Abstract
The nonlinear thermal behavior of laser-heated gold nanoparticles in aqueous suspension is determined by time-resolved optical spectroscopy and x-ray scattering. The nanoparticles can be excited transiently to high lattice temperatures owing to their large absorption cross-section and slow heat dissipation to the surrounding. A consequence is the observation of lattice expansion, changed optical transmission, vapor bubble formation or particle melting. The heat transfer equations are solved for two limiting cases of heat pulses shorter and longer than the characteristic cooling time. The results of pulsed excitation with femtosecond and nanosecond lasers are explained by the theoretical prediction, and the bubble formation is interpreted by a spinodal decomposition at the particle–liquid interface. It is shown that both the laser spectroscopy and x-ray scattering results agree qualitatively and quantitatively, underlining the validity of the comprehensive model.
GENERAL SCIENTIFIC SUMMARY Introduction and background. Noble metal nanoparticles have emerging applications as very selective targets in biological matter. By heating the labelled particles with ultrashort laser pulses, very localized damage to tissue can be induced. For a deeper understanding of these processes, the thermal kinetics and the mechanism and energetics of bubble formation needs to be understood. Yet optical experiments have given inconclusive results.
Main results. To track the structural modifications and thermal kinetics in a gold nanoparticulate suspension we have comparatively employed time-resolved x-ray scattering to track the particle lattice and the bubble formation process with sub-nanosecond time resolution and time-resolved optical spectroscopy. By quantitatively relating the observed excitation to thermodynamical calculations it was possible to describe the behaviour within one single model based only on observable input parameters.
Wider implications. This work should provide confidence on the actual state of laser-heated nanoparticles, including the mechanism of bubble nucleation by a spinodal threshold process. Consequently, it should be sufficient to know quantities such as heat capacities, spinodal temperature, absorption cross section or thermal interface resistance of particles in order to estimate quantitatively the thermal state for nanosecond laser excitation.