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One pulse good, two pulses better

Monday, October 14, 2013

Luca Giannessi, email:
Giovanni De Ninno, email:
Maya Kiskinova, email:

Two-pulse two-colour free-electron laser provide a self-standing source for pump-probe experiments.

Understanding the exotic properties of matter driven to extreme non-equilibrium states by interaction with very intense VUV/X rays, has become possible with the advent of ultrabright free electron lasers (FEL). Development of different photon correlation schemes, with temporal and spatial resolution determined only by the FEL pulse duration and wavelength, are key steps towards accessing ultra-fast dynamic phenomena. The dynamics is initiated by the first “pump” pulse, which generates carriers at time scales shorter than carrier diffusion and electron-phonon scattering. The evolution of the transient states is then monitored by a second “probe” pulse arriving at variable and defined time delay. Tuning the pulse wavelengths to atomic resonances opens an unprecedented opportunity to add selectively elemental sensitivity to the mesurement, which is essential for exploring ultrafast processes in morphologically complex multicomponent materials.

Addressing the growing interest in using multi-color FEL pulses for ultrafast science the scientists at Elettra-Sincrotrone Trieste demonstrated the possibility of operating FERMI FEL in regimes suited to perform two-color pump-probe experiments in the XUV or X ray domain.

In the standard single pulse operation mode the electron bunch of FERMI-FEL is seeded with a single laser pulse with peak intensity tuned to maximize the emission from the central part of the electron beam. The adopted seed-scheme of FERMI-FEL has allowed generation of two-color pulses, using the methods illustrated in Figure 1 (a) and (b). The first two-color FEL scheme (Figure 1 (a)) exploits the FEL saturation dynamics to split the pulse in two parts by seeding the electron bunch with a powerful laser pulse, carrying a significant frequency chirp. At seeding peak intensities above a given power threshold, the pulse degrades the micro-bunching in the central part of the electron beam,  emitting only from the tails, separated in time, of the seeded area. The spectrum of the splitted FEL pulses shows that these have a wavelength difference of 0.2 nm, with a time separation in the range 90-160 fs. We expect to reach 50 - 30 fs in the future.

Figure 1: (a) Generation of two-color pulses using powerful seed laser pulse which carries significant frequency chirp. The right panel shows the wavelength split as a function of seed power. (b) Generation of two-color pulses using two independent seed laser pulses with slightly different central wavelengths. The right panel shows sequence of consecutive two-color spectra where the green dash lines highlight the intentional suppression of one of the FEL pulses.

The second two-color FEL scheme, illustrated in Figure 1(b), uses two independent seed laser pulses with slightly different central wavelengths,   and  , with variable time separation and intensity ratio. The two electron bunch seeded regions emit two independent, temporally separated FEL pulses at the harmonics of the seed wavelengths   and   respectively. The time separation between these two-color pulses can be controlled by tuning the delay between the input seed pulses. By blocking one of the seed laser arms one can easily switch between single and double FEL emission. The time separation depends on the laser pulse length and on the effective electron bunch extension. Presently the time separation can be varied in the range 150 fs – 800 fs and can be extended beyond 1 ps in the future.

Figure 2 (Left) Two-color FEL pulses, l1 and l2, tuned across the Ti M-resonance, impinge on a Ti grating with a temporal separation, Dt. (Right) Diffraction patterns corresponding to single color ‘pump’ and ‘probe’ pulses and to two-color ‘pump’-‘probe’ pulses (delayed by 500 fs) for different flux (F) regimes: low-F = 10-30 mJ/cm2, high-F = 2 J/cm2.

The potential of the second twin-pulse seed scheme to explore transient states of matter, stimulating and probing electronic transitions from core levels is demonstrated by a pilot pump-probe experiment with Ti grating structure deposited on a Si3N4 window, sketched in Figure 2. The selected wavelengths of both the pump (l1 = 37.2 nm) and probe (l2= 37.4 nm) pulses are within the slope region of the Ti M2/3 absorption resonance, where the Bragg peak intensities and positions are very sensitive to the instantaneous Ti ionization state. The results displayed in Figure 2 show that at low ‘pump’ and ‘probe’ intensities, the diffraction pattern is a simple sum of the ‘pump’ and ‘probe’ Bragg peaks. Using a very intense ‘pump’ pulse, the diffraction pattern undergoes an abrupt change due to dramatic loss of the ‘probe’ Bragg peak intensity. Since the sum of the delay time (~500 fs) and pulse duration (~100 fs) is shorter than those of hydrodynamic expansion and ablation, this result can be explained only by dramatic changes in the Ti electronic structure, namely highly ionized states of Ti atoms that pushes the M- resonance to shorter wavelengths.

This pilot experiment shows that multi-color studies with element specificity, can be carried out with sub ps time resolution, essential for addressing many gaps in our knowledge for interactions between atomic constituents or spatially separate phases or units in matter.