Metal Plasmonics

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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.

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                                            (J. Phys. Chem. C 2016, 120, 20471)

<Demo video of laser-induced conversion of Au powders into particles>

Recently, surface plasmons have been used to control colors of materials. This is possible since controlling the particle's shape and size determines the types of surface plasmons that can couple to it and propagate across it. This in turn controls the interaction of light with the surface. These effects are illustrated by the historic stained glass which adorn medieval cathedrals. In this case, the color is given by metal nanoparticles of a fixed size which interact with the optical field to give the glass its vibrant color. In modern science and technology, these effects have also been engineered for both visible light and microwave radiation. Much research goes on first in the microwave range because at this wavelength material surfaces can be produced mechanically as the patterns tend to be of the order a few centimeters. To produce optical range surface plasmon effects involves producing surface patterns which have features of <400 nm. This is much more difficult and has only recently been demonstrated in metal thin film patterns fabricated on substrates by means of e-beam lithography or focused ion-beam. However, these processes are extremely expensive and also not scalable. We are developing a low-cost, scalable method of fabricating nano-scale surface patterns on bulk metals to produce tunable structural colors from them. 

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