Direct measurement of the pulse duration and frequency chirp of seeded XUV free electron laser pulses

Abstract

We report on a direct time-domain measurement of the temporal properties of a seeded free-electron laser pulse in the extreme ultraviolet spectral range. Utilizing the oscillating electromagnetic field of terahertz radiation, a single-shot THz streak-camera was applied for measuring the duration as well as spectral phase of the generated intense XUV pulses. The experiment was conducted at FLASH, the free electron laser user facility at DESY in Hamburg, Germany. In contrast to indirect methods, this approach directly resolves and visualizes the frequency chirp of a seeded free-electron laser (FEL) pulse. The reported diagnostic capability is a prerequisite to tailor amplitude, phase and frequency distributions of FEL beams on demand. In particular, it opens up a new window of opportunities for advanced coherent spectroscopic studies making use of the high degree of temporal coherence expected from a seeded FEL pulse.

THz streak camera performance for single-shot characterization of XUV pulses with complex temporal structures

“…Retrievals of the chirp of HHG pulses indicate a reproduction of the decrease of the chirp parameter with increasing harmonic order [33]. Analyses of SASE FEL [30,37] and seeded FEL [38] photon pulses reveal a linear chirp that is attributed to the linear energy chirp in the electron bunch.…”

“…The period of the laser streaking field has to be longer than the XUV pulse duration. Therefore, THz field is used for diagnostics of femtosecond XUV pulses [30][31][32][33]36,38] and near infra-red (NIR) field is used for diagnostics of attosecond pulses [29,[39][40][41][42]. The presence of the laser field introduces an additional momentum to the electrons that are released into the continuum state during the interaction of the XUV pulse with the atoms.…”

“…The streaking speed α is determined from the analysis of streaking spectrograms as described in section 5.2. The Gaussian approximation has been implemented for the retrieval of SASE FEL pulses at FLASH [30,37], HHG pulses [32,33] as well as seeded FEL pulses [38] with several tens of femtoseconds pulse duration. All studies report good agreement between retrieved and expected photon pulse durations.…”

Stimulated resonant inelastic x-ray scattering with chirped, broadband pulses

“…Also FEL pulses in the x-ray regime, which typically have a spectral bandwidth on the order of 1 % to several percent of the central photon energy [2,55,56], can cover the energy gap between ground state and valence-excited states (condition (II)(a)). Moreover, they can be chirped [57,58]. For instance at FLASH in Hamburg [1], the pulses could potentially be chirped in such a way that the arrival times of frequencies in the range up to ∼ 4 % of the photon energy are stretched over a time interval on the order of tens of femtoseconds [59].…”

Temporal characterization of a plasma-based seeded XUV laser using a laser-dressed photoionization technique

“…Temporal information about the XUV pulse can be retrieved in a single-shot from the streaked spectrum. The streaking technique has been successfully demonstrated with XUV free-electron lasers, using a THz dressing pulse 6 . If tXUV >> Td satellite peaks called sidebands appear on each side of the main photoline, when the XUV pulse and the dressing pulse are overlapped in time.…”