In many examples the calculation is run for a single wavelength, eg. 633 nm. This is fine if we want to study monochromatic light interaction with some structure. However, in many cases a spectral dependence of some quantity is needed.

If we use a database of optical constants, given wavelength by wavelength, still the most straightforward calculation is to run the solver for each wavelength separately. Even if there is at present no graphical user interface for running such calculations, GSvit works with simple text files and one can write a script for running it across multiple wavelengths relatively easily. This is how majority of spectral calculations in this documentation was obtained.

If we use no spectral dependence of optical constants at all, use optical models that are given by some functional dependence of dielectric properties, e.g. the metal models like Drude + critical points, or if we fit our optical constants to be in this form, we can run the calculations with broadband excitation, significantly shortening the computation time and not needing any script at all.

Broadband calculations in FDTD are based on use of a broadband excitation, so instead of feeding our source with a sine function we use something with wider spectrum, like Gaussian function. Then we analyze the results using Fourier transform to get the spectral dependence.