Hello Arun,
We have an introduction on PhC devices and determination of the PBG within the manual installed on your machine with the product, it can also be found over at <a href=”https://optiwave.com/optifdtd-manuals/fdtd-photonic-crystal-and-photonic-band-gap-introduction/”>https://optiwave.com/optifdtd-manuals/fdtd-photonic-crystal-and-photonic-band-gap-introduction/</a>.
This would be the best place to start.
Scott