Citrate salts (CSs), as one type of organic salts, have been widely used in the food and pharmaceutical industries. Accurate and quantitative detection of CSs in food and medicine is very important for health and safety. In this study, an asymmetric double-opening ring metamaterial sensor is designed, fabricated, and used to detect citrate salts combined with THz spectroscopy. Factors that influence the sensitivity of the metamaterial sensor including the opening positions and the arrangement of the metal opening ring unit, the refraction index and the thickness of the analyte deposited on the metamaterial sensor were analyzed and discussed from electromagnetic simulations and THz spectroscopy measurements. Based on the high sensitivity of the metamaterial sensor to the refractive index of the analyte, six different citrate salt solutions with low concentrations were well identified. Therefore, THz spectroscopy combined with a metamaterials sensor can provide a new, rapid, and accurate detection of citrate salts.

A simple and efficient THz metamaterial based on an asymmetric double split square array resonator was designed and fabricated. The sensitivity and Q value of the metamaterial sensor were theoretically analyzed to be 278 GHz/RIU and 11, respectively. Three typical biological samples with different concentrations were measured to validate the sensitivity of the THz metamaterial. For the bovine serum albumin (BSA) protein solution, the lowest detectable concentration (LDC) can reach to 1 μM, which is comparable to most recent reports. For the Staphylococcus aureus bacteria, the LDC is 1 × 10³ cells/mL, and for the K-citrate solution, the LDC is 0.1 mM. Our studies indicate that the THz metamaterial may be effectively applied in the low-concentration detection of biological samples.

A terahertz broadband polarization converter based on the metasurface of a double-split ring resonator is proposed. The structural parameters of this device have been optimized in numerical simulations. The results show that in the frequency range from 0.49 to 1.88 THz (bandwidth of 1.39 THz) the proposed device rotates incident linearly polarized waves through 90°. The polarization conversion rate is more than 80%. The coefficient of reflection for cross polarization is greater than 90% with a transmission loss of 1 dB. The surface current distribution of this structure was simulated and analyzed at three frequencies that give high conversion rates of the polarization. The mechanism underlying this high conversion rate is presented, and the dependence of the conversion rate on incident angle and polarization angle is stimulated and analyzed. The results show that the conversion performance of this device is good for incidence angles ranging from 0 to 30° and polarization angles ranging from − 10 to 10°. Compared with previous designs, the polarization converter has a simple structure and a broad operational bandwidth. It has potential applications in the field of terahertz polarization modulation.

Active lenses with focal tunable properties are highly desired in the modern imaging systems from the visible to the microwaves. In this paper, we demonstrate a terahertz (THz) lens with electrically switchable focal length. It is composed of a large-birefringence liquid crystal (LC) layer infiltrating a dielectric metasurface. When the birefringence of LC is tuned with an external bias, the phase shift of a single meta-unit will change. With parameter sweep using the finite-different time-domain (FDTD) simulation method, meta-units with varying geometries are optimized to achieve a focal length switchable metalens. The numerical results show that the focal length can be switched between 8.3 mm and 10.5 mm at bias OFF and ON states, respectively, which is consistent with the design. A feasible fabrication procedure of the lens is further discussed. Such a device can be designed beyond the THz band to the visible or the microwaves, and may be widely applied in integrated imaging systems.

A double-strip array-based metasurface that supports the sharp quasi-bound states in the continuum (quasi-BICs) is demonstrated in terahertz regions. By tuning the structural parameters of metal strips, the conversion of BICs and quasi-BICs is controllable. The simulated results exhibit an achieved maximum Q-factor for quasi-BICs that exceeds 500, corresponding to a bandwidth that is less than 1 GHz. The optical response of quasi-BICs is mainly affected by the properties of substrates. Resonant frequencies decrease linearly with increasing refractive index. The bandwidth of quasi-BICs decreases to 0.9 GHz when n is 2.2. The sharp quasi-BICs are also sensitive to changes in material absorption. Low-loss materials show higher Q-factors. Thus, the selection of a suitable substrate material will be beneficial in achieving resonance with a high Q value. The sensitivity of DSAs for molecules is assessed using a thin membrane layer. The DSAs show high sensitivity, which achieves a frequency shift of 70 GHz when the thickness of the membrane is 10 μm, corresponding to a sensitivity of 87.5 GHz/RIU. This metasurface with sharp quasi-BICs is expected to perform well in THz sensing.

Terahertz (THz) technology has unique applications in, for example, wireless communication, biochemical characterization, and security inspection. However, high-efficiency, low-cost, and actively tunable THz modulators are still scarce. We propose a broadband tunable THz beam deflector based on liquid crystals (LCs). By a periodic gradual distribution of the orientation of the LC in one direction, a frequency-independent geometric phase modulation is obtained. The LC device with this specific orientation distribution was obtained through ultraviolet polarization exposure. We have verified the broadband beam deflection in both the simulation and experiment. The device can achieve a good spin-coupled beam deflection effect in the 0.8–1.2 Thz band, and the average polarization conversion efficiency exceeds 70%. Moreover, because the electro-optical responsivity of LCs is excellent, graphene transparent electrode layers introduced on the upper and lower substrates enable the deflection modulation to be switched and dynamic tuning to be achieved.