Plasmons

Plasmons

A metal is an arrayed archipelago of protons in a sea of electrons, extending in three dimensions.

A pebble dropped into a pool pushes water molecules away where it hits the water, causing them to tumble into their neighbours which then jostle and displace other molecules down the line, which shows up as a ripple on the surface. Similarly, when light falls upon an electron, the incoming energy results in a minute displacement. In most such instances, the electron bobs for a bit and the wave dies out in place.

However, the incoming light can be precisely manipulated to generate a disturbance that runs across the surface of the metal like a wave. As this wave propagates through the electron gas (the traditional term for the ‘sea of electrons’), the electrons themselves become sparse in some areas and packed more closely together in others.

At the same time, the protons exert an attractive force on the electrons, trying to pull them back into their original positions. This back-and-forth causes a coherent oscillation in the electron gas, of the same frequency as the light that caused it to start in the first place.

It is instructive to remember that while electrons and protons are true particles in the sense of ‘matter that occupies space and has mass’, many other ‘particles’ in physics are merely labels of convenience for systems that move and interact as a unit. One famous example is the phonon (a ‘particle’ of vibration, such as sound, that can be tracked as it travels through a medium). The oscillations in the electron gas are called plasmons.

Electron motion is a good deal slower than light (and the faster electrons go the more massive they get – a consequence of special relativity), so maintaining the frequency means plasmons need to have much shorter wavelengths. This results, then, in plasmons being associated with extremely powerful, highly localised electric fields.

Just as sound cannot by definition escape the atmosphere, plasmons are limited by the edges of the metallic surface – meaning also that the nature of plasmons on nanoparticles can be controlled by manipulating the geometry and size of those particles. Plasmons can decay when electrons stop oscillating regularly and scatter on encountering crystal defects.

In the very specific conditions under which plasmons are generated, the momentum that was carried by the impinging photons parallel to the surface is transferred to the electrons. This is sensitive to a variety of factors, including the transparent substrate surrounding the metal, the wavelength of the light, and the angle of incidence. Because plasmon generation is so very sensitive to minute differences in the refractive index of the surrounding substrate, it can be used as a protein detector. The idea is that as the protein adsorbs (‘ad-’, not ‘ab-’!) on to the metal’s surface, the angle of incidence at which light generates a surface plasmon changes.

This has been exploited to manufacture detectors that can detect a picomolar concentration of proteins: a single molecule of protein in a trillion

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