Generation of Ultrashort Coherent Vacuum Ultraviolet Pulses Using Electron Storage Rings: A New Bright Light Source for Experiments

We demonstrate for the first time that seeded harmonic generation on electron storage rings can produce coherent optical pulses in the vacuum ultraviolet spectral range. 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.

Figure 1

Schematic layout of the CHG setup implemented on the Elettra SR. The SR is filled with a relativistic electron bunch, which interacts in the modulator with an external (Ti:sapphire) laser of wavelength $\lambda$. In the magnetic chicane, electrons are subdivided in micro-bunches separated by $\lambda$. Finally, in the radiator, they emit coherently at $\lambda /n$, $n$ being an integer number. The radio-frequency cavity is used to provide electrons with the energy they lose during one turn of the ring.

Figure 2

Intensity of the UV pulses vs acquisition time. 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 ${10}^4$ (see text). In (a) the radiator is tuned for circular polarization; in (b) the radiator is tuned for linear polarization.

Figure 3

Quadratic dependence of the coherent harmonic detected using a PMT vs (normalized) bunch current. Dots represent experimental data; the curve is a fit obtained using a quadratic function.

Figure 4

(a) Spectrum of the coherent emission at 132 nm (linear polarization). The integration time is 1 ms; the spectrum is obtained after subtraction of the background due to spontaneous emission. (b) Spectrum of spontaneous and coherent emission for the case in which the radiator is tuned at 203 nm, i.e., slightly mismatched with respect to the second harmonic of the seed laser (198.5 nm).

Figure 5

Acquisition of the coherent signal during 200 ms (i.e., 200 turns pulses).