Generation of two-color FEL pulses and first two-color pump-probe experiments at FERMI free electron laser.

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 statesis 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, 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 seeded-FEL FERMI in regimes suited to perform two-color pump-probe experiments in the XUV or X ray domain.
In the standard single pulse operation mode of FERMI the electron bunch is seeded with single laser pulse with a peak intensity tuned to maximize the emission from the central part of the electron beam. The adopted seed-scheme has allowedgeneration 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 FERMI saturation dynamics to split the pulse in two partsby seeding the electron bunch with a powerful laser pulse, carrying a significant frequency chirp. At seeding peak intensitiesabove a given power threshold, the pulse degrades the micro-bunching in the central part of the electron beam, emitting two separated in time FEL pulses only from the tails of the seeded area. The spectrum shows that the split FEL pulses have wavelength difference of ~ 0.2 nm. The time separation can vary in the range 90-160 fs and weexpect to reach 50 - 30 fs in the future.
The second two-color FEL scheme, illustrated in Figure 1(b), uses two independent seed laser pulses with slightly different central wavelengths, λ₁^seed and λ₂^seed, 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, λ₁^seed/N and λ₂^seed/N, respectively. One can easily switch between single and double FEL emission by blocking one of the seed laser arms. The time separation between these two-color pulses can be controlled by tuning the delay between the input seed pulses and 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 1:   (a) Generation of two-color pulses using powerful seed laser pulsewhich 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 spectrawhere the green dash lines highlight the intentional suppression of one of the FEL pulses.

The potential of the second twin-seed pulse 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 gratingstructure deposited on a Si₃N₄ window, sketched in Figure 2.The selected wavelengths of both the pump (λ₂= 37.2 nm) and probe (λ₂ = 37.4 nm) FEL pulses are within the slope region of the Ti M_2/3 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_2/3 resonance to shorter wavelengths.
This pilot experiment shows thatmulti-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.

Figure 1:   (Left) Two-color FEL pulses, λ₁and λ₂, tuned across the Ti M-resonance, impinge on a Ti grating with a temporal separation, Δt. (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.