Enhanced plasmonic coloring of silver and formation of large laser-induced periodic surface structures using multi-burst picosecond pulses

We report on the creation of angle-independent colors on silver using closely time-spaced laser bursts. The use of burst mode, compared to traditional non-burst is shown to increase the Chroma (color saturation) by ~50% and to broaden the lightness range by up to ~60%. Scanning electron microscope analysis of the surfaces created using burst mode, reveal the creation of 3 distinct sets of laser induced periodic surface structures (LIPSS): low spatial frequency LIPSS (LSFL), high spatial frequency LIPSS (HSFL) and large laser-induced periodic surface structures (LLIPSS) that are 10 times the laser wavelength and parallel to the laser polarization. Nanoparticles are responsible for each plasmonic color and their distributions are observed to be similar for both burst and non-burst modes, indicating that the underlying structures (i.e. LIPSSs) are responsible for the increased Chroma and Lightness. Two-temperature model simulations of silver irradiated by laser bursts show significant increase in the electron-phonon coupling coefficient which is crucial for the creation of well-defined ripple structures. Finite difference time domain (FDTD) simulations of the colored surfaces show the increase in Chroma to be attributable to the HSFL arising in burst mode.