ИСТИНА

Intense femtosecond laser interaction with the solid target is accompanied with channel formation, generation of hot microplasma inside it, generation harmonic of laser radiation. The presence of this plasma is indicated by X-ray emission (>2.5 keV). The second harmonic generation process (SHG) could be used as a tool for diagnosis of plasma parameters – SH amplitude changing and spectral modifications depend on the presence of ambient gas, laser pulse frequency and contrast. The goal of this work was to obtain information about parameters of plasma inside the channel and near target surface by investigating spectra of SH generated during channel formation in a solid target by intense fs-laser radiation. Experiments were carried out with radiation of Cr:F and Ti:S lasers (wavelength 1,24 and 0,8 um respectively), as targets were used CaF2, quarz, Al. We discovered that spectra of SH generated in a laser induced channel in a target placed in vacuum or air is broadened and blue-shifted. The value of a blue shift is non-monotonous during the channel formation and reaches a maximum of 15nm. The spectrum of the fundamental radiation also undergoes corresponding changes. At the same time we observed similar dynamics of X-ray yield and SH signal amplitude that indicates an increase of plasma temperature inside the channel. The signal of third harmonic was detected in the same conditions, that testifies to the cloud of ablated matter arising inside the channel. Comparative experiments measuring the spectra of SH generated by the interaction of laser radiation with a clean target surface are also performed. Efficiency of SHG generated at the surface of target placed in vacuum was ~10-4. Experimentally studied the possibility of diagnosing the depth of the channel, based on the image of SHG generated in the channel and reflected back to the CCD. The image size is determined by the position of the region in which the SHG is generated with respect to the position of the focus of lense. Since the focus of the lens is adjusted on the undisturbed surface of the target, then the size of the image depends on the depth of the formed channel. We obtained images of the SH generated in the channel of the target and its dynamics depending on channel formation.