In the paper, we discuss the field enhancement effect in terahertz nano metasurfaces. The unit cell of the metasurfaces consists of a metallic split ring resonator (SRR) connecting with a wire. When the gap of SRR varies from micron-scale to nano-scale, the field enhancement factor in the gap achieves an order-of-magnitude increase in nano metasurfaces compared to that of micron metasurfaces. We then apply the nano metasurfaces to electric field sensing by assembling a layer of graphene film. In the simulation, the conductivity of graphene is tuned by varying the scattering time (relaxation time) corresponding to the varying external voltage. Compared to the structure without graphene film, the transmission of the graphene-based metasurfaces will be modulated by graphene conductivity. And the conduction effect of the graphene-based metasurfaces with nanogap under the same voltage is much better than that of the structure with micron gap, due to the extreme high field enhancement of the former. Based on this study, we can further optimize the nano metasurfaces for high sensitivity sensing, which can be applied in biological/chemical sensors or nonlinear devices.
We propose an ultra-thin Huygens lens with high focusing efficiency and robustness that is composed of a unit cell with a pair of C-shaped split-ring resonators (CSRRs). Compared with the single-layer CSRR lens, the focusing efficiency of the Huygens metasurface lens improves 90% at the best focal lengths and three times at the central frequency. Our device has robustness against geometrical parameters, such as the difference of the opening angle, the mismatch of the opening angle, and the mismatch of the radius, which reduces the manufacturing difficulty. The Huygens lens has unprecedented potential for real-time, quick, electromagnetic wave processing such as holography, beam steering/splitter, imaging, and launcher control.
In this work, a five-band metamaterial absorber (MMA) for temperature sensing application in terahertz region is analyzed. The MMA is composed of three layers. The bottom layer is the metallic film, the middle dielectric layer is the indium antimonide (InSb) and the top layer is the metallic pattern, in which five resonance peaks are generated. With utilizing the dielectric thermo sensitive property of InSb, the resonant absorption is tunable by varying temperature. The electric current on the MMA is investigated to better understand the physical mechanism of the resonances, revealing the resonances attributed to the high-order magnetic resonances. The multi-band absorber is insensitive to the polarization angle, and be with ultrathin thickness of structure. This design of the MMA provides a new approach for electromagnetic stealth, sensing and imaging.
We propose a terahertz metalens based on Huygens’ metasurface which can realize the focused field enhancement compared with the single-layered metasurface. The metalens consists of two-layered well-arranged metallic C-shaped split-ring resonators array separated with the dielectric layer. After investigating the relationship between the transmission characteristics and the geometrical parameters of the metasurface, we demonstrate that the Huygens’ metasurface can efficiently enhance the transmission amplitude while inducing the phase gradient within the supercell. Due to higher transmission amplitude of the Huygens’ metasurface, the electric field of the focus is enhanced 117% at 0.8 THz. Our results may offer a new avenue to design efficient metalens, which is promising in developing metasurface-based integrated devices for the terahertz imaging.
We present thermal control of electromagnetically induced transparency (EIT) by actively modulating the dark mode in terahertz (THz) metamaterials, including a cut wire and a split-ring resonator (SRR). By integrating indium antimonide (InSb) into the SRR and increasing the temperature, the active modulation of EIT is realized. The coupling mechanism is numerically analyzed through the coupled oscillator model, and the result of fitting the EIT intensity agrees well with the simulation results when the temperature changes from 200 to 240 K. By analyzing the electric field distribution, the physical mechanism is the change in the damping rate of the dark mode resonator due to the increase in InSb temperature. Our work has practical significance in designing tunable THz functional devices.
We design all-dielectric band stop filters with silicon subwavelength rod and block arrays at terahertz frequencies. Supporting magnetic dipole resonances originated from the Mia resonance, the all-dielectric filters can modulate the working band by simply varying the structural geometry, while eliminating the ohmic loss induced by the traditional metallic metamaterials and uninvolved with the complicated mechanism. The nature of the resonance in the silicon arrays is clarified, which is attributed to the destructive interference between the directly transmitted waves and the waves emitted from the magnetic dipole resonances, and the resonance frequency is determined by the dielectric structure. By particularly designing the geometrical parameters, the profile of the transmission spectrum can be tailored, and the step-like band edge can be obtained. The all-dielectric filters can realize 93% modulation of the transmission within 0.04 THz, and maintain the bandwidth of 0.05 THz. This work provides a method to develop THz functional devices, such as filters, switches and sensors.
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