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

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 femtosec ond FEL pulses,which we exploit for fine self-driven spectral tun ability 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 account!ng for ultrafast ionization and delayed thermal response of highly excited electrons above and below threshold,respectively.