# distinguishing (n,k) material and sample data for broad band source

#1

Hai…
I plotted the transmission spectra of a sampled data as well as (n,k) material by giving the same value for real and imaginary part of refractive index for a broad band source. Even though the refractive index will vary along the wavelength for (n,k) material for a broad band source , the transmission spectra of both are same and when i plotted in the same graph they are overlapping.Why it so?

#2

Hi,

I would tend to suspect the material fit: for both the sampled data and (n,k) materials, the properties are fitted over the wavelength range. You can check the fit with the material explorer (https://kb.lumerical.com/en/index.html?materials_material_explorer.html).

Feel free to share your simulation files so we can have a look.

#3

Hai,
I imported a sampled data by giving the value 2 as real and .002 as imaginary in the wavelength range of 0.4-0.7 micrometer and plotted the graph in material explorer.But the material data curve and FDTD curve were not fitting whereas both coincided at a particular wavelength.Then what will be the value of refractive index will it be taking?How can i keep a constant refractive index in the broad band source for sampled data?

#4

Ok, thanks for the clarification. The value that is used for the calculation is the fit. That fit is calculated from the permittivity (and not the index). Moreover, the real and imaginary parts are interdependent, and it can be very difficult (sometimes impossible) to get a good fit for both.

In this case, if you look at the permittivity, you will see a perfect fit for the real part, but not so good for the imaginary part:

I was wondering, is there a specific reason why you would like to model a material with a constant index? Most materials are quite dispersive.

#5

Hai,
Thank you for the reply.What iam planing to do is,i want to plot the transmission spectra of a sampled data with particular refractive index.Then i want to change the imaginary part and have to analyse the change in transmission spectra.If the imaginary part is not fitting as you said then how can i compare the transmission data?

#6

What kind of structure are you interested in? If you want to calculate the transmission spectrum of a layer of this material, you can use the “stackrt” command instead (https://kb.lumerical.com/en/index.html?ref_scripts_stackrt.html). You can easily do a loop over the imaginary part, and calculate the spectrum for each value, over the bandwidth.

Alternatively, you can set the simulation for a single wavelength, and do a sweep over the wavelength to calculate the spectrum at each wavelength and keep the index constant over the bandwidth.

The last possibility I can think of would be to play with the fit parameter, to reduce the error on the imaginary part (you can increase the “imaginary weight” in the “Advanced settings”, and the number of coefficient, in the material explorer). But that will not give you a constant index over the bandwidth.