Tomographic measurement of high harmonic generation
Yao-Li Liu1*, Shin-Chi Kao1, Yi-Yong Ou Yang1, Zhong-Ming Zhang1, Jyhpyng Wang1,2,3, Hsu-hsin Chu1,3
1Department of Physics, National Central University, Zhongli, Taiwan
2Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
3Center for High Energy and High Field Physics, National Central University, Zhongli, Taiwan
* Presenter:Yao-Li Liu,
We demonstrate a complete tomographic analysis backed up by measurement of the HHG process in an Ar gas jet driven by 805-nm near-IR pulses. The local Ar gas density, plasma density, and driving pulse beam profile are measured individually to obtain the neutral gas dispersion, plasma dispersion, geometrical phase shift, and intrinsic dipole phase variation. With these data the 3-D phase-matching profile is obtained, which enables us to find the optimal condition for simultaneous maximum output and maximum efficiency. The tomographic measurement of the HHG growing process is achieved by using a transverse disruptive pulse to control the interaction length, where the disruption of HHG based on the ``selective zoning'' mechanism. The measurements show that the phase-matching condition along longitudinal and transverse directions are both non-uniform, and together they determine the overall harmonic yield and its far-field beam profile. Calculation of HHG and its propagation incorporating the measured 3-D phase-matching profile confirms that under the optimal condition, the major contribution of the final harmonic yield comes from the short-trajectory emission in the central region of the driving laser beam. The calculation reproduces the HHG growth curve precisely with no fitting parameter as well as distinctive features of the far-field beam profiles. Such a detailed analysis can provide critical insights for high efficiency HHG beyond extreme-UV, especially in achieving quasi-phase matching in highly dispersive media by periodic transverse selective zoning for HHG with higher conversion efficiency and shorter wavelength.

Keywords: Tomography of High-harmonic generation, phase-matching, coherent extreme ultraviolet, selective zoning method