Lam R K, Raj S L, Pascal T A, Pemmaraju C D, Foglia L, Simoncig A, Fabris N, Miotti P, Hull C J, Rizzuto A M, Smith J W, Mincigrucci R, Masciovecchio C, Gessini A, Allaria E, De Ninno G, Diviacco B, Roussel E, Spampinati S, Penco G, Di Mitri S, Trovò M, Danailov M, Christensen S T, Sokaras D, Weng T-C, Coreno M, Poletto L, Drisdell W S, Prendergast D, Giannessi L, Principi E, Nordlund D, Saykally R J, Schwartz C P
Department of Chemistry, University of California, Berkeley, California 94720, USA.
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Phys Rev Lett. 2018 Jan 12;120(2):023901. doi: 10.1103/PhysRevLett.120.023901.
Nonlinear optical processes at soft x-ray wavelengths have remained largely unexplored due to the lack of available light sources with the requisite intensity and coherence. Here we report the observation of soft x-ray second harmonic generation near the carbon K edge (∼284 eV) in graphite thin films generated by high intensity, coherent soft x-ray pulses at the FERMI free electron laser. Our experimental results and accompanying first-principles theoretical analysis highlight the effect of resonant enhancement above the carbon K edge and show the technique to be interfacially sensitive in a centrosymmetric sample with second harmonic intensity arising primarily from the first atomic layer at the open surface. This technique and the associated theoretical framework demonstrate the ability to selectively probe interfaces, including those that are buried, with elemental specificity, providing a new tool for a range of scientific problems.
由于缺乏具有所需强度和相干性的可用光源，软X射线波长下的非线性光学过程在很大程度上仍未得到探索。在此，我们报告了在费米自由电子激光产生的高强度、相干软X射线脉冲作用下，在石墨薄膜中碳K边（约284 eV）附近观察到软X射线二次谐波产生。我们的实验结果以及伴随的第一性原理理论分析突出了碳K边以上共振增强的效应，并表明该技术在中心对称样品中对界面敏感，二次谐波强度主要来自开放表面的第一原子层。这项技术和相关的理论框架证明了能够以元素特异性选择性地探测界面，包括那些埋藏的界面，为一系列科学问题提供了一种新工具。
