Discussion Forum

Hello,

I am using comsol 3.5a to simulate light (532 nm, Nd:YAG laser) scattering from a waveguide with a square cross-section (1 micrometer times 1 micrometer). The waveguide is made of polymer (refractive index 1.58) and it is supported by a layer of the same polymer with the thickness of 0.3 micrometer. Beneath the polymer layer, there is SiO2 substrate (refractive index 1.45). The case is modeled in a 2D geometry.

I tried the mode of RF Module/In-Plane TE Waves/Scattered TE waves. However, there is a problem in definition of the excitation field. Since the scatter (the waveguide) is placed on a polymer layer on a SiO2 substrate, the background field (the excitation field) is not as simple as exp(-j*k0_rfweh*y). The effect of the infinite multilayered substrate should be taken into account. However, it is not straightforward to get the analytical expression of the (Fresnel formulation) background field in the presence of the polymer layer and the SiO2 substrate. On the other hand, it is either incorrect to simply put a plane wave onto the whole structure including the scatter and the supporting layers since the supporting layers are then limited in lateral size and it also causes scattering.

I was suggested by some online discussion that I can perform a 2-step calculation. First, I calculated the background field with the infinite supporting layers (by setting periodic boundary conditions) but without the scatter. Second, I stored the solution (Solve Manager/Store Solution) of the first step and then used it as an initial value (Solve Manager/Initial Values/Stored Solution) for the second-step calculation. However, I found this method not so correct because the initial value is not regarded as the excitation field by Comsol so that this 2-step calculation just gives the same result as simply putting a plane excitation wave, exp(-j*k0_rfweh*y), onto the scatter and the supporting layers which also serve as scatters due to its limited lateral size.

Would you please say something in detail about how to define a numerical background excitation field in the calculation of scattered field if anyone knows. Or would you please help me find out an alternative way to solve my problem. Thank you so much!

Sincerely,
Jing Zhou