| dc.description.abstract | The interaction between laser and gas to excite high-harmonics generation (HHG) is an important method of generating X-rays. The laser hits the gas target to generate free electrons, which will release short-wavelength photons during the process of acceleration and recombination with the parent ions. Currently, the researches related to the HHG mainly focuses on how to extend the cutoff photon energy and improve conversion efficiency.
In order to extend the cutoff photon energy of HHG, gaseous ions with higher ionization energy can be used as the interacting medium, which can increase the ponderomotive potential and thus the cutoff photon energy. For example, helium gas can be selected as the gas target, and HHG can be generated by singly ionized helium. The cutoff photon energy can reach keV hard x-ray range. However, the highly ionized plasma not only represents a high density HHG sources, but it also means that plasma dispersion is difficult to balance. Therefore, achieving high efficiency HHG remains a big challenge. Here we propose to utilize the HHG intrinsic dipole phase variation to balance the plasma dispersion and the geometrical phase shift. Then the phase-matching condition can be achieved and the conversion efficiency can be increased.
In this study, the propagation of laser in a helium target is simulated by solving the two-dimensional axisymmetric cylindrical coordinate with advective-diffusion electromagnetic (EM) envelope equation, combined with Keldysh′s optical field ionization model, and the simulation results are used to calculate the phase matching conditions, HHG yield. We used 405-nm, FWHM 50-fs driving laser to interact with helium gas. The simulation results show that the output yield of the 95th harmonic (4.26-nm) reaches 66% of the perfect phase matching condition, and the 169th harmonic (2.4-nm) output also reaches 78% of the perfect phase matching condition. This simulation study demonstrates that numerical experiments can know the effect of laser and gases’ parameters on phase matching conditions before conducting experiments, thereby improving the efficiency and success rate of experiments. | en_US |