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). Schemes of PPCs with equal effective refractive index and structure period: 1D PPC (a), and 3D opal-like PPC (b). Courtesy of Kazan Federal University. 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. (a): Transmission spectra of 1D photonic crystal (PC) and PPC. Dashed line is the spectrum of PC. Thick line is the spectrum of PC with the 30-nm Au layer. Red line is the spectrum of PC with the 30-nm Au and 270-nm buffer layers. The thin line is the calculated transmission spectrum of the 30-nm Au layer. (b): Intensity of transmission peak of the 1D PPC for different values of the thickness of the Au layer. (d): Transmission spectra of 3D PC and PPC. Dashed line is the spectrum of PC. Thick line is the spectrum of PC with the 40-nm Au layer (p-polarization). Red line is the spectrum of PC with the 40-nm Au and 280-nm buffer layers. Thin line is the spectrum of PC with the 40-nm Au layer (s-polarization). (e): Intensity of the transmission peak of the 3D PPC plotted as a function of the thickness of the Au layer. Courtesy of Kazan Federal University. OLPPCs with the hybrid mode of the optical states could be used in high-polarization-sensitive sensors, the researchers believe. “We assume that the hybrid mode can be useful for improving the control of light in PPCs. New types of resonators based on OLPPCs can be used for the strong interaction of light and matter,” researcher Artyom Koryukin said. (a): Transmission spectrum of PPC with the 30-nm Au layer. (b): Transmission spectra of the stand-alone 40-nm Au continuous layer. (c): Transmission spectra of PPC with caps. Solid line is the spectrum of PC with the 40-nm Au caps. Dashed line is the spectrum of stand-alone 40-nm Au caps. Courtesy of Kazan Federal University. The group is planning to create a theoretical description of the model for these processes. Additionally, it will investigate possible applications where OLPPCs could be effective, such as strong light-matter interactions with a single photon source. The research was published in Plasmonics (https://doi.org/10.1007/s11468-018-0880-6).