SR-FEL Highlights

Different methods for generating harmonics are compared between them, and a detailed characterization of the emitted light is performed for different polarizations. Our results also contribute to the debate about the possible presence of a coherent on-axis signal in helical undulators. In this respect, we provide an experimental confirmation of recent theoretical studies that predict no coherent on-axis signal.

In the figure:
Measured angular distribution of the second harmonic in the case of CHG (red) and NHG (black) with helical undulators. Measurements are well fitted by theoretical curves (dashed line, CHG case, continuous line, NHG case).

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The setup is based on the frequency up-conversion of a high-power external signal (provided by a Ti:Sapphire laser) and makes use of a relativistic electron bunch as resonating medium.The produced VUV pulses have peak power in MW range, variable polarization, high shot to shot stability and control of the timing parameters at the ps level. In this paper, we present the first characterization of the temporal and spectral features of the emitted light. The radiation can be exploited for new experiments in the fields of dynamical phenomena, non-linear physics, magnetism and biology.

In the figure:
Open dots are the CHG amplitude as a function of the relative delay between seed and electron bunch, dashed line is the electron bunch temporal profile as measured with the streak camera, the solid line is the squared bunch profile. The quadratic dependence confirms the coherent character of the emission.

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The experiment is performed at Elettra, where coherent pulses are generated at 132 nm, with a duration of about 100 fs. The light source has a repetition rate of 1 kHz and adjustable polarization; it is very bright, with a peak power several orders of magnitude above that of spontaneous synchrotron radiation. Owing to high stability, the source is used in a test photoemission electron microscopy experiment. We anticipate that seeded harmonic generation on storage rings can lead to unprecedented developments in time-resolved femtosecond spectroscopy and microscopy. 

In the figure:
Intensity of the UV pulses vs acquisition time. The pulse at the center corresponds to the coherent harmonic signal, while the small peaks at ±1000 ns correspond to the synchrotron emission of the undulators. The signal was acquired using a photomultiplier (PMT) placed downstream a monochromator. Note that the PMT does not allow to resolve the sub-ps temporal scale on which the coherent pulse evolves. This, in turn, does not permit direct detection and therefore appreciation of the effective amplitude difference between the seeded and the spontaneous signals, their true ratio being a factor about 10⁴ .

Red trace: the radiator is tuned for circular polarization; black trace: the radiator is tuned for linear polarization.

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