Surface plasmon polariton
Surface plasmon polaritons (SPPs) are electromagnetic waves that travel along a metal–dielectric or metal–air interface, practically in the infrared or visible-frequency.
An SPP will propagate along the interface until its energy is lost either to absorption in the metal or scattering into other directions (such as into free space).
Application of SPPs enables subwavelength optics in microscopy and photolithography beyond the diffraction limit.
It also enables the first steady-state micro-mechanical measurement of a fundamental property of light itself: the momentum of a photon in a dielectric medium.
This momentum mismatch is the reason that a free-space photon from air cannot couple directly to an SPP.
This method, while less frequently utilized, is critical to the theoretical understanding of the effect of surface roughness.
At low k, the SPP behaves like a photon, but as k increases, the dispersion relation bends over and reaches an asymptotic limit called the "surface plasma frequency".
The surface plasma frequency is the asymptote of this curve, and is given by In the case of air, this result simplifies to If we assume that ε2 is real and ε2 > 0, then it must be true that ε1 < 0, a condition which is satisfied in metals.
Electromagnetic waves passing through a metal experience damping due to Ohmic losses and electron-core interactions.
This condition is satisfied at a length[12] Likewise, the electric field falls off evanescently perpendicular to the metal surface.
Nanofabricated systems that exploit SPPs demonstrate potential for designing and controlling the propagation of light in matter.
The resulting enhanced sensitivity of light to external parameters (for example, an applied electric field or the dielectric constant of an adsorbed molecular layer) shows great promise for applications in sensing and switching.
Current research is focused on the design, fabrication, and experimental characterization of novel components for measurement and communications based on nanoscale plasmonic effects.
In addition to building functional devices based on SPPs, it appears feasible to exploit the dispersion characteristics of SPPs traveling in confined metallo-dielectric spaces to create photonic materials with artificially tailored bulk optical characteristics, otherwise known as metamaterials.
[5] Artificial SPP modes can be realized in microwave and terahertz frequencies by metamaterials; these are known as spoof surface plasmons.
[13][14] The excitation of SPPs is frequently used in an experimental technique known as surface plasmon resonance (SPR).
In SPR, the maximum excitation of surface plasmons are detected by monitoring the reflected power from a prism coupler as a function of incident angle, wavelength or phase.
[16] The ability to dynamically control the plasmonic properties of materials in these nano-devices is key to their development.
[17] This approach has been shown to have a high potential for nanoscale light manipulation and the development of a fully CMOS- compatible electro-optical plasmonic modulator.
CMOS compatible electro-optic plasmonic modulators will be key components in chip-scale photonic circuits.
The electric field is stronger at the interface because of the surface plasmon resulting in a non-linear optical effect.
For visible and near-infrared light, the only plasmonic materials are metals, due to their abundance of free electrons,[22] which leads to a high plasma frequency.
[24] The table below shows the quality factors and SPP propagation lengths for four common plasmonic metals; Al, Ag, Au and Cu deposited by thermal evaporation under optimized conditions.
[25] The quality factors and SPP propagation lengths were calculated using the optical data from the Al, Ag, Au and Cu films.
[10] Silver exhibits the lowest losses of current materials in both the visible, near-infrared (NIR) and telecom wavelengths.
Gold has the advantage over both silver and copper of being chemically stable in natural environments making it well suited for plasmonic biosensors.
[27] Aluminum is the best plasmonic material in the ultraviolet regime (< 330 nm) and is also CMOS compatible along with copper.
[22] These include transparent conducting oxides, which have typical plasma frequency in the NIR-SWIR infrared range.
Some materials have negative permittivity at certain infrared wavelengths related to phonons rather than plasmons (so-called reststrahlen bands).
If surface plasmons are excited in the Kretschmann geometry and the scattered light is observed in the plane of incidence (Fig.
