A five-band absorber based on graphene metamaterial for terahertz ultrasensing

We design and propose a five-band absorber based on graphene metamaterial for the terahertz (THz) sensing field. The localized surface plasmon resonances (LSPR) of patterned graphene are excited, contributing to five tunable ultra-narrow absorption peaks, which are specified by the electric field distributions. Moreover, the absorber is insensitive to different polarization modes and incident angles. When increasing the Fermi level of the patterned graphene, which is composed of a round ring and a square ring connected by four thin wires, the resonant frequencies exhibit distinct blue shifts. For refractive index sensing, due to the addition of a continuous dielectric groove, the theoretical results show that the maximum averaged normalized sensitivity, Q factor, and FOM can reach 0.647 RIU-1 (refractive index unit, RIU), 355.94, and 215.25 RIU-1, indicating that the sensing performances are further enhanced compared with previous works. As a result, the proposed structure may provide a new method to realize ultrasensing in the THz region.

Rabi-like splitting and refractive index sensing with hybrid Tamm plasmon-cavity modes

Rabi-like splitting and self-referenced refractive index sensing in hybrid plasmonic-1D photonic crystal structures have been theoretically demonstrated. The coupling between Tamm plasmon and cavity photon modes are tuned by incorporating a low refractive index spacer layer adjacent to the metallic layer to form their hybrid modes. Anticrossing of the modes observed at different values of spacer layer thickness validates the strong coupling between the two modes and causes Rabi-like splitting with different splitting energy. The modes coupling has been supported by coupled mode theory. Rabi-like splitting energy decreases with increasing number of periods (N) and refractive index contrast (η) of two dielectric materials used to make the 1D photonic crystals, and the observed variation is explained by an analytical model. Angular and polarization dependency of the hybrid modes shows that the polarization splitting of the lower hybrid mode is much stronger than that of the upper hybrid mode. On further investigation, it is seen that one of the hybrid modes remains unchanged while other mode undergoes significant change with varying the cavity medium. This nature of the hybrid modes has been utilized for designing self-referenced refractive index sensors for sensing different analytes. For η=1.333 and N=10 in a hybrid structure, the sensitivity increases from 51 nm/RIU to 201 nm/RIU with increasing cavity thickness from 170 nm to 892 nm. For the fixed cavity thickness of 892 nm, the sensitivity increases from 201 nm/RIU to 259 nm/RIU by increasing η from 1.333 to 1.605. The sensing parameters such as detection accuracy, quality factor, and figure of merit for two different hybrid structures ([η=1.333, N=10] and [η=1.605, N=6]) have been evaluated and compared. The value of resonant reflectivity of one of the hybrid modes changes considerably with varying analyte medium which can be used for refractive index sensing.