software to fit optical spectra - Quantum Materials Group

For a reliable extraction of ε (ω)
it is often crucial to fit spectra of different types
simultaneously. An example, when the analysis of a single quantity might be rather shaky, is
the already mentioned Kramers-Kronig transformation of reflectivity. It is well known, that the
experimental reflectivity uncertainties and the ambiguity of extrapolations to high and low
frequencies in this case can boost the error bars of the resulting dielectric function (or,
equivalently, of the optical conductivity). As a rule, it helps a lot to supplement the reflectivity
measurement with the ellipsometry or transmission spectra. In RefFIT the user can freely
choose the collection of datasets, which should be fitted simultaneously.
An indicative list of the built-in models includes the Drude-Lorentz model, the dielectric
function of a BSC superconductor, the extended Drude model, ellipsometry, reflection and
transmission of multi-layer and anisotropic samples. Of course, it is hopeless to predict all the
experiments that a solid-state optical spectroscopist might wish to do. Therefore, new models
are added to RefFIT from time to time in order to meet the emerging needs.
The fitting is always a try-and-error activity, which requires a lot of human efforts and
intuition. Sometimes one can spend hours and even days before a proper model is found. In
order to ease this procedure, RefFIT was designed in such a way that a user can see the results
of his manipulations with parameters on the graphs in real time.
Once a good model is found, it might be necessary to apply it many times in the same
way to different datasets, e.g. to study temperature dependence of spectra. It is obviously not
practical to continue doing it ‘by hand’. Do not worry: the built-in macro language greatly
facilitates the execution of lengthy routine calculations in RefFIT.
1.3. About this manual
This manual consists of three major parts.
In Chapter 2 some theoretical aspects of the optical data fitting are considered. It starts
with the description of the celebrated Levenberg-Marquardt algorithm for a non-linear
modeling, which is the ‘computational engine’ of RefFIT. Then the physical constraints on the
dielectric functions are discussed, which have to be met by a realistic model. We also describe
in details a novel method of the so-called variational dielectric functions (RefFIT is likely the
first program to use this technique). Finally, the analysis of differential (modulation) spectra is
discussed.
Chapter 3 is a tutorial that shows, step-by-step, how to tackle typical fitting problems
with RefFIT. The examples are selected in order to cover the most essential features of RefFIT.
Finally, in Chapter 4 you can find the full reference information about RefFIT, including
the description of all its elements (datasets, models, graphs etc.), the exact formulas used to
calculate optical quantities, the macro language and so on.
This manual is not meant to be an optics course, so I assume that the reader is familiar
with optical spectroscopy. A basic acquaintance with Windows is also assumed.
The screenshots, which I used in the text, are made on my laptop, running under
Windows XP. If you have another Windows version, then the appearance of some graphics on
your screen might look slightly different.
Guide to RefFIT Page 6