Non-linear polarization rotation of femtosecond laser radiation with intensity of the order of 1013 W/cm2 in BaF2 crystal under plasma formation conditions was investigated. Abnormal behavior of cross-polarized wave generation efficiency on laser pulse energy may be explained by influence on the process of polarization anisotropy of fifth order non-linearity.
Femtosecond laser radiation self-channeling and second harmonic generation under plasma formation conditions were investigated. Laser pulses with sub- and microjoule energy were tightly focused into the volume of a KDP crystal, Estimated laser pulse intensity was of the order 1013 W/cm2 in the focal spot exceeding damage and plasma ignition thresholds in the crystal. Threshold of plasma channel formation measured in the experiment corresponds to critical power of self-focusing 1.5f0.1 MW for KDP crystal. Plasma channels with length up to 150 μm were observed in the volume of the KDP crystal. We developed an algorithm allowing estimating electron density and temperature in the plasma channel using experimental dependence of laser energy transmittance through the crystal on incident laser energy. For laser pulse energy 1 μJ estimated electronic density in the plasma channel is of the order of l020 cm-3 (that is about one tenth of the critical plasma density value) and mean electronic temperature is about 3 eV. Free plasma electrons leaded to refraction index increasing by 5 %, extinction coefficient was -25 cm-1. The maximum measured efficiency of second harmonic generation was 1.2 %.
Experiments on single pulse plasma channel formation in non-linear KDP crystal by tightly focused (NA = 0.47) fundamental and doubled Cr:forsterite laser radiation with energy of 0.1 ÷ 10 μJ and 100 fs pulse duration were carried out. We propose the simple model of non-linear absorption of femtosecond laser radiation in plasma channel. This model allows estimate laser intensity in the channel and plasma parameters.
It was demonstrated experimentally that liquid with low pressure of saturated vapor can be used as target for femtosecond laser plasma formation. Plasma formed at free liquid surface (vacuum oil) in vacuum by femtosecond laser pulse with intensity above 1016 W/cm2 is similar to high-temperature plasma formed at solid target surface. So, from x-ray and ionic time-of-flight measurements the temperature of hot electrons for oil target is 6 ± 3 keV and for crystalline silicon target is 4 ± 1 keV. The optics diagnostic of liquid surface relaxation has shown that upper repetition rate of laser pulses for interaction with non-perturbed liquid surface is 10 Hz.
Making use of time-of-flight and mass-spectroscopic methods we investigated influence of the film on the target surface at vacuum of 10-5 Torr onto ions emission from plasma created by femtosecond laser pulse with itensity of 2 • 1016 W/cm2. It was shown that the highest energy per charge (8.5 keV) acquire protons, while basic target ions (Si, Ti) gain less energy. Heating by laser nanosecond pulse advancing femtosecond pulse by 0.1 - 100 ms with energy density up to 20 J/cm2 allows for effective surface cleaning due to removal of molecules containing hydrogen, carbon and oxygen. By the contrast to the instantaneous resistive heating the pulsed laser cleaning provides for higher heating temperatures and can be used for any solid targets in the regimes of thermal and plasma cleaning.
We have observed self-guiding of a single femtosecond visible laser pulse in the bulk of transparent nonlinear media (SiO2, KDP) and in the water. The dependence of filament length on laser pulse energy was measured. Continuous open-ended channels and frozen modifications of the matter were observed in transparent two-component condensed medium (thin glass plate placed in water).
Recent advances in hard x-ray production from porous silicon targets have been reported. Experimental data on efficient hot electron generation in plasma, created by 200 fs laser pulses at intensity in excess of 1016 W/cm2 are discussed. The assessment of the hot electron temperature in single laser shot was made using simple modification of the well known x-ray filter method. X-ray quanta with energy of approximately 80 keV were observed at the 'moderate' intensity of 2 X 1016 W/cm2, revealing the existence of the second non-thermal electron component with the 'temperature' of 30-50 keV. Finally, we discuss the feasibility to design 2.5 MeV neutron source with peak flux of more than 1020 neutrons using deuterium enriched nanowire metallic arrays.
Novel scheme providing for 7 J Nd-glass subpicosecond MultiTeraWatt laser pulse shortening down to 200 fs with intensity rise time of 100 fs for 2 order of magnitude and prepulse suppression down to quantum noise level are proposed. Non-linear optical two-stage converter consists of sequential second harmonic generator and stimulated raman scattering oscillator-amplifier.
We present results on plasma formation in porous silicon (cluster like solid with mean cluster size of 3 nm, mean density 0.1 - 0.2 of crystalline silicon) by femtosecond laser pulses at intensity above 10 TW/cm2. We deduced hot electron temperature as high as 8 keV and fast ions of at least 2 MeV energy.
Stimulated Raman Scattering (SRS) of nano- and picosecond light pulses in molecular gases is widely used for wavelength conversion, pulse shortening and other purposes [1]. For powerful femtosecond light pulses such non-linear phenomena as self- and crossmodulation, group velocity dispersion, etc. can strongly affect the quality of output stocks pulse. In this paper we analyse SRS of powerful femtosecond light pulses and show, that SRS could provide high-efficient conversion if the specific range of pulse and medium parameters is chosen. We also discuss the possibility of prepulse suppression and front edge sharpening that is extremely important for femtosecond-light-pulses-with-matter interaction.
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