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Dielectric Function (Read 11330 times)
Gerrit-Jan Linker
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Dielectric Function
11.07.12 at 21:50:57
Dielectric Function
The dielectric function of a material describes the electrical and optical properties versus frequency, wavelength, or energy.  
It describes the polarization (electric polarizability) and absorption properties of the material.  
The dielectric function ε, consists of two parts ε1 and ε2, or e1 and e2:
ε(λ) = e1(λ) ± ie2(λ)   ε is the complex dielectric function.
Note e2 is sometimes written as a positive quantity, or sometimes as a negative quantity, so the equation above contains ± to cover either case.
The quantity e1 represents how much a material becomes polarized when an electric field is applied;
e2 represents absorption in a material.
When the induced dipole oscillations in a material become large it is possible for the material to start absorbing energy from the applied field.  When absorption occurs the quantity e2 becomes important.
When a material is transparent e2 is zero, but becomes nonzero when absorption begins.  
When written together e1 and e2 form what is called the complex dielectric function.  It is called complex because the two parts are considered together and written using complex number representation:
ε(λ) = ε1(λ) ± iε2(λ).
It is important to consider both e1 and e2 together since they affect each other, meaning the shape of e2 cause corresponding changes in the shape of e1 and vice-versa.  This is known as the Kramers-Kronig relation between the real (e1) and imaginary (e2) parts of the dielectric function.
In short, the dielectric function describes what an electric field such as an oscillating light wave does to material.
    ε1 = volume polarization termà Dipoles created.
    ε2 = volume absorptionà vibrational energy lost as heat in insulating materials, or electrons become free carriers in solar cells, for example.
The dielectric function is related to the refractive index of a material by the equation
ε = n2
Or as complex numbers:
(ε1 ± iε2) = (n ± ik)2
See also:
Dielectric, Electric susceptibility
Dielectric Constant
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« Last Edit: 12.07.12 at 07:42:17 by Gerrit-Jan Linker »  

Gerrit-Jan Linker
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