Learning Forum

[jie li]

I want to apply FDTD to simulate the absorption rate of polytetrafluoron (Teflon, PETE) ,

but I find that the refractive index of PETE is about 1.38, which is close to the silica（SiO2）. So the simulation results show that the material is nearly transparent, but in reality the material has extremely high reflectivity。

Is there a problem with my material modeling method? and How to model this kind of polymer。

thank you so much！

[Guilin Sun]

You may consider the following two issues:

1: when you say high reflectivity, what is the condition? how thick is the polymer? is it on SiO2? 

2: did you use lossy material data, eg, real(n)+i*imag(n)? absorption happens only when the material has loss, eg the imaginary part of the refractive index is non-zero. If it is pure dielectric, there is no absorption.

Have you measured the refractive index? from my experience, the imaginary part is difficult to measure, if not using polarimetry. Please check the actual refractive index you used, and make sure the thickness is porperly set to compare with experiment.

 

  

[jie li]

Thank you for your answer.

1、I learned that the PTFE thickness is in the millimeter level, is a single substance, there is no SiO2.
2、In my eyes, PTFE is an insulator, should be regarded as a pure dielectric, I did not see in the paper there is a imaginary part of its refractive index (in terahertz band，refractive index of about 1.4, dielectric constant of about 2) (I am not familiar with this knowledge, please forgive my wrong expression).
3、What bothers me is that PTFE is pure white and SiO2 (quartz glass) is transparent in real life, but their refractive indices are close (resulting in the reflectivity calculated by me using Fresnel's formula is also close).
I wonder if my material model was applied incorrectly, or if the FDTD method is not suitable for simulation。

[Guilin Sun]

From what you said, I would suggest you to use stackrt stackrt - Script command as there is no nano structure.

For the high reflectivity, it should be from the cavity effect. thus the thickness is important. Two transparent materials such as PTFE on SIO2 can have very low transmission or reflection, depending on their thickness.

FDTD solves directly the Maxwell's Equations. As long the physics can be described by the Equations you should be able to simulate it with FDTD, provided that you have powerful computer and correct materials/geometry.  In your case using stackrt should give you correct result.