Optimization of cut-off frequency in high harmonic generation in noble gas

Abstract

High harmonic generation is a phenomenon that occurs when an atom is subjected to a strong laser field. The laser field must have an intensity of at least ${10}^{14}$ $\mathrm{W}/{\mathrm{cm}}^{2}$ to cause tunnel ionization, which releases an electron from the nucleus and allows it to gain energy by moving in the laser field. Upon recombining with the parent ion, the electron radiates its kinetic energy as photons in the XUV region. In this study, In order to find the optimal intensity of each atom, we first simulate and investigate the ionization probability of the noble gas atom. The simulation results show that the saturation intensity of tunnel ionization is about 3.8, 6, 8.1 and 12.5 × ${10}^{14}$ $\mathrm{W}/{\mathrm{cm}}^{2}$ for xenon, argon, neon and helium gases, respectively. Using these results, we have been able to obtain the appropriate intensities to produce optimal attosecond pulses for each atoms. Then, according to the saturation intensity of gas ionization, we simulate the high-order harmonic spectrum in three intensities of 3, 5, and 8 × ${10}^{14}$ $\mathrm{W}/{\mathrm{cm}}^{2}$ and investigate the effects of atomic and laser parameters on the cut-off point and plateau region of high harmonic generation spectrum. The simulation results show that the intensity and time profile of attosecond pulses are highly dependent on ${I}_{P}$, and only the atoms that are in the range of tunnel ionization produced attosecond pulses. Also, our simulation results indicate that increasing the intensity leads to higher cut-off points and plateau regions for the harmonics. Furthermore, we found that helium had the highest cut-off point and plateau region at all three intensities, making it the most efficient noble gas for high harmonic generation.