Direct Measurement of the Phase-Matching Condition of High-Harmonic Generation

Yao-Li Liu^1*, Shih-Chi Kao¹, Yi-Yong Ou Yang², Chung-Min Chang², Jyhpyng Wang¹, Hsu-Hsin Chu²

¹Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
²Department of Physics, National Central University, Taoyuan, Taiwan

* Presenter:Yao-Li Liu, email:nem2050@gmail.com

It is well known that high-harmonic generation (HHG) is a promising method to generate ultrashort coherent extreme ultraviolet or soft x-ray. Its conversion efficiency is not only determined by the single-atom response but more importantly also determined by the phase-matching condition of the overall process.
In this paper, we used 40-fs, 810-nm mid-infrared pulses focused onto an argon gas jet to drive HHG. The interacting region was investigated by a transverse probe beam. By using a wavefront sensor to resolve the wavefront distortion of the probe pulse, the density of the argon gas and the density of the plasma produced in the HHG process were measured. Then the phase mismatch accumulated along the direction of HHG propagation was obtained.
On the other hand, by adjusting the probe pulse delay to be coincident with the driving pulse on the argon jet, the HHG process can be suppressed based on the mechanism called “selective zoning”. The transverse probe pulse perturbs the phase of the driving pulse and, thereby, turns off the harmonic generation at the overlap region. Therefore, by using a knife edge to control the probe pulse exposure location, the tomography of HHG was achieved. Therefore, the growth of HHG yield with the propagation of the driving pulse was resolved.
Under the optimized condition of the 31^th harmonic (26.13 nm), the phase measurement and the yield measurement conformed to each other. The HHG yield increased rapidly in the first half of the interacting region, where the accumulated phase mismatch was less than π. After that, in the rear half of the interacting region where the phase mismatch is larger than π, the HHG yield stopped growing. At the same time, other harmonics were also measured. The dephasing effect was stronger for higher harmonic orders. The phenomenon also verified the theoretical prediction.
These results revealed the phase-matching condition of HHG in situ and pointed out the way to the generation of higher harmonics with the quasi-phase matching method.

Keywords: high-harmonic generation, selective zoning, wavefront sensor, ultrashort coherent EUV