Vinamra,
Our support team reached out to you in regards to these questions through the support email a few days ago clarifying these points as well as asking for more details. I have sent you a clarifying email through our support channel but will copy its contents here for other users.
Points 1 / 2
You have not clarified what material you are trying to create so it is difficult for us to clarify for you what range of numbers you are referring to as we support dielectric, nonlinear, and dispersive materials. We also support isotropic and anisotropic variants of these. There are in fact quite a few different potential answers depending on what material you are trying to create. Our OptiFDTD Techincal Background pdf does profied detailed breakdowns of the materials supported as well as their paramters.
In regards to wavelength rangesf you are using Lorentz-Drude dispersive materials and you wish to fit to your own data set then the wavelength range over which you fit should include the wavelength range you intend to simulate as extrapolation is potentially risky depending on the fit. If you are loading LD materials from the material library you will not a wavelength range in their name which is the wavelength range over which the fit was done and the range over which your simulation should use it.
Point 3
The input field and its central wavelength are used for a number of purposes. The central wavelength is typically only used for calculations involving dispersive materials and injection. Note that for example if performing injection of a pulsed source if a refractive index is required the refractive index at the central wavelength is used. IF you are referring to normalization functions within Analyzer these use the input plane designated as the key source during simulation setup to normalize but it does not necessarily use just the central wavelength.
There is no scattering principal restrictions on an FDTD simulation so there is no requirement that the structure be less than the exciting wavelength. Typically the primary constraint in an FDTD simulation is on your mesh. 1) It must be less than 1/10 of a wavelength (in the highest refractive index material), and typically and 2) The mesh must be small enough such that your features are properly resolved, i.e. a 0.1 micron mesh is not adequate for spheres of radius 0.05 or even 0.2 micron.
Point 4
Currently our 3D simulation only supports propagation along the z axis. You can do X and Z propagation in 2D simulations or you could arrange your structure such that z propagation would duplicate your propagation from the top position. We will be releasing an update to the product which will include an updated input plane capable of improved directional support.
Scott