when we introduce a defect layer in the middle of the photonic crystal (binary, ternary, or any other structure) a resonant mode appears somewhere within the photonic bandgap. I have noticed that the FWHM of these resonant modes decreases as the number of cells in the photonic crystal increase! Anybody knows why!
You have orchestrated nice relationship among various variables, which is real nice answer, but I am seeking a physical explanation why that happens
Thank you for sharing your thoughts
Loosely speaking, the FWHM represents how spread the resonant mode is around a certain range of frequencies. If it's wide, it means the photonic crystal is not doing a good job at isolating the resonant mode, a.k.a. it's allowing into the bandgap not only one specific frequency like ideally it should, but a whole range of them.
This improves if you increase the number of cells around the defect, because they contribute to select/filter only one frequency to be allowed inside the bandag, hence the resonant peak gets thinner around it (FWHW decreases). In textbooks, when they have a single, pure resonant frequency it's because they are considering an infinite crystal.
Obviously in practice you have fabrication limitations and scattering losses of the structure itself that will limit how much the quality factor can improve.
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