In his research, Willems combined static experiments with synchrotron radiation at BESSY II with femtosecond time-resolved experiments on a laser-based high harmonic generation source at MBI. Via measurements of both the real and imaginary part of the magnetic dichroic contribution to the refractive index in iron, cobalt and nickel at the 3p (M-edge) resonances, he has laid the experimental foundation for an improved theoretical ab-initio description of these spectra. This subsequently enabled a time-dependent theoretical description of the magnetic response to an optical stimulus on the femtosecond time scale. This time scale was experimentally accessed in Willems’ PhD work via pump-probe experiments using a high harmonic source, providing soft x-ray pulses of 30 femtosecond duration in resonance with the respective 3p core transitions. Willems could probe the dynamic response of multi-element magnetic systems with this approach, providing new insight into spin-flip and interatomic transport related mechanisms active during the manipulation of magnetization via ultrashort laser pulses.
PhD Thesis, Felix Alexander Willems,“Ultrafast optical demagnetization dynamics in thin elemental films and alloys: foundations of and results from helicity-dependent and time-resolved XUV spectroscopy”, Technische Universität Berlin (2019)