analysis optical spectrum result

[gaurav1201]

To,
    respected sir,
I want to know the optical properties in optical spectrum i.e. absorption coefficient, dielectric constant, refractive index (n)  graph relation with each other for a material having the direct band gap of 3-4 eV and their refractive index, imaginary dielectric constant and absorption coefficient origin start at equal to its bandgap value. The graphs of R.I.  and imaginary dielectric constant are looking like same as absorption coefficient.  while structure having zero band-gap their refractive index and imaginary dielectric constant and absorption coefficient origin start from zero energy gap but their graphs behaviour are different from optical absorption coefficient. why so. 
Please answer me.
 Thank you sir
 
 
 

[Petr Khomyakov]

Please describe your problem(s) more clearly, also giving some information on the systems of your study, and posting the corresponding scripts and explanatory figures.

Now I can only say that having zero gap means that you have a metal system which should exhibit strong optical absorption and dielectric response, since the electric field is supposed to be zero in metals due to their high DOS, giving rise to efficient screening of the electric field.

[gaurav1201]

sir,
I am working on optical properties of ZnS nanotube armchair(3, 3) (4, 4) and chiral(4, 2) and zigzag(5, 0) all are showing band gap between 3.67 to 4 eV. All are the semiconductor. When I am calculating optical properties of all these nanotubes, I am getting the graph of the absorption coefficient, dielectric constant and refractive index (w.r. t. to energy eV). the origin of all these graphs starts from equal to its band gap of the nanotube. The dielectric constant and refrective index graph looks same as the absorption coefficient. But when  I adsorbed phosphorous on all nanotubes and  found zero eV band gap
also calculating optical properties as above for all adsorbed  phosphorous ZnS nanotubes. the origin of all the graph absorption coefficient imaginary dielectric constant and refractive index start from 0 eV. but peaks refractive index and imaginary  dielectric constant in the  graph are not same as like as absorption coefficient graph. while it is posible in case of pristine ZnS nanotube.       

[Petr Khomyakov]

alpha (absorption coefficient) is proportional to the imaginary part of dielectric function, Im[epsilon], by definition. If you want to get the dielectric constant, then you have to plot Re[epsilon], i.e., real part of the dielectric function.

I note that you should be cautious in using the Optical spectrum analysis object for single nanoribbons and nanotubes, since the optical spectrum is actually defined in QuantumATK for bulk-like 3D structures, see https://docs.quantumwise.com/manuals/Types/OpticalSpectrum/OpticalSpectrum.html - the \Xi is given per unit cell volume. If you have an array of nanoribbons or nanotubes, then it should be fine to use that object.