Non-linear self-driven spectral tuning of Extreme Ultraviolet Femtosecond Pulses in monoatomic materials

Abstract

Self-action nonlinearity is a key aspect – either as a foundational element or a detrimental factor – of several optical spectroscopies and photonic devices. Supercontinuum generation, wavelength converters, and chirped pulse amplification are just a few examples. The recent advent of Free Electron Lasers (FEL) fostered building on nonlinearity to propose new concepts and extend optical wavelengths paradigms for extreme ultraviolet (EUV) and X-ray regimes. No evidence for intrapulse dynamics, however, has been reported at such short wavelengths, where the light-matter interactions are ruled by the sharp absorption edges of core electrons. Here, we provide experimental evidence for self-phase modulation of femtosecond FEL pulses, which we exploit for fine self-driven spectral tunability by interaction with sub-micrometric foils of selected monoatomic materials. Moving the pulse wavelength across the absorption edge, the spectral profile changes from a non-linear spectral blue-shift to a red-shifted broadening. These findings are rationalized accounting for ultrafast ionization and delayed thermal response of highly excited electrons above and below threshold, respectively.

Nonlinear harmonics of a seeded free-electron laser as a coherent and ultrafast probe to investigate matter at the water window and beyond

“…In order to characterize simultaneously the FEL emission at both the fundamental (233 eV) and third nonlinear harmonic wavelength (700 eV) on a shot-to-shot basis, we used two high-resolution spectrometers placed in series (Fig. 2) one (PRESTO) [53] integrated in the common photon transport line [54] of the FERMI experimental hall, the other (WEST) recently installed downstream of the EIS-TIMEX end station [55].…”

“…2. Sketch of the photon transport line, including the intensity monitors, the filter revolvers, the calibrated photodiode, and the two spectrometers (PRESTO [53] and WEST [55]) used in the experiment. dominates in the first part of the radiator, where as a result of the seeding there is a non-negligible bunching also at the harmonic of the resonant wavelength.…”

XUV plasmonic waveguides by near-zero index heterostructures

“…Moreover, nonlinear interaction of matter with XUV radiation enables advanced photonic applications, e.g. wave-mixing [24,25], second-harmonic generation [26,27], saturation of absorption [28] and self-induced spectral tuning [29]. However, the inherently weak light-matter interaction at XUV photon energies, combined with the limited power of current XUV tabletop sources, hamper their disruptive potential for ultrafast spectroscopy and extreme nonlinear optics.…”