The Mode Interaction In Between The Unit Cells Of THz Metamaterials

Recently, with the development of metamaterials in terahertz technology, metamaterials in terahertz technology play a more and more important role. One of the hottest point is to achieve a cost-effective metamaterial with broadband response and low loss. Owing to above reasons, we investigate the mechanism of mode interaction in metamaterials of different unit cell. With the help of experiment and numerical simulation, the electromagnetic response of metamaterials and the origin of mode interaction are revealed. Our results will be significant to improve the performance of THz metamaterials. The highlights of our works are listed as follows:1. A teeter-totter effect of terahertz(THz) resonant modes in C-shaped complementary split-ring resonators(CSRRs) is observed. The intensity of the lower-frequency resonance modes increases monotonically with the CSRR gap width, which is accompanied by a monotonic decrease in the intensity of the higher-frequency resonance modes. The origin of the dual resonant modes is numerically explained by the electromagnetic energy density distribution and surface current analysis. The inductive-capacitive resonance dominates the lower frequency mode, while the dipole oscillation dominates the higher frequency mode. By tuning the gap of the CSRRs, an equilibrant transmittance of above dual resonance modes can be designed. This teeter-totter effect promises a possible application of CSRRs as potential dual-bandpass filters in the THz-region.2. We investigate the terahertz electromagnetic responses of fractal meta-atom induced by different mode coupling mechanisms. Two types of meta-atom based on concentric rectangular square(CRS) resonators are presented: independent CRS(I-CRS) and junctional-CRS(J-CRS). In I-CRS, each resonator works as an independent dipole so as to result in the multiple resonance modes when the fractal level is above 1. In J-CRS, however, the generated layer is rotated by π/2 radius to the adjacent CRS in one meta-atom. The multiple resonance modes are coupled into a single mode resonance. The fractal level increasing induces resonance modes redshift in I-CRS while blueshift in J-CRS. When the fractal level is below 4, the mode Q factor of J-CRS is in between the two modes of I-CRS; when the fractal level is 4 or above, the mode Q factor of J-CRS exceeds the two modes of I-CRS. Furthermore, the modulation depth(MD) decreases in I-CRS while it increases in J-CRS with the increase in fractal levels. The surface currents analysis reveals that the capacitive coupling of modes in I-CRS results in the modes redshift, while the conductive coupling of modes in J-CRS induces the mode blueshift. A high Q mode with large MD can be achieved via conductive coupling between the resonators of different scales in a fractal meta-atom.3. We investigate an evolution of plasmon-induced transparency(PIT)-like behavior induced by resonance detuning in hybrid planar metamaterials at terahertz(THz) region. Each unit cell of MMs contains two types of dipole oscillation resonators: a cut-wire(CW) and a pair of U-shaped resonators in mirror symmetry(SU). The hybridization of above resonators(CW/SU) split the transparent tip into dual side modes in THz transmission spectrum. The surface currents and electric energy distribution reveals that the near-field coupling between cut-wire and U-shape resonators results in inductive-capacitive(LC) resonance, which dominates the low frequency mode; while the high frequency mode attribute to the coupled dipole oscillation. The reduction of the length of cut-wire give arise to a dipole oscillation detuning, which enhance the LC resonance via near-field coupling, while attenuate the constructive inference of triple dipole oscillators. By control the detuning appropriately, a transparent tip can be created in between the dual side modes. However, such a transparent tip is unable to induce slow group delay. Aforementioned PIT-like behavior can support the design of hybrid planar metamaterials in application of dual-band filters or buffer in the THz-region.