Tunable terahertz metamaterial with multi-resonance characteristic for refractive index sensing application

A tunable terahertz (THz) metamaterial (TTM) composed of double connected semicircle-shaped rings and two segregated metallic bars is presented. Through the manipulation of the radius of connected semicircle-shaped rings (R), the transverse electric (TE)-polarization resonance is red-shifted from 1.225 to 0.669 THz. The first and second transverse magnetic (TM)-polarization resonances can be systematically red-shifted from 1.225 to 0.669 THz and from 1.787 to 1.162 THz, respectively. By increasing the R value from 15.0 to 27.5 µm, the tuning trend of the free spectral range is linear over the range of 0.50 THz. Moreover, the TTM device shows polarization-dependent characteristics, which can be modulated by manipulating the distance between the semicircular-shaped ring structure and the metallic bar. The electromagnetic responses of the TTM device can be effectively tuned, enabling the possibility to switch between single-resonance in the TE mode, and dual-, triple-, and quad-resonance in the TM mode. The experiment and simulation results are agreed well. The TTM device can be employed as a sensor for evaluating the change of surrounding refractive index. The sensitivities between resonances and refractive indices are linear in the TE and TM modes. Therefore, TTM devices can be used for single-, dual-, triple-, and quad-resonance switching, polarization switching, and high-efficiency environmental sensing applications.