An analogue of electromagnetically induced transparency (EIT) in terahertz range with an all-dielectric metasurface is investigated numerically. The unit cell of the silicon-based metasurface is composed of a ring and a rectangular bar. It is found that due to the interaction between the dark mode and the bright mode supported by the ring and the bar, respectively, EIT-like resonance can be realized in this structure. By choosing proper geometrical parameters, the sharp EIT-like transmission peak can be always observed with transmittance higher than 99% and highest Q-factor about 1890. With this high quality EIT resonance, the figure-of-merit for the refractive index sensing of this metasurface is about 260, which is larger the previous reported metal-based metasurfaces. The transmittance property of the EIT-like resonance can be manipulated by changing the structure parameters. This high quality EIT metasurface has great applications in the design of slow-light low-loss equipment, optical modulations, and optical sensing.
Metamaterials has become a hot research field in recent years due to its unique artificial electromagnetic proper- ties. It has the potential to realize high-performance temperature sensors because its resonance features are highly dependent on the surrounding environment. In this paper, we designed a metamaterial sensor with two-layer grating structure and studied the relationship between its resonance dip and the incident angle of electromagnetic wave. By fixing the incident angle at a proper value, we realized a highly sensitive temperature sensor. The measured temperature sensitivity and limit of detection are about 0.766%/°C and 0.154°C, respectively.
The influence of water vapor absorption is inevitable in optical path from the THz source to detector without nitrogen gas, resulting signal distortion at the tail of THz pulse. In the frequency spectrum, the unwanted water-vapor absorption lines will emerge, making it difficult to identify the possible specific absorption lines of the measured sample near the water-vapor absorption lines. To eliminate water-vapor influences can greatly expend the applications of THz-TDS system in the most common open-air environment. In this paper, we used a Support-Vector-Machine algorithm (SVM) in the recent advanced machine learning technology to recover the actual THz signal by removing the influence of water vapor degradation. The learning and prediction of the water vapor absorption effects is completed by iterative training process of the SVM algorithm. After the SVM model is built, we found that it can effectively eliminate the fluctuations of the THz-TDS signal obtained in the open-air environment, thus the corresponding water-vapor absorption peaks in the frequency spectrum are greatly suppressed. We also compared the signal recovering ability of our SVM algorithm with traditional Back- Propagation(BP) neural network algorithm with the same training data as well as the same training time and found that the SVM algorithm outperforms the traditional BP neural network algorithm. To furtherly verify the generalization ability of our SVM model, the THz signals measured under different humidity are set as the inputs of the SVM model. It turns out that the SVM algorithm can still effectively eliminate the water-vapor absorption effects.
The highly hysteretic insulator-to-metal phase transition (IMT) of vanadium dioxide (VO2) enables an effective path to actively tuning terahertz (THz) wave, which holds great promise for the next generation optical memory devices. In the THz range, existed VO2-based memory device are driven by electric, which suffers some problems, such as slow write time at scale of several seconds, complicated circuit fabrication for electric conduction, and so on. Here, we propose an all optically-controlled THz wave memory device with inexpensive, straightforward and scalable fabrication of VO2. We demonstrate the reconfigurable THz-wave memory device with 1 bit and 2 bits programmed by two continuous lasers and its potential write time of tens of microseconds driven by ultrafast amplifier laser. This work paves the way for robust multifunctionality in optically-controlled terahertz switching, photonic memory, and ultrafast terahertz optics. And combining our memory device with spatial light modulator, its functionality can be further extended into spatial dimension, such as being a programmable and reconfigurable spatial THz wave modulator for the cutting-edge THz ghost imaging.
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