Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
Department of Advanced Materials Science, The University of Tokyo, Kashiwa 277-8561, Japan.
Phys Rev Lett. 2018 Jun 1;120(22):223902. doi: 10.1103/PhysRevLett.120.223902.
Nonlinear optical frequency conversion has been challenged to move down to the extreme ultraviolet and x-ray region. However, the extremely low signals have allowed researchers to only perform transmission experiments of the gas phase or ultrathin films. Here, we report second harmonic generation (SHG) of the reflected beam of a soft x-ray free-electron laser from a solid, which is enhanced by the resonant effect. The observation revealed that the double resonance condition can be met by absorption edges for transition metal oxides in the soft x-ray range, and this suggests that the resonant SHG technique can be applicable to a wide range of materials. We discuss the possibility of element-selective SHG spectroscopy measurements in the soft x-ray range.
非线性光学频率转换一直面临着向极紫外和 X 射线区域推进的挑战。然而，极低的信号强度使得研究人员只能对气相或超薄薄膜进行传输实验。在这里，我们报告了软 X 射线自由电子激光从固体反射光束的二次谐波产生（SHG），这种增强是由共振效应引起的。观察结果表明，双共振条件可以通过软 X 射线范围内过渡金属氧化物的吸收边来满足，这表明共振 SHG 技术可适用于广泛的材料。我们讨论了在软 X 射线范围内进行元素选择 SHG 光谱测量的可能性。
