Second Harmonic Generation using a seeded soft X-ray laser

Nonlinear interactions between light and matter are crucial for widespread applications in physical sciences, life science and engineering. Not only enables the nonlinear response of matter the generation of laser light at virtually any wavelength from Terahertz to X-rays, it is also the basis for nonlinear spectroscopies that enable unique insight into properties and dynamics of matter itself. Most notably, second
harmonic generation and sum-frequency generation spectroscopy in the near infrared and optical range have enabled intriguing insights into surface properties and their influence on chemical reactions. Here, we report the first generation of second harmonic emission in the extreme ultraviolet (XUV) at the titanium M_2,3-edge with a high-harmonic seeded soft X-ray laser in a single-hot manner with an input photon energy of 37.8 eV, bringing nonlinear XUV spectroscopy with atomic specificity to the table-top. Comparing the experimental observations with real-time time dependent density functional theory calculations, we find a strong surface sensitivity in titanium, caused by a resonant enhanced transition. This proof-of-principle experiments paves the ways for future research enabled by element and chemical sensitivity at surfaces and buried interfaces in the lab scale.