Metal nanoparticle colloids provide a unique combination of optical, magnetic, electronic and thermal properties finding applications in various fields ranging from optoelectronics to cancer therapy. Chemical synthesis of such nanoparticles has been refined to a high degree of control of crystallinity, shape and size distribution. Among them, gold nanoparticles are of particular interest because they host plasmonic resonances that enhance their interactions with light. Their size and shape imposes the resonance wavelengths which, depending on the application, must be tunable. For this, the synthesis processes are generally adapted on a case-by-case basis, preventing the development of a generic recipe that can cover a wide range of applications. We present here a different approach and propose to modify the size distribution of post-synthesis nanoparticles by laser irradiation in the presence of a gold salt to tune their resonance wavelengths. Plasmonic resonances driven by femtosecond laser irradiation are used to tune the redox activity of AuCl - 4 ions in the presence of gold nanorods which leads to 1 a reshaping of nanoparticles. All chemical intermediates and products are monitored by in situ UV-visible spectroscopy, ex-situ electronic microscopy analysis, and then compared to 3D simulations in order to unveil the underlying photochemical processes.

As such, these results not only pave the way for the fine tuning of the plasmon resonance of gold colloids over a wide spectral range, which can have an impact on many application areas such as plasmonic colours, sensors or absorbers but also feeds into the very active field of plasmonic photocatalysis.