Hello Malik,
Run your PWEM calculation using the TM polarization and you will find that there is 2 bandgaps. The one I believe you are looking for is the one from [w/2pic] 0.517826 to 0.547916.
I realize that the reference you are likely using probably says you should be looking at the TE polarization. It is important to note that there are different conventions for TE and TM. For a 2D simulation with infinite Y, OptiFDTD uses TE (Ey, Hx, Hz) and TM (Hy, Ex, Ez). While this may differ from some references it is inline with many waveguide based reference.
On another note, the PWEM calculation in OptiFDTD does not take into consideration the power of the source so the bandgap you are calculating is based on the linear permitivity of 15.813 that you entered.
Scott