The user has two options for input of the refractive index and absorption index for materials forming bulk (3D) regions.

One way is direct input of the values, and another way is input of parameters for a fixed parametric dependence for these
properties on the emission wavelength.

The advantage of direct input is that the user specifies exactly the value used in computations. However, in this case the user
takes responsibility to update the value as the wavelength is changed.

For input via parametric dependence, the program automatically updates the refractive index and absorption index as the wavelength
is changed. However, the user need accept pre-defined fixed equation for the wavelength dependence of the properties.

For the refractive index, the simplest form of Sellmeier equation is used (http://en.wikipedia.org/wiki/Sellmeier_equation) .
Compared the above link, the parameters are A<->B1, lambda1^2 <-> C1 .

For the absorption index, absorption due to the free carriers is taken into account. Please note this equation is valid for the
quant energy well below the energy gap of the semiconductor (otherwise, a much more complex dependence appears). The contribution
of the free carriers is proportional to the carrier concentration (it is natural) and to the second power of the wavelength (that
is important for the wavelength dependence of alpha). The factor a includes some basic constants and the carreir mobility; please
refer to Eq. 10.4 of SiLENSe Physics Summary or the below link:

http://books.google.ru/books?id=OX2cHKJWCKgC&pg=PA98&lpg=PA98&dq=free+carrier+absorption&source=bl&ots=DrJ5ZJYr85&sig=tt8hCyxrr_ACQjrDtsl0Rpf8ZKg&hl=ru&ei=MnMPTaO7OsaDOuruxKgJ&sa=X&oi=book_result&ct=result&resnum=12&ved=0CGEQ6AEwCw#v=onepage&q=free%20carrier%20absorption&f=false