Plasmonics

PLASMONICS

• Plasmonics or nanoplasmonics refers to the generation, detection and manipulation of signals at optical frequencies frequencies along metal-dielectric interfaces in the nanometer scale.

• Inspired by photonics, plasmonics follows the trend of miniaturizing optical devices and find applications in sensing, microscopy, optical communications, and bio-photonics.

• Plasmon is the collective oscillation of conduction electrons in metals. The resonance energy depends on the particle size and shape, and the permittivity (dielectric constant) of the material itself and surrounding materials.

Principle

Plasmonics typically utilizes so-called Surface Plasmon Polaritons (SPPs) that are coherent electron oscillations travelling together with an electromagnetic wave along the interface between a dielectric (e.g. glass, air) and a metal (e.g. silver, gold).

The SPP modes are strongly confined to their supporting interface, giving rise to strong light- matter interactions. In particular, the electron gas in the metal oscillates with the electro- magnetic wave. Because the moving electrons are scattered, ohmic losses in plasmonic signals are generally large.

It limits the signal transfer distances to the sub-centimeter range, unless hybrid optoplasmonic light guiding network, or Plasmon gain amplification are used.

Surface Plasmon Resonance (SPR)

The collective oscillation of the free electrons with respect to the fixed positions of the positively charged nuclei (acting as the core) is called plasmon.

In the case of a metallic nanoparticle where there is a large fraction of atoms as the surface atoms, the free surface electrons can collectively oscillate to produce the surface plasmon (SP). But due to the quantum confinement, the surface plasmon is localized and it is called the Localized Surface Plasmon (LSP).

When a nanoparticle is exposed to light, its oscillating electric vector of light can induce a collective oscillation of free electrons of the metal nanoparticles.

In other words, the incident light can cause a oscillating a dipole moment. Thus, this leads to charge separation generating a dipole oscillation along the direction of the electric field of the light. Thus is explained for a spherical nanoparticle in fig. 4.39.

For a particular nanoparticle, the amplitude of light induced collective oscillation of free surface electrons reaches maximum at a specific frequency of the incident light.

It produces the surface plasmon resonance (SPR) and the light is absorbed (called plasmon absorption).

In other words, when frequency of the incident electromagnetic radiation matches the inherent frequency of surface plasmon, SPR occurs and the light absorption is observed. (Fig.4.41)

Uses of Plasmonics

• Plasmonic gold and silver nanoparticles have unique optical, electrical, and thermal properties and hence are used in applications such as antimicrobial coatings and molecular diagnostics. Color engineering.

• The unique optical properties of metal nanoparticles are very useful in color engineering.

• Plasmonics (or nanoplasmonics) is a young topic of research, which is part of nanophotonics and nano-optics. Plasmonics concerns to the investigation of electron oscillations in metallic nanostructures and nanoparticles.