet seeding (200 nm), increased electron beam energy, and preservation of an ultralow relative slice energy spread at the 10 − 5 level. The emitted radiation at such short wavelengths still exhibits the typical narrow bandwidth and wavelength stability of the HGHG scheme enabling, for example, time-resolved x-ray absorption spectroscopy measurements. Longitudinally coherent emission at 1 nm is obtained as the third harmonic of the fundamental FEL emission. Despite the lower intensity typical of nonlinear harmonic emission, these results demonstrate that external seeded FELs can be extended to nanometer wavelengths, establishing deterministic, fully coherent soft x-ray generation and surpassing the previous wavelength operation limits. This advance opens new opportunities for coherent control and nonlinear x-ray experiments that require both high photon energy and longitudinal coherence, and demonstrates a promising pathway toward further extension of externally seeded FELs to shorter wavelengths.

Two-photon absorption laser-induced fluorescence (TALIF) is a well-established diagnostic technique for measuring the density of neutral atoms in plasmas. We demonstrate a new setup for a femtosecond-TALIF (fs-TALIF) diagnostic in fusion-relevant hydrogen plasmas in the Resonant Antenna Ion Device (RAID). The system is designed to produce high-energy laser pulses with a broad spectral width, suitable for exciting both atomic hydrogen and krypton in known TALIF schemes without the need to retune the laser wavelength. The setup consists of a commercial fs laser, an in-house-developed optical harmonic generator, and the fluorescence detection system on the plasma device. The system produces laser pulses at the atomic hydrogen two-photon absorption line at 205 nm, with pulse energies up to 145 μJ, spectral widths of 1.2 nm (1/e2 width), and pulse durations of 250 fs (FWHM). The pulse parameters are stable on a pulse-to-pulse basis and over longer time periods. We characterize the effect of the transport of the pulses to the plasma device through 19 m of atmospheric air and a thin MgF2 window. We show a one-dimensional fs-TALIF profile in a krypton gas in RAID, in which the laser is collimated (3 mm diameter) and tuned to the hydrogen resonance. This result shows the potential of the system to be used as a fs-TALIF diagnostic for calibrated atomic hydrogen density measurements.

In seeded free-electron lasers (FELs), harmonic up-conversion of an external seed laser can be used to generate coherent emission at short wavelengths. The harmonic generation process is sensitive to the electron beam slice energy spread, which limits the highest harmonic conversion factor. We present here an experimental study of the impact of the slice energy spread on the FEL performance in two different harmonic generation schemes: high-gain harmonic generation and echo-enabled harmonic generation (EEHG). The slice energy spread is controlled by means of a laser heater installed in the low energy region of the linear accelerator that drives the FEL. The measurements of the FEL pulse energy as a function of the electron beam slice energy spread were performed at different harmonics. Our results confirm the robustness of EEHG to the slice energy spread and open the way toward fully coherent FEL pulses in the soft x-ray domain.

Ultrafast time-resolved photoelectron spectra are reported for the vacuum-ultraviolet (VUV) photoionization of acetylene following excitation to the Ã1Au state via UV absorption at 200 nm. The excitation energy lies above the lowest dissociation threshold to C2H X̃2Σ+ + H, as well as above the threshold for adiabatic dissociation of the Ã1Au state to form C2H (Ã2Π) + H. The time-dependent mass spectra and photoelectron spectra provide insight into the intramolecular decay processes of the Ã1Au state. In addition, photoelectron spectra of the Ã1Au state with VUV light access both the X̃2Πu and Ã2Σg+ states of the ion, as well as the predicted, but previously unobserved, 1 2Πg state, which corresponds to a two-hole, one-particle configuration that lies in close proximity to the Ã2Σg+ state. The 1 2Πg state is split into 2A2 + 2B2 and 2Ag + 2Bg states in the cis and trans configurations, respectively. Electronic structure calculations, along with trajectory calculations, reproduce the principal features of the experimental data and confirm the assignment of the 1 2Πg state.

The operation of modern free-electron lasers (FELs) necessitates precise knowledge of electron beam properties at the undulator to ensure the level of control required by increasingly demanding experiments. In seeded FELs, where only electrons interacting with the seed laser contribute to the process, it is crucial to determine the local values of these properties. We present a novel method, based on accurate modeling of the FEL process in high-gain harmonic generation, to accurately retrieve the electron beam slice energy spread, current and laser-induced energy modulation. Understanding these values is essential for enabling advanced FEL schemes and optimally setting advanced seeding schemes such as echo-enabled harmonic generation. We describe the method and provide an experimental application to the FERMI FEL-1, where a slice energy spread in the range 40–100 keV with a few keV accuracy is measured.

Generation of high energy few-fs pulses in the ultraviolet (UV) still represents challenges due to compression and phase control difficulties in this spectral range. Presented here is a pulse compression approach utilizing cross-phase modulation within a thin solid-state medium induced by a strong, spatially and temporally controllable near-infrared (NIR) pulse acting on a weaker, 400 nm UV pulse. Through this method, four-fold compression is attained within a single fused silica plate, resulting in a 13 fs UV pulse with preserved beam quality. With some further technical adjustments, this method’s applicability could be extended to deep or even vacuum UV, where direct compression is difficult.

Wave packet interferometry with vacuum ultraviolet light has been used to probe a complex region of the electronic spectrum of molecular nitrogen, N2. Wave packets of Rydberg and valence states were excited by using double pulses of vacuum ultraviolet (VUV), free-electron-laser (FEL) light. These wave packets were composed of contributions from multiple electronic states with a moderate principal quantum number (n ∼ 4–9) and a range of vibrational and rotational quantum numbers. The phase relationship of the two FEL pulses varied in time, but as demonstrated previously, a shot-by-shot analysis allows the spectra to be sorted according to the phase between the two pulses. The wave packets were probed by angle-resolved photoionization using an infrared pulse with a variable delay after the pair of excitation pulses. The photoelectron branching fractions and angular distributions display oscillations that depend on both the time delays and the relative phases of the VUV pulses. The combination of frequency, time delay, and phase selection provides significant control over the ionization process and ultimately improves the ability to analyze and assign complex molecular spectra.

An important trend in extreme ultraviolet and soft X-ray free-electron laser (FEL) development in recent years has been the use of seeding by an external laser, aimed to improve the coherence and stability of the generated pulses. The high-gain harmonic generation seeding technique was first implemented at FERMI and provided FEL radiation with high coherence as well as intensity and wavelength stability comparable to table-top ultrafast lasers. At FERMI, the seed laser has another very important function: it is the source of external laser pulses used in pump–probe experiments allowing one to achieve a record-low timing jitter. This paper describes the design, performance and operational modes of the FERMI seed laser in both single- and double-cascade schemes. In addition, the planned upgrade of the system to meet the challenges of the upgrade to echo-enabled harmonic generation mode is presented.