Terahertz (THz) waves have great potential applications in communication, imaging, and spectroscopy fields. Effective THz modulators are highly desired to realize those functionalities. Wherein, as a kind of artificial composite material, THz metamaterials can achieve extraordinary responses to the electromagnetic wave through the geometric structure design. Nevertheless, normal metamaterials have no tunability once they have been designed and fabricated. To overcome this issue, active medias have been explored to enable the expected modulation of metamaterials under the external stimuli. Among them, phase transition materials are often used in dynamically tunable THz devices due to their intriguing properties. Particularly, vanadium dioxide (VO2) has attracted attention owing to the reversible physical properties and can exhibit insulator-to-metal transition (IMT) behavior at near room temperature. Here, we explore the strength of the resonance response and the change of spectral lineshape caused by the size variation in the metamaterial unit cell. On this basis, adding VO2 thin film can realize broadband modulation during the IMT process. Furthermore, by incorporating the VO2 patches in the gold microstructure can further achieve the dual modulation of amplitude and frequency simultaneously. The design of VO2 hybrid metamaterial can break the single function limitation of traditional metamaterial modulators, reduce material loss, and open up a new path for the development of multifunctional THz modulators.
Metamaterials are artificial materials with unique electromagnetic properties and could exhibit a strong electromagnetic resonance. Resonant metamaterials are widely used in the field of biosensors because they are sensitive to the changes of surface dielectric environment. In this study, we demonstrated a highly sensitive detection method of 4-Mercaptohydrocinnamic acid using terahertz metamaterials fabricated on silicon substrate with metal arrays of the three bars structure. We measured the transmission spectra of the metamaterial to detect the 4-Mercaptohydrocinnamic acid at three different densities by the terahertz time-domain spectroscopy system. With the density of 4-Mercaptohydrocinnamic acid increasing, resonance dips exhibit the red-shift phenomena and transmission at the resonance frequency simultaneously reduce. The movement of resonance dips and the change of transmission show that the three bars resonators are sensitive to the density of analyte. The finite-difference time-domain simulation shows a good agreement with the experimental data, and the simulation of surface current and electric field distributions at resonance dips can further understand the resonance modes in transmission spectra. Our study provides new prospect into the application of terahertz metamaterials in biosensing.
With strong optical response from subwavelength metamaterial structures in terahertz, plasmon-induced transparency (PIT) has attracted considerable attention in terahertz modulators and biosensing devices. Here, we tune PIT effect by the destructive interference of two bright modes. We design a terahertz metamaterials structure with triple U-shaped resonators (TUR) arrays. In the vertical direction, double U-shaped resonators (DUR) are arranged downward, single-U resonator (SUR) is upward. We change the length of the SUR, the inner arms of the DUR and the horizontal distance of the DUR in order to observe the terahertz transmission spectrum. It is found from the results that with the length of the SUR increases, the resonant frequency has an obvious red-shift, the absorption of the low-frequency resonance increases and the nonresonance absorption peak gradually decreases. As the arms of the DUR increase and the distance between them decreases in horizontal, the frequency of resonance dip has a red-shift, and the transmittance of the non-resonant region increases slightly. To explore the influence of dielectric environment on the resonance characteristics of the terahertz metamaterials, we have further performed the simulations with the applied surface analyte of different refractive index. The results show that with the increase of the refractive index of the surface of the analyte, the resonance frequency has a more significant red-shift. Our obtained results could provide the idea for designing terahertz modulators and sensitive biosensing devices.
Metamaterials has shown outstanding flexibility and functionality in optics and electromagnetics, which attracted plenty of interest. Nested ring resonators have a wide range of applications in terahertz (THz) spectroscopy, sensing and communication. Hence, we design a kind of metamaterials structure which can modulate THz transmission and resonance. It consists of a circular split ring resonator (CSRR) inside a closed square ring resonator. The single CSRR has an inductive-capacitive (LC) resonance and the single closed square ring resonator has a dipole resonance. After nesting two resonators, the resonance mode is changed from single mode to double modes. The results show that the amplitude transmission of non-resonant region is related to the gap opening and the asymmetry of structure. The amplitude transmission of resonance region depends on the conductivity of substrate. With the gap opening of CSRR increases, the amplitude transmission of non-resonant region increase. Meanwhile, the frequency of resonance has an obvious blue-shift. With the bottom edge distance of the two resonators decreases(the asymmetry increases), the amplitude transmission of the non-resonant region increases gradually and the low frequency of resonance has a red-shift and the high frequency of resonance has a blue-shift. To further analyze the influence of conductivity of substrate on amplitude transmission, we change the conductivity of substrate during the simulation. The results demonstrate that with the conductivity of the substrate increases, the resonance absorption peak decreases until disappears, the amplitude transmission of the non-resonant region decreases. Our results may have the potential applications in THz modulator.
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