Separating Non-linear Optical Signals of a Sample from High Harmonic Radiation in a Soft X-ray Free Electron Laser

Second harmonic generation (SHG) is a unique non-linear optical effect that can be used to investigate chemical states of molecules at surfaces/interfaces. By the use soft X-rays, SHG gains element selectivity through inner-core excitation resonance. However, it challenging observe signals separately from second-order light generated undulator. Here, we report new ellipsometry method for X-ray suppress contribution second-harmonic radiation source. Through measurements GaAs(100) crystal, demonstrate pure obtained horizontally polarized component. The present generally applicable regardless incident photon energy and hence absorption edge targeted materials. If combined with filters blocking equipped phase shifters, allows one obtain further information such as tensor components susceptibility.