Bright EUV source of coherent radiation driven by near- and mid-IR femtosecond laser system for THz assisted HHG

High-order harmonics generation (HHG) driven by the femtosecond multi-band laser system, based on Cr:Forsterite (1.24 um) and Fe:ZnSe (4.5 um) crystals, has been realized in an argon gas jet. The assessed vacuum intensity at the target region is at the level of $10^{14}-10^{15} \mathrm{~W} / \mathrm{cm}^{2}$, corresponding Keldysh parameter is at the level of 0.2, that indicates the tunneling regime of ionization. The measured harmonics orders are in the range of 43-71 eV with total generated photon number at the level of $5 \times 10^{8}$ photons per pulse, energy of 4.4 nJ and corresponding conversion efficiency of $3 \times 10^{-6}$. It is experimentally established, that the increase of gas jet pressure leads to blue-shifting and double-peaking in harmonics spectrum due to the nonlinear propagation effects of generating radiation. The effective generation of THz radiation by the 1.24 -um laser pulses of Cr:Forsterite laser system and extension of the generation wavelength to the mid-IR (4.5 um) range by driving of Fe:ZnSe laser system pave the way toward the HHG generation by near- and mid-IR radiation in the presence of strong THz field.