The IMD optical constant database is located in the directory called nk.dir, in the imd directory. Optical constant files are designated by a '.nk' extension, and are ASCII files containing three columns of numbers: wavelength (in angstroms), n, and k. Comment lines (beginning with a semicolon) may precede the data.
Each 'nk' file in nk.dir corresponds to one material, although there is usually more than one nk file for any given material. The naming convention I have adopted for nk files is Material_reference.nk. For example, the file Si_palik.nk corresponds to the optical constants for silicon, taken from the optical constant handbook by Palik [18]; the file Si_llnl_cxro.nk corresponds to other data for silicon, in this case data in the X-ray region compiled from the LLNL and CXRO atomic scattering factors (see section A.3 below.) Source reference information is usually included as a comment at the beginning of the file. In addition to these source-specific files, for every material I have created compilation files that cover as wide a range in wavelength as possible. The compilation files include no reference in the name. For example, the file Si.nk is a compilation of the data from Si_llnl_cxro.nk and Si_palik.nk, and spans the range from 0.124 angstroms to 3.33e6 angstroms.
To use a specific file for a given material, enter the full name of the file (excluding the .nk extension) in the Material fields in the relevant ambient, layer, or substrate widgets (be sure that the Optical constants file name button is checked, rather than the Composition and density button.). For example, to use the data for silver published by Hagemann, et al, specify Ag_hagemann, instead of just plain Ag.
The nk files contained in the nk.dir directory are listed in the file called AAACATALOG.TXT, which is also located in nk.dir. This file is displayed when you select Materials/Optical Constants -> Browse Optical Constants Database from the IMD menu bar.