Post by Baron von Lotsov on Mar 28, 2024 13:34:37 GMT
This is a little difficult to describe, but I'll do my best.
A plasmon is a thing called a quasiparticle. In a typical material you get a 3D array of molecules and a surface is similarly a 2D array of molecules. Going back a long way in physics when one looks at the mathematics of these arrays, due to their regular spacing you can create standing waves and these waves have a certain energy. The energy waves in a 3D material behave as if they were particles of the energy of the wave and these were called phonons. A phonon is therefore described as a quasiparticle. It's not a physical thing, but a physical effect equivalent to the behaviour of a particle, so we can use this model to aid understanding and calculation. A plasmon is a similar principle, but where the phonons were related to vibrational waves of molecules, plasmons are related to electrons and quantum waves in a regular pattern of electrons.
wiki explains the mechanism of these electron waves as :
Plasmons can be described in the classical picture as an oscillation of electron density with respect to the fixed positive ions in a metal. To visualize a plasma oscillation, imagine a cube of metal placed in an external electric field pointing to the right. Electrons will move to the left side (uncovering positive ions on the right side) until they cancel the field inside the metal. If the electric field is removed, the electrons move to the right, repelled by each other and attracted to the positive ions left bare on the right side. They oscillate back and forth at the plasma frequency until the energy is lost in some kind of resistance or damping. Plasmons are a quantization of this kind of oscillation.
OK so the next bit is the case of a 2D surface of these electrons where the plasmon is called a surface plasmon polariton, which is defined as
Surface plasmon polaritons (SPPs) are electromagnetic waves that travel along a metal–dielectric or metal–air interface, practically in the infrared or visible-frequency. The term "surface plasmon polariton" explains that the wave involves both charge motion in the metal ("surface plasmon") and electromagnetic waves in the air or dielectric ("polariton").[1]
They are a type of surface wave, guided along the interface in much the same way that light can be guided by an optical fiber. SPPs have a shorter wavelength than light in vacuum at the same frequency (photons).[2] Hence, SPPs can have a higher momentum and local field intensity.[2] Perpendicular to the interface, they have subwavelength-scale confinement. 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).
They are a type of surface wave, guided along the interface in much the same way that light can be guided by an optical fiber. SPPs have a shorter wavelength than light in vacuum at the same frequency (photons).[2] Hence, SPPs can have a higher momentum and local field intensity.[2] Perpendicular to the interface, they have subwavelength-scale confinement. 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).
so next up we see what properties these have and it turns out they have something called surface plasmon resonance
Surface plasmon resonance (SPR) is a phenomenon that occurs where electrons in a thin metal sheet become excited by light that is directed to the sheet with a particular angle of incidence, and then travel parallel to the sheet. Assuming a constant light source wavelength and that the metal sheet is thin, the angle of incidence that triggers SPR is related to the refractive index of the material and even a small change in the refractive index will cause SPR to not be observed. This makes SPR a possible technique for detecting particular substances (analytes) and SPR biosensors have been developed to detect various important biomarkers.[1][2]
there is a good little diagram on this page which shows you how you can make a detector to detect a very precise band of energy absorption in a sample substance for determining its chemical properties. Each molecule has a whole range of different frequencies it will absorb or emit a photon and this varies on the molecular structure, but where you have complex molecules this method gives you the accuracy you need to tell one energy abortion line from its adjacent one.
So now we all know what SPR is (hopefully!) then lets see what else it can be used for.
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