Lloyd David T, O'Keeffe Kevin, Anderson Patrick N, Hooker Simon M
Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK.
Department of Physics, Swansea University, Singleton Park, Swansea, SA2 8PP, UK.
Sci Rep. 2016 Jul 28;6:30504. doi: 10.1038/srep30504.
High harmonic generation (HHG) is an established means of producing coherent, short wavelength, ultrafast pulses from a compact set-up. Table-top high-harmonic sources are increasingly being used to image physical and biological systems using emerging techniques such as coherent diffraction imaging and ptychography. These novel imaging methods require coherent illumination, and it is therefore important to both characterize the spatial coherence of high-harmonic beams and understand the processes which limit this property. Here we investigate the near- and far-field spatial properties of high-harmonic radiation generated in a gas cell. The variation with harmonic order of the intensity profile, wavefront curvature, and complex coherence factor is measured in the far-field by the SCIMITAR technique. Using the Gaussian-Schell model, the properties of the harmonic beam in the plane of generation are deduced. Our results show that the order-dependence of the harmonic spatial coherence is consistent with partial coherence induced by both variation of the intensity-dependent dipole phase as well as finite spatial coherence of the driving radiation. These findings are used to suggest ways in which the coherence of harmonic beams could be increased further, which would have direct benefits to imaging with high-harmonic radiation.
高次谐波产生（HHG）是一种利用紧凑装置产生相干、短波长、超快脉冲的既定方法。桌面型高次谐波源正越来越多地被用于使用诸如相干衍射成像和叠层成像术等新兴技术对物理和生物系统进行成像。这些新颖的成像方法需要相干照明，因此，表征高次谐波光束的空间相干性并了解限制该特性的过程都很重要。在此，我们研究了在气体池中产生的高次谐波辐射的近场和远场空间特性。通过SCIMITAR技术在远场测量强度分布、波前曲率和复相干因子随谐波阶数的变化。使用高斯-谢尔模型，推导出产生平面中谐波光束的特性。我们的结果表明，谐波空间相干性的阶数依赖性与强度依赖偶极子相位变化以及驱动辐射的有限空间相干性所引起的部分相干性一致。这些发现被用于提出进一步提高谐波光束相干性的方法，这将对利用高次谐波辐射进行成像有直接益处。
