Hi Ahmad,
Since Lumerical solvers are photonic solvers, if the index change is due to a chemical reaction between the thin film and the gas then you won't be able to simulate this. But if you can get the perturbed index due to the interaction, then you can include this perturbed index to calculate the modal properties. Would you be able to confirm if the index change is due to a chemical interaction? Thank you!
Regards,
Amrita
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