Metamaterials (MM) are artificially designed structures in which resonant subwavelength elements act as unit cells. However, metallic resonators in terahertz frequency range often have high losses. To address the issue of losses, enhancing the quality factor of resonances in MMs is an important field of study. Toroidal excitations are a class of electromagnetic excitations that take place when magnetic moments develop an end-to-end formation. The toroidal excitation has the advantage of lower radiative losses. One unique way of decreasing linewidth of resonances is to couple the MM resonance to the first-order lattice mode of the resonance. In our study, we enhance the quality factor (Q) of a toroidal resonance in a THz MM by coupling it to the lattice mode of the MM. Our proposed MM displays a broad toroidal resonance at 0.5 THz. On coupling the toroidal mode to the lattice mode by setting a P=170 μm, a 64% increase in the Q of the resonance is observed The Q is calculated to be 47. The lattice coupling is confirmed by varying the periodicity at 0.5 THz and observing modulation of resonances. Such an increase in the Q could be useful in the design of highly sensitive THz sensors for chemical and biological sensing.
The metamaterial is an arrangement of artificial structural elements designed to achieve advantageous and unusual electromagnetic properties. In the unit cell level, metamaterials are composed of an array of small structured elements called split ring resonators (SRRs). Recently, a lot of emphases has been given to the realization of terahertz metamaterials owing to its significance in the construction of terahertz photonic components. In this context, near-field coupling in terahertz metamaterials is extremely crucial. The short-range coupling in metamaterials occurs via the electric and magnetic fields due to the close proximity of the neighboring resonators. The electric field mainly couples through the gaps of SRRs, while the magnetic field couples through the circumference. In this work, we experimentally investigate near-field gap to gap capacitive coupling between a pair of single split gap ring resonators (SRRs) in a terahertz metamaterial. This has been achieved by manipulating the near field electric interactions via changing one resonator split gap with respect to the other resonator split gap for several inter resonator separations. Introducing asymmetry by changing the split gap in one resonator with respect to the other resonator, results in the split in the fundamental resonance mode when operated in the strong near-field coupled regime. The split occurs because of the strong near field capacitive/electric interactions between the resonators. We have further calculated Q factor for the lower and higher resonance modes for different inter resonator separations. The modulation of resonances in capacitive coupled planar terahertz metamaterial systems studied through this work has great potential in manipulating and controlling electromagnetic waves which can ultimately result in novel applications for terahertz frequency domain.
We have numerically demonstrated refractive index sensing using terahertz metamaterials comprised of single split gap resonators. Sensing capabilities of odd and even order resonance modes are precisely investigated. In this scheme the top surface of metamaterials array is covered with a thin layer of polyimide whose refractive could be changed manually. The sensitivity and corresponding figure of merit (FoM) of several lowest order modes are examined with respect to the different thicknesses of the coated polyimide film. We have investigated the electric field distributions at different resonances for the MMs. Although we have mainly focused on refractive index sensing but this study could be extremely useful for the development of metamaterials based sensing devices, bio-sensors etc.
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