Femtosecond laser filaments in gold colloidal solutions: supercontinuum and ultrasonic tracing

Focused 100-femtosecond, 800-nanometer laser pulses with variable supercritical powers were fragmented into multiple millimeter-long filaments in pure water and gold nanoparticle (Au-NP) hydrosols of variable extinction coefficients. Power-dependent supercontinuum (SC), acquired at the blue shoulder of the laser pulses, overlapped with the Au-NP plasmon resonance, and as a result, it was damped by two orders of magnitude, rather than visibly enhanced by plasmonic NPs via non-linear light–NP interactions. Its damping factor increased with the increasing Au-NP extinction due to dissipative linear absorption of both the SC emission and pump-pulse attenuation in the colloidal solutions, with the latter process damping the underlying χ3 interactions in the filaments. The minor optical extinction of the Au NPs at the 800-nanometer pump wavelength pertained to the filamentation process, with the overall dissipative losses ultrasonically probed via a thermoacoustic generation mechanism in terms of the peak laser power, exhibiting different absorption mechanisms for the pure water/diluted Au-NP hydrosols, and for the more concentrated colloids.