Filamentation of an ultrashort laser pulse in a medium with artificial nonlinearity

The introduction of a small amount of subwavelength gold nanoparticles (extinction coefficient $\sim 0.01$–$1$ cm$^{-1}$) to water allows the efficient control of filamentation owing to nonlinear dissipative losses of a primary ultrashort pump laser pulse and secondary broadband radiation (supercontinuum), because the intrinsic extinction coefficient in the multiple filamentation regime is below $0.04$ cm$^{-1}$. Optical emission and photoacoustic spectroscopy studies have shown that an increase in losses in a colloidal solution sublinearly increases the power of an ultrashort laser pulse for the nonlinear generation of supercontinuum with the same intensity, and it also increases the filamentation threshold near the supercontinuum generation threshold. At the same time, supercontinuum emission is significantly enhanced in the region of plasmon resonance of nanoparticles against the background of broadband nonlinear absorption. The results demonstrate new possibilities of controlling the nonlinear optical characteristics of media for the generation and filamentation of ultrashort light pulses in various spectral ranges.