software to fit optical spectra - Quantum Materials Group

∆S ω) ≈ α ( ω)
∆ε
( ω)
+ α ( ω)
∆ε
( ω)
.
Equation 2-23
( 1 1
2 2
Then one can associate an extra (differential) model ε diff (ω)
with ∆ ε and adjust its parameters
in order to fit ∆ S(ω
) , using Equation 2-23. The error bars of ∆ S(ω
) have to be estimated
separately. In the fitting with the differential model, the role of the base model is to provide
only the (fixed) derivatives α ( ) and α ( ) .
1 ω
2 ω
Formally, ε diff (ω)
has to be treated as a usual dielectric function, because the physical
requirements to it are almost the same as the usual requirements to the dielectric functions (see
2.2.1). For instance, it is important that ε diff (ω)
satisfies the Kramers-Kronig relation. The
only obvious difference is that ∆ ε 2 can be negative (unlike ε 2 ), and both ∆ ε1
and ∆ ε 2 must
vanish at very high frequencies.
Guide to RefFIT Page 24