Thank you for responding. I was also thinking about the possibility of cavity formation. I have used periodic boundary conditions on the x-axis and PML boundary conditions on the y-axis. The source is 2.5 um away from the structure and the top PML boundary is 1.75 microns away from the source. The bottom PML boundary is 2.75 microns away from the silicon. I have used 650 nm wavelength light. So, a cavity formation is difficult to justify in this setup.
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