ИСТИНА

Focusing of the super-intense laser pulses on the surface of a solid target provides for relativistic electrons, which give rise to a wide range of nonlinear optical phenomena, such as the broadband continuum generation and the attosecond waveforms synthesis. In this context, the generation of high-order harmonics[1, 2] is not only a promising way to obtain ultrashort X-ray pulses but also a unique tool for diagnosing plasma when it is exposed to ultra-high-power laser pulses. The use of long-wavelength radiation in the mid-IR range makes it possible to increase the ponderomotive energy of electrons in proportion to the square of the wavelength [3, 4]. This paper presents the results of an experimental study of the spectra of high-order harmonics generated in the field of laser pulses with a central wavelength of 800 nm and 3900 nm on the solid surfaces. We observed harmonics up to the 51st and 21st order from the mid-IR and near-IR radiation, respectively (Fig. 1). The generation efficiency for different types of targets and different chirps of exciting laser pulses was thoroughly analyzed.