Draw grating with two method, the same parameters, different results


Simulation of a grating coupler using two ways . In case of other conditions are the same, First is by increase the rectangles of SI on the SI waveguide, set(‘material’,‘Si (Silicon) - Palik’); second is by add a short Si waveguide on Si waveguide, etch on the short Si waveguide to make the grating coupler. set(‘material’,‘etch’); or set(“index”,air); I try both of them.

In the parameters box in the chart to the right, shows the structural parameters of Si rectangular blocks and blocks of air, you can see they are the same. Background refractive index of both stuctures is 1.
I would like to ask what is the reason, thank you!
Result is as follow.


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