Model Assistance
I am combing posted topics into a one. I am simulating the scattering from an AFM (Atomic Force Microscope) tip in the THZ frequency range. As a model for my AFM I for this post have used a cone much simpler and very similar to my full simulation. The cone is a rough approximation of an AFM tip without the cantilever. Since the height of the AFM is on the order of the incident wavelengths (~15um), antenna resonances can be seen in the scattered far field. In this way it is sometimes referred to as a nano antenna. What I am really interested in though is placing a dielectric slab beneath the the apex of the cone. As the cone is brought closer to the dielectric, through a near field mediation between cone apex and sample, the scattered far field changes with the introduction of a dielectric. It is possible to unravel this change and determine local optical properties of the underlying dielectric. What I would like to prove is that instead of simulating a platinum coated silicon cone, I can simply replace the multilayer with a PEC. I am simulating my platinum as a dielectric because my meshing requirements near the apex of the cone are such that the skin effect approximation displays errors since the mesh is much smaller than the thickness of the coating. I am afraid to simulate the platinum as a metallic layer for this reason. My first attempt at simulating the multilayered cone was with no dielectric underneath the cone and by placing a smaller cone inside the original cone and simulating the platinum region and silicon dielectric region with the SEP. I am unsure if this is the correct way to go about simulating the platinum/silicon layers so I attempted to use the FEM. But since the effective wavelength in platinum is quite small the meshing requirements for the platinum exceed my computational capacity. Simply put I need to know that I can reliably simulate a PEC instead of the multilayer. Any suggestions?
Answers
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Hello pmcardle,
It looks like you have scared everyone off with this questions. ;-) Since I have been involved with answering your other questions, I'm not sure how much more I can add. But, I can make some comments and hopefully it helps you with your task.
In my experience with permittivity measurements, you would calibrate the systems with air / free space and the the material parameters are then extracted by looking at the change due to introducing a dielectric medium. The question you need to ask yourself is if the change in the field distribution (more concentrated at the tip due to the dielectric) significantly changes the current distribution of the probe. If not (as I expect), using PEC is good enough, since you are interested in the difference and not the actual values.
As a rule, I always try to start with the easiest possible model and once I can prove that my concept works (in your case, that the measurement change by the probe with and without the dielectric can be used to extract the material parameters, I would investigate possible sources of error (as an example, change the PEC to a skin effect approximation, even if you are not sure if it is valid) and repeat the simulations to see if the resulting material extraction changes or if it is unaffected (calibrated out). My suspicion is that you can model using PEC, but this needs to be tested at some point during your research.
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