Calculation of high-harmonic generation for water window regime X-ray using wavelength 405 nm pump laser

Cheng Chang Hsu^1*

¹ISAPS, National Cheng Kung University, Tainan, Taiwan

* Presenter:Cheng Chang Hsu, email:nckulaserplasmalab@gmail.com

High-harmonic generation (HHG) driven by ultrashort intense laser pulses has been demonstrated as a reliable, coherent light source from extreme-ultraviolet (EUV) to x-ray in recent years. The produce of HHG can be explained by three-step model: I- Tunnelling Ionization, the electrons are tunneling ionization from the atom. II- Acceleration, electrons are accelerated by the laser electric field. Half an optical cycle after ionization, the electron will reverse direction as the electric field changes. III- Recombination, electrons will accelerate back towards the parent atoms and release high photon energy. The maximum photon energy with HHG is given by cut-off photon energy of Ecutoff= Ip + 3.17Up, where Ip is ionization energy and Up is ponderomotive potential. In order to extend the cut-off energy, it is a promising method that ions as the medium of interaction, for the ionization energy of ions is higher than atoms, and it requires strong laser pulses to ionize, resulting in higher ponderomotive potential. However, in the process of the interaction, the severe phase-mismatch problem caused by the plasma dispersion limits the conversion efficiency.
In this paper, we proposed to focus a laser into a gas-filled capillary to achieve the phase-matching in a highly-ionized plasma via the dispersion balance of plasma and intrinsic dipole phase by adjusting the laser intensity, gas density, and capillary radius. In order to further enhance the conversion efficiency, we use shorter wavelength laser source as driving for HHG, it is done by using a 7-mJ, 30-fs, 405-nm driving pulse focused into a neon-filled capillary waveguide with 20-μm radius, 30-mm length, and 3.9 × 10¹⁵ /cm³ gas density, the 129th HHG (3.14nm) wavelength can be generated efficiently. The relative conversion efficiency reaches 99.7% to the perfect phase-matching condition. In the end, we design the installation of the capillary and use ANSYS to simulate the dynamic behavior of gas inside the capillary.

Keywords: High-Harmonic-Generation, three-step model, simulation