Fabrication and Characterization of The Gas-Filled Capillary Waveguide for Laser-Driven High-Harmonic Generation

Shih-Chi Kao^1*, Jia-Wen Gu¹, Kai-Xiang Li¹, Jian-He Lin¹, Hsu-hsin Chu¹

¹Department of Physics, National Central University, Taoyuan, Taiwan

* Presenter:Shih-Chi Kao, email:blackdiamondkao@gmail.com

Laser-driven high-harmonic generation (HHG) from noble gases is an ultrashort, coherent light source with wavelengths ranging from extreme ultraviolet to soft x-ray. To extend the interaction length for higher conversion efficiency, the gas-filled capillary waveguide is a suitable option [1].

Here, we present the details of the fabrication procedures for the gas-filled capillary waveguide assembly and the complete diagnosis of its quality. The optimized guiding conditions are verified using a CW laser beam and low-energy laser pulses, consistent with the theoretical EH₁₁ guiding mode. A frequency-domain interferometer is constructed to measure the plasma density in the capillary produced by intense laser pulses. It is found that the effect of ionization-induced defocusing increases the capillary guiding loss, and alters the guiding mode. With a driving laser intensity of 5 x 10¹⁴ W/cm² and a resulting plasma density of 2.5 x 10¹⁷ cm⁻³, the attenuation coefficient reaches 0.03 mm^-1. Such attenuation of the driving laser pulse can be utilized to control the intrinsic dipole phase of HHG, enabling phase-matching of HHG at shorter wavelengths [2,3].

[1] IEEE Photonics Technology Letters, 32, 1393 - 1396 (2020)

[2] Physical Review A, 104, 023112 (2021)

[3] Optics Express, 30 (2), 1365 (2022)

Keywords: ultrafast optics, laser plasma, waveguide