Free electron lasers (FELs) generate high intensity, ultrafast and coherent X-Ray pulses that are tunable to the elemental edges of interest. We develop and use nonlinear X-Ray techniques at FELs to investigate ultrafast dynamics in solids. Specifically, X-Ray Second Harmonic Generation (XUV-SHG) has been our main spectroscopic tool. In principle, XUV-SHG is particularly attractive as it unites the core-level specificity of X-Ray spectroscopies with the unique properties of second harmonic generation.
We have studied bulk non-centrosymmetric crystals using XUV-SHG [1,2]. In these crystals due to ion displacements in the unit cell, emergent order such as ferroelectricity appears which makes them an ideal candidate for XUV-SHG studies. In the case of the polar ferroelectric LiOsO3, we demonstrated that the polar phase establishes as a result of Li ion displacements, while by investigating the polarization of XUV-SHG emitted from a well-studied ferroelectric, LiNbO3, we established that the long-accepted theories of angular anisotropy in optical SHG is applicable to XUV-SHG. Along with these studies, we also contributed to the investigation of tabletop sources that achieve XUV-SHG  and demonstrated the capability of XUV-SHG to investigate challenging buried interfaces .
1-E. Berger, S. Jamnuch, C. Uzundal, C. Woodahl, H. Padmanabhan, P. Manset, Y. Hirata,I. Matsuda, V. Goplan, Y. Kubota,et al., “Direct observation of symmetry-breaking in a ‘ferroelectric’ polar metal,” (2020), arXiv:2010.03134.
2-C. B. Uzundal, S. Jamnuch, E. Berger, C. Woodahl, P. Manset, Y. Hirata, T. Sumi, A. Amado, H. Akai, Y. Kubota, et al., “Polarization-Resolved Extreme Ultraviolet Second Harmonic Generation from LiNbO3” (2021), arXiv:2104.01313
3-T. Helk, E. Berger, S. Jamnuch, L. Hoffmann, A. Kabacinski, J. Gautier, F. Tissandier, J. P. Goddet, H.-T. Chang, J. Oh, C. D. Pemmaraju, T. A. Pascal, S. Sebban, C. Spielmann, M. Zuerch. “Table-top Nonlinear Extreme Ultraviolet Spectroscopy” (2021), arXiv:2009.05151
4-C. P. Schwartz, S. L. Raj, S. Jamnuch, C. J. Hull, P. Miotti, K. Lam, D. Nordlund, C. B.Uzundal, C. D. Pemmaraju, L. Foglia,et al., “Angstrom-resolved Interfacial Structure in Organic-Inorganic Junctions,” (2020), arXiv:2005.01905.