I think this may be caused by the enhanced fields when you increase the thickness of the metal. Therefore, the matter-light interaction increases and the stop-band (i.e. the bandgap) works for broader frequency range. This is a qualitative understanding. You may check it using field profiles at these frequencies.
Without knowing the details of the crystal structure you are referring to, as a rule of thumb photonic band gaps increase with the refractive index difference. Metals have large (often negative) permittivities relating to a large refractive index. On increasing its thickness you probably increase the relative gradient in the refractive indices of the components of your structure.
The photonic band gap can be enhanced by many methods. It can be enhanced by increasing the refractive index contrast of the layers. The band gap can also be enlarged in a chirped or disordered photonic crystal. Moreover, when we use the heterostructured photonic crystal, a considerably enhanced band gap can be obtained.
A lot of possibilities are there. Certain region can only by knowing all parameters and wavelength/frequency region used. The permittivity of metal is complex and have dependence on frequency. The permittivity or dielectric constant of metal have negative value too. So, the metal dielectric photonic crystal has large dielectric contrast which also cause large photonic badgap in certain region.
The width of photonic band gap can be changed by many parameters.
These parameters are such as refractive indices of layers, thicknesses of layers, number of periods,...and so on.
In case of metal-dielectric PC, I believe that the enhancement of photonic band gap widh is related with the refractive index difference.
Finally, I think that the type of metal effect on this.
My regards, Malek G. Daher
- Badges
- Science topic
- Similar topics
- Phytochemistry
- Extracts