Optical harmonics generation under the interaction of intense (up to $10^{14}$ W/cm$^2$) mid-infrared femtosecond laser radiation of a Fe:ZnSe laser system with a dense laminar gas jet

Low-order (fifth, seventh, and ninth) harmonics have been generated under the interaction of intense (${10}^{14}$ W/cm$^2$) femtosecond mid-infrared radiation of a laser system based on a Fe:ZnSe crystal (wavelength is $4.55\, \mu$m, pulse duration by the FWHM level of intensity is $160$ fs, and the pulse energy is up to $3.5$ mJ) with an argon jet (pressure is up to $10$ bar) in the tunneling ionization regime (Keldysh parameter is $\gamma = 0.2$). The maximum energy efficiencies of the $5^{\mathrm{th}}$, $7^{\mathrm{th}}$, and $9^{\mathrm{th}}$ harmonic generation are $2 \times 10^{-7}$, $6 \times 10^{-9}$, and $3 \times 10^{-10}$, respectively. It has been established that nonlinear effects of propagation of generating radiation under an increase in the pressure of the gas jet begin to significantly affect the process of generation.