Generation of few-femtosecond superradiant extreme-ultraviolet free-electron laser pulses
Foreword
The typical time duration of the light pulses produced at FERMI is in the range 20-90 fs. Moreover, as the FEL process is driven by an external seed laser, FERMI pulses are easily synchronized with femtosecond accuracy with an external reference and have good longitudinal coherence, therefore making this facility a valuable tool for time resolved experiments. Nevertheless, shorter and powerful pulses with a time durations below 10 fs would allow the study of, among other interesting phenomena, conical intersections in photochemistry and short-lived nonlinear phenomena resolving the hole-electron recombination (Auger effect).
The time duration of the pulse produced by a seeded FEL working in HGHG mode, like FERMI, is directly proportional to the seed duration. Nevertheless, the simple approach of shortening the seed is not sufficient because of the finite-gain bandwidth of the conversion process.
Here the FERMI team and collaborators succeeded in demomstrating the feasibility of a new approach, the superradiance cascade (SRC) proposed by Giannessi et al. for producing very short FEL pulses. This process exploits the FEL dynamic process itself to beat the gain bandwidth limit, where the FEL pulse propagates and grows as a self-similar solitary wave, undergoing longitudinal compression into saturation and superradiance, along a cascade of undulators resonant at progressively higher harmonics of an initial seed. The production of such short pulses implies a higher peak power than in the standard high-gain harmonic generation (HGHG) mode.
Results
The FERMI team performed a three-stage superradiant cascade (SRC) starting from an ultraviolet (UV) seed pulse and reaching the EUV spectral range. The emitted final wavelength of 14.7 nm was chosen as the 18thharmonics of 264.4 nm seed, to be achieved through a triple harmonic jump over the 6 available radiators (x3, x2 and x3). In this experiment the third harmonic of the Ti:Sa is optimized to produce the shortest and most powerful pulse available now available for seeding at FERMI. Starting from a short and powerful seed makes it easier to enter into the superradiant regime:
The SRC configuration was compared with the standard HGHG two-stage cascade, tuned to reach the same final wavelength in a double harmonic jump (6 x 3). The switching between the two configurations entailed tuning of the undulator gaps and dispersive sections, while preserving the same electron beam properties and seed duration.
The pulse duration was directly inferred by an autocorrelation measurement based on a time-resolved Two Photon Above Threshold Ionization (ATI) experiment carried out on Argon.
The FWHM pulse durations, derived from autocorrelation traces both for the SRC and the HGHG FEL configurations are δtFEL = 4.7 ±0.6 fs in SRC mode, in agreement with the Fourier Transform Limit (FTL) of the experimental spectrum and with the simulations carried out with FEL numerical tools (GENESIS 1.3). In HGHG mode it is δtFEL = 22 ± 4 fs.
From the measurement of the spectral width, it is inferred that the FTL pulse duration in HGHG mode should be about 15 fs (FWHM). This value is smaller than the measured one, suggesting a residual non-linear phase chirp of the seed. On the contrary, the theory for superradiance predicts that the duration of the superradiant spike can be deduced as the FTL of the spike spectral width. In our case: ~5 fs (FWHM), which is in reasonable agreement with the actual measurement.
The FERMI photon diagnostic system included an ionization chamber which was used to monitor the pulse energy in the two configurations. The obtained averages were 7.7 ±1.8 μJ and 23.5 ±5.2 μJ in SRC and HGHG mode respectively; the resulting shot-to-shot relative stability in the two configurations is thus comparable (24% in SRC versus 22% in HGHG).
In other experiments a superradiant pulse at the 12th harmonic of the seed has been produced starting from a laser pulse produced by an optical parametric amplifier. In this case it was possible even to tune the central wavelength of the output radiation between 20.52 and 20.64nm and to study the nonlinear behavior of the He 2S2P (Fano resonance)
Perspectives
In SRC mode pulses shorter than those allowed by the gain bandwidth of the corresponding FEL amplifier have been obtained while preserving or even exceeding the saturation peak power.
Moreover, pulse energy stability is similar in both HGHG and SRC modes, again a feature inherent to seeded FELs.
During the proof-of-principle experiment we could transport the light simultaneously from the three stages at 88 nm, 44 nm and 14.7 nm, down to the experimental chamber. The absence of dispersive sections along the cascade ensures sub-femtosecond-level synchronization also between the different colours, that is the various harmonics could be phase locked. Thus pump-probe experiments can be performed, either with a synchronized optical laser or with the EUV light of the intermediate stage (at 44 nm in the example presented here), which is expected to be only around two or three times longer than the measured pulse.
All of the above features are potentially beneficial for present and future users. That is why it is foreseen that the SRC configuration be implemented very soon as one of the operation modes of FERMI for user experiments.
