I am trying to see how the normalized transmission value in the through port of an optical ring resonator behaves when the PCM material's(GST) amorphization of GST is increased. My structure is a Si bus and ring waveguide on a SiO2 substrate where the PCM material i.e. GST is embedded on the on the Si ring waveguide. The expected transmission value in the though port should be like the attached figure. The figure depicts that the normalized transmission value should be decreased from 0.5 approx. (at 100% crystallisation/0% amorphization) to 0.1 approx. (at 0% crystallisation/100% amorphization).

But, my transmission value starts from 0.7 and becomes 0.45 at 50% amorphization. Then, it remains close to 0.45 even at 100% amorphization. I have used exact dimensions and material properties.

[N:B: The results of normalized transmission V/s degree of crystallization in the GST cell from the paper i am following is attached. I am simulating in Lumerical FDTD, where three ports are used in INPUT, Through and Drop port. I am peforming the simulation at 1529 nm wavelength (center) having 100 nm width]

Can anyone help how to resolve this issue in Lumerical FDTD?