1.Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China 2.University of Chinese Academy of Sciences, Beijing 100049, China 3.School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China
Fund Project:Project supported by the National Basic Research Program of China (Grant No. 2017YFA0402300) and the National Natural Science Foundation of China (Grant No. 11934004)
Received Date:01 July 2020
Accepted Date:10 August 2020
Available Online:25 November 2020
Published Online:05 December 2020
Abstract:Femtosecond extreme ultraviolet (XUV) light pulses play an important role in investigating the ultrafast dynamics of atoms and molecules, and are complementary to the conventional large facilities like synchrotron radiation and free electron laser. We build a table-top femtosecond extreme ultraviolet light source based on the high-order harmonic generation (HHG) process of gaseous medium in a strong laser field. We implement HHG by focusing an intense IR laser into a 5 cm long gas-filled hollow waveguide, instead of the conventional tightly focusing geometry with gas jet. Inside the waveguide, the laser peak intensity is nearly constant and the gas pressure is well-controlled, making it possible to maintain the phase matching condition over an extended distance. And a fully coherent high harmonic beam builds up along the waveguide, leading to a dramatically higher HHG efficiency. Monochromatic XUV light pulses are obtained by spectral selection of the HHG through employing the conical diffraction method of grating. With this geometry used, the pulse broadening caused by wave front tilting during the diffraction can be strongly suppressed, especially for the case of grazing incidence. And the femtosecond temporal character of the light pulse can be preserved while keeping a high reflectivity. The temporal broadening of the XUV light pulse in our setup is estimated to be within 100 femtosecond. By using different noble gases, photons with energy values ranging from 20 eV to 90 eV are produced. For the 27th-order harmonic centered at 41.9 eV, the flux is measured to be 1 × 1010 photons per second, with an energy spread of 0.4 eV. In order to investigate the ultrafast dynamic behaviors of gaseous atoms and molecules with an HHG-based XUV source, we develop a reaction microscope with ultrahigh vacuum of about 10–11 mbar. The combination of HHG-based XUV with the newly developed reaction microscope provides a unique tool for studying the XUV photon and atom/molecule interaction. A series of experiments has been successfully carried out on the platform and the system shows good performance. Keywords:extreme ultraviolet (XUV) photon source/ high order harmonic generation (HHG)/ conical diffraction/ reaction microscope
高次谐波产生的基本原理, 可以采用Corkum等[19,20]提出的半经典“三步模型”来形象地说明. 但是三步模型理论只能描绘高次谐波产生的单原子图像, 无法考虑实际工作气体中大量原子的宏观效应. 而宏观效应则决定了高次谐波的实际输出强度, 只有当不同原子释放的高次谐波相干叠加增强, 即达到相位匹配时, 才能实现输出强度的最大化, 如图2(a)所示. 图 2 高次谐波过程中的相位匹配 (a) 在相位匹配条件下, 不同原子释放的谐波相干叠加增强; (b) 使用气体射流产生高次谐波的紧聚焦模式; (c) 使用充气的中空波导管产生高次谐波 Figure2. Phase matching of the HHG: (a) Radiations from different atoms add up constructively when phase matched; (b) tightly focusing geometry of HHG with a gas jet; (c) HHG in a gas-filled hollow waveguide.