Investigation of Laser Induced Structure formation and resultant fluorescence

• Quantification of formation of Laser Induced Structures (LISs).
• Investigation of fluorescence dynamics of LIS.
• Exploring reasons behind generation of selectively fluorescence active LIS edges.
• Photo-physical characterization of LIS.
• Discovery of fluorescence quenching due to presence of water.

The formation mechanism of 3D micron-sized fluorescent structures generated in silver nanoparticle containing sodium citrate dihydrate films, during exposure to focused laser radiation, was investigated. Microscopic and thermochemical data indicate that heat accumulates at the nanoparticle surface. The heat causes local melting and an increase in temperature beyond the decomposition point of the immediate surrounding layer. In turn this leads to the rapid release of volatile gases (H2O and CO2). These expanding gases push the melted liquid pool outward, from the center of the focal volume, leaving behind a trough-like structure with elevated edges. It was observed that the edges of the structures were fluorescent. The fluorescence mechanism was investigated using atomic force, scanning electron and Confocal Fluorescence Microscopy. The observed fluorescence was attributed to the decomposition of sodium citrate dihydrate to sodium citrate. The presence of water acts to quench fluorescence in the bulk film, but near regions that experienced heat, the water is driven off.