High harmonic generation near the low-frequency edge of a plateau under nonlinear propagation of 1.24-$\mu$m near-infrared femtosecond laser radiation in a dense argon jet

High ($15$–$25$) harmonic generation in the vacuum ultraviolet spectral range ($83$–$50$ nm) has been realized by focused ($\mathrm{NA} = 0.033$) near-infrared femtosecond laser radiation (wavelength $\lambda = 1.24$ $\mu$m) with a vacuum intensity of $\sim 7.5 \times 10^{14}$ W/cm$^2$ irradiating a dense gas jet. It has been shown experimentally that the use of such a high-numerical aperture focusing requires high (up to $10$ bar) gas jet pressures to optimize phase matching. The use of the dense gas jet results in a noticeable manifestation of nonlinear propagation effects for generating radiation, which affect the generation process through the change in the phase matching conditions. Furthermore, it has been shown that the prechirping of the generating pulse makes it possible to compensate a chirp appearing due to self-phase modulation and to increase the harmonic generation efficiency because of the nonlinear compression of the generating pulse. This approach has allowed $17$th ($73$ nm) harmonic generation with an energy of $2$ pJ in a pulse and a generation efficiency of $5.4 \times 10^{-9}$. The estimates obtained have shown that this radiation can be used for single-pulse maskless photolithography in the extreme ultraviolet range.