Plasma edge
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A plasma edge is a characteristic structure in a reflection spectrum . With solids it occurs at the point in the spectrum where the plasma resonance occurs. There the real part ε 1 of the dielectric constant has a zero.
With metals (example: silver ) the effect can be explained well according to the Drude theory of free electrons in the conduction band , see plasma resonance .
Applications
The plasma edge effectively divides the reflection spectrum into two parts: At shorter wavelengths the reflection is very low, at higher wavelengths it is much higher. This can be used technically.
One application is a solar selective absorber . This is intended to solve the problem that a solar absorber does get hot due to solar radiation, but a noticeable proportion of this energy is emitted again as thermal radiation before the energy can be used for technical purposes. The plasma edge selects here in the right order: At small wavelengths, specifically in the visible, especially in the green spectral range, the reflection is low and the absorption is high. At longer wavelengths, the near infrared , where the heat radiation takes place, the reflection is high and therefore the radiation is low. However, there are only very complex to manufacture materials (mostly highly doped semiconductors with a large band gap such as tin-doped indium oxide , aluminum- doped zinc oxide or highly doped germanium ) that have the plasma edge in the right place (the transition from visible light to infrared) so that their practical application in solar absorbers is rather rare.