Advances in instrumentation for FEL-based four-wave-mixing experiments

Nanoscale transient gratings excited and probed by extreme ultraviolet femtosecond pulses

“…The optical system for EUV TG experiments presents critical aspects, for instance, the mandatory use of reflective optics, which have required huge technical efforts before making EUV TG experiments a reality; further technical details on the setup can be found elsewhere ( 20 ). The three EUV pulses were generated from a single FEL pulse using the edges of two splitting flat mirrors (PM1 and PM2) working in the vertical and horizontal plane, respectively.…”

“…Here, we report the first EUV pump/EUV probe TG (EUV TG) measurements and demonstrate vibrational and thermal lattice responses in silicon and silicon nitride at L TG ’s down to 28 nm. The EUV TG capability has been implemented in the TIMER beamline ( 20 ) at the FERMI FEL. Our results reveal thermal transport kinetics that can now be studied incisively, and we discuss the potential of this new class of experiments for studying structural dynamics, transport phenomena, and magnetism on the nanoscale.…”

Exploring the multiparameter nature of EUV-visible wave mixing at the FERMI FEL

“…The main difference between the two EUV-FWM setups at FERMI, described in detail elsewhere, 11,28 is the probe wavelength and consequently the accessible momentum range |k| in TG experiments. Indeed, the optical probe limits the accessible exchanged momentum at mini-TIMER to |boldkex|<2|boldkopt|≈0.05 nm−1, while TIMER can access |k| values as large as 1 nm −1 by using an EUV probe.…”

“…Indeed, far from electronic resonances, the third order nonlinear susceptibility scales as the inverse of the frequency squared for each of the incoming beams, causing a significant decrease in the cross sections when moving from visible to EUV photon energies 24 . These limitations have been recently overcome in two dedicated setups that were developed and implemented at the FERMI FEL source 26 with the aim of performing noncollinear EUV FWM experiments based on the TG scheme: the compact EUV-optical wave mixing setup mini-TIMER and the purely EUV-FWM beamline TIMER 11 . TG experiments are a particular class of FWM where two of the incoming pulses, called the “pump,” have the same frequency and are overlapped in time ( τ 1 = 0) on the sample at an angle to generate a transient standing wave that effectively acts as a diffraction grating for the third pulse, the “probe.” The interference pattern of the two pumps excites impulsive modes with momentum |k|=4π sin (θ)/λex, where 2 θ is the pump crossing angle and λ ex is their wavelength.…”

An approach for realizing four-wave-mixing experiments stimulated by two-color extreme ultraviolet pulses

“…different photon frequencies (ω1 and ω2), can stimulate processes inaccessible by the TG scheme, as for instance coherent Raman scattering, [21][22][23][24] hence substantially broadening the range of potential applications. [5][6][7][8]14,24 In the experiment hereby discussed we employed our non-collinear split-delay-recombination setup ("mini-TIMER"), [25][26][27] previously used for realizing EUV TG, [14][15][16][17][18] and the two-color seeded FEL emission ("twin-seed mode") 24,28 available at the FERMI FEL1 source, 29 to demonstrate a time-resolved FWM response driven by the interaction between two EUV pulses at different photon frequencies.…”