Femtosecond laser pulses interacting with atoms

Abstract

We theoretically investigated a phenomena arising from the interaction of femtosecond pulsed lasers with an Hydrogen atom. The phenomena of interest to us, is the High or der Harmonic Generation (HHG) of soft and hard-Xray using linearly polarized two-color femtosecond laser pulses. This HHG process is a commonly employed technique to pro duce ultrashort intense light spanning through the ultraviolet to the x-ray region of the Electromagnetic Spectrum (EM). The development of HHG which has opened fascinat ing research at sub- atomistic scale is however been delimited by its lower higher-order harmonics, and the possibility of obtaining a single burst of attosecond (which is of high important, since significant amount of the laser energy are carried by single pulses) are been compromised. We began addressing this challenge, by solving the 1D non-relativistic Schr¨odinger equation for an H atom using the dipole approximation, whose solution was obtained by the split operator method. We separately computed the HHG spectrum due to a Titanium-Sapphire (fundamental field) and an arbitrary pulsed laser (secondary field) in the far-visible region. When the fundamental Ti:Sapphire laser field with the param eters; 800 nm, 2×1014W/cm2 and 10 cycles, were allowed to irradiate the H atom, we observed a train of 1330 as pulses extending from the 8th - 36th harmonic order, and whose energy measured 12 eV -55.8 eV . By considering the interaction of a similar Ti:Sapphire laser, with a different intensity of 4×1014W/cm2, we observed a train of 677 as pulses extending from the 25th - 75th harmonic order and whose energy measured 39 eV - 116 eV . When the secondary laser source in the far visible region with the parameters; 400 nm, 5×1014W/cm2 and 20 cycles irradiated the H atom, a train of 1209 as pulses extending from the 28th - 50th harmonic order, and whose energy corresponds to 87 eV - 155 eV was generated. Furthermore, by synthesizing the fundamental and secondary laser, and upon irradiation with the H atom, an HHG spectrum with a broad plateau whose energy spans through the ultraviolet region up to the hard X-ray region was observed. The broadness of the plateau implies the generation of short pulses, which also represents a clear indica tion of the possibility of obtaining a single attoseconds pulse. Though by considering a time delay in the synthesized laser there weren’t obvious change of our results, we finally obtained a continuum of abrupt end at the 685th order corresponding to a single pulse of 1.044 keV of energy with a short time duration of 169 as.