Plasmonics is the study of the interaction between electromagnetic field and free electrons in a metal. Free electrons in the metal can be excited by the electric-field component of light to have collective oscillations. Surface plasmon resonance (SPR) is the resonant oscillation of conduction electrons at the interface between a metal and a surrounding dielectric (air, water, etc) stimulated by incident light.
Metal nanoparticles have a wide variety of application areas including electronic devices, displays, solar cells, and bio-sensors. In particular, gold nanoparticles have attracted significant attention in such medical and laboratory fields as cancer therapy and detection of infectious agents. This is primarily due to their superior biocompatibility and unique optical properties. The distinct localized SPR and surface enhanced Raman scattering (SERS) effects of Au nanoparticles can be effectively utilized in biomedical sensing and imaging. It is well known that the optical properties of metal nanoparticles are very sensitive to their shape and size. Various chemical methods have been developed to control the size and shape of metal nanoparticles, which include citrate reduction method, Brust-Schiffrin method, and seeded-growth method. High-quality Au nanoparticles have also been synthesized. However, the chemical approaches used to produce high-quality metal nanocrystals are often energy-intensive, employ toxic chemicals, and require high temperatures. It is of practical significance to develop an environment-friendly and cost-effective method to produce nanoparticles that have a regular shape and uniform size. Pulsed laser fragmentation in liquids have emerged as a “green” technique for the synthesis of nanoparticles. Au powders of arbitrary shape and size are a good starting material, since their price is less than 1/100 of that of commercial Au nanoparticles. We demonstrate that Au powders with arbitrary shapes can be converted into highly stable nanoparticles with a narrow size distribution by a nanosecond Nd-YAG pulse laser, as shown below.