With the rapid increase in the research and application of the terahertz radiation, it is highly desirable to develop advanced devices for manipulating the terahertz waves. Recently, terahertz plasmonic structures supporting plasmon polaritons have exhibited great potential for extreme light confinement and manipulation applications. Typically, surface plasmon polaritons get excited and localized when terahertz waves irradiate a periodic metallic surface, thus producing a resonance peak in the spectrum. Despite the extraordinary transmission properties of the typical plasmon structures, multifunction plasmon devices have not been realized. Various efforts that have been proposed to improve the coupling of the surface plasmon polaritons, such as altering the geometric shapes and the interval between the structural units, have several drawbacks that limit their practical applications.
Recent research revealed that the tradeoff could be improved by designing the manipulation devices using hybrid plasmonic structures of dielectric films and metal surfaces. This has led to the development of double-layer metal grating arrays exhibiting low propagation loss and improved surface plasmon polaritons coupling. Unfortunately, the dielectric film responsible for the formation of the hybrid mode in the multi-layered structures has not been fully understood. Besides, this requires a thorough understanding of the field coupling in the multilayer hybrid plasmonic slabs in the terahertz frequencies. To this note, a group of researchers: Dr. Dejun Liu, Dr.Xiaohu Wu, Professor Feng Liu from Shanghai Normal University and Professor Lin Chen from University of Shanghai for Science and Technology developed a new composite slab and investigated its feasibility at THz frequencies. The aim was to analyze the role of the dielectric film and near-field coupling in the hybrid slab. Their work is currently published in the journal, Optics Express.
In their approach, the slab comprised of double-layer metallic gratings and dielectric films for supporting the two resonant modes. The origin of the hybrid mode was determined based on the electric field vectors and charge distributions. The authors also investigated the effects of different structural parameters on the resonant peaks as well as the effects of grating width on the plasmonic band. The results were validated by comparing the experimental and simulation results.
Results showed that unlike the single/double-layer metallic grating, the dielectric film-induced sharp Fano resonance at a frequency of 0.189THz, thus resulting in a significant increase in the transmission of the transverse magnetic waves. The slab exhibited a bandgap of 40% and two resonant peaks, corresponding to the symmetric plasmonic and hybrid modes, in the transmission spectrum. The low-frequency resonant mode (0.176 THz) originated from the symmetric plasmonic mode while the high-frequency resonant mode (0.332THz) was induced by mixing the dielectric and plasmonic modes. Furthermore, altering the structural parameters such as the refractive index and thickness of the dielectric films exhibited remarkable effects on the spectral response of the plasmonic slabs. Similarly, the plasmonic bandgap could be manipulated by tuning the grating width.
In summary, the study reported a terahertz composite plasmonic slabs based on double-layer metallic gratings and dielectric films. The slab exhibited two resonant modes and a broad bandgap. The origin of the resonant modes and the effects of the structural parameters on the resonant peaks were clarified. Moreover, the experiment results agreed well with the numerical results to confirm the feasibility of the study. In a statement to Advances in Engineering, Professor Lin Chen said the reported composite plasmonic slab is a potential candidate for the design of high-performance optoelectronic devices in the higher frequency regions.
Liu, D., Chen, L., Wu, X., & Liu, F. (2020). Terahertz composite plasmonic slabs based on double-layer metallic gratings. Optics Express, 28(12), 18212.