ИСТИНА

We demonstrate that the clustering of dense gases and supercritical fluids plays a crucial role in the process of the supercontinuum generation under femtosecond filamentation. The high-density gases and supercritical fluids have anomalously high nonlinear refractive index due to the clustering. Besides the nonlinear properties of fluids can be tuned by the change in pressure or temperature. The clustering leads to the formation of two regions, where the supercontinuum generation is the most efficient: the region of the subcritical pressures and the vicinity of the Widom line that divides the supercritical region into liquid-like and gas-like supercritical fluids. These regions are distinguished by the best balance between the highest nonlinear refractive index and the lowest density fluctuations. In these regions, on the one hand, the nonlinear properties are maximal. However, on the other hand, the clustering leads to the increase of density fluctuations. To increase the efficiency of the supercontinuum generation medium with a synthesized nonlinearity can be applied. The binary mixtures represents such type of media, by combining noble gas with high nonlinear refractive index and molecular gas with high molecular part of nonlinear refractive index it is possible to increase the width of the spectrum (about 1.5 times). Besides the clustering is more efficient in mixtures that can additionally increase the efficiency of the supercontinuum generation. We demonstrate that the efficiency of the SC generation in the IR wing in mixture CO2+Xe is twice as high then in the case of the supercontinuum generation in a single component.