As part of their research into optical states of plasmonic-photonic crystals (PPCs), scientists at Kazan Federal University investigated three-dimensional opal-like plasmonic-photonic crystals (OLPPCs), focusing on why OLPPCs do not admit light of certain wavelengths. (This is called the photonic bandgap — it is the range of light wavelengths where propagation through a crystal is difficult).
The primary conditions for passing a light beam with the wavelength of the photonic bandgap and a certain polarization through an OLPPC are the continuity of the gold layer, with a thickness of about 40 nm, and the use of polarized light, said the team.

The researchers modeled light transmission through photonic crystals with a continuous gold layer on their surfaces. They modeled different versions of PPCs and were able to define the conditions of existence of a polarization-sensitive photonic bandgap transmission peak in the OLPPC. They also studied the condition of efficient excitation of the hybrid plasmonic-photonic mode in such structures.

The researchers found that transmittance of light across a PPC was accompanied by excitations of the optical Tamm states. One-dimensional PPCs had light transmission pass bands inside the photonic bandgap in both polarizations, but 3D PPCs did not have light transmission pass bands inside the photonic bandgap, they said, because of a noncontinuous gold layer (shaped like separate nanocaps or nanocrescents on the surface of a PPC). The OLPPCs that were studied had a light transmission pass band inside the photonic bandgap with certain polarization, due to the excitation of the hybrid mode of the optical states.