Terahertz metamaterial sensors have received extensive attention in biosensing applications. However, sensitivity toward terahertz frequencies emitted by liquid samples remains challenging because of the strong absorption of terahertz waves by water. Here, we present a highly sensitive terahertz sensor based on a three-dimensional double I-type metamaterial integrated microfluidic channel. The designed sensor produces an inductive-capacitive (LC) resonance with a high quality factor of approximately 72, while demonstrating a maximum sensitivity of 832 GHz/RIU. Furthermore, we analyzed the relationship between the resonance frequency and ethanol concentration. These findings would promote the application of terahertz technology in label-free and rapid biomedical sensing as well as substance detection.

As a harmless detection method, terahertz has become a new trend in security detection. However, there are inherent problems such as the low quality of the images collected by terahertz equipment and the insufficient detection accuracy of dangerous goods. This work advances BiFPN at the neck of YOLOv5 of the deep learning model as a mechanism to improve low resolution. We also perform transfer learning, thereby fine-tuning the pre-training weight of the backbone for migration learning in our model. Results from experimental analysis reveal that mAP@0.5 and mAP@0.5:0.95 values witness a percentage increase of 0.2% and 1.7%, respectively, attesting to the superiority of the proposed model to YOLOv5, which is the state-of-the-art model in object detection.

In traditional laser-induced breakdown spectroscopy (LIBS) applications, the line intensity and analysis capability are susceptible to plasma shielding. To investigate the shielding effects on the characteristics of Cu plasma in air, a ~120-picosecond laser with a wavelength of 1064 nm was employed to produce plasma. The plasma temperature and electron density were calculated under the condition of local thermal equilibrium (LTE) and optically thin, while the relationships between the line intensity, plasma temperature and electron density were analyzed. Moreover, the LTE condition was validated by the McWhirter relation, plasma relaxation time and diffusion length, and the optically thin condition was observed through the variation in line intensity. The results indicated that when the focal point was below the target surface, the plasma shielding was the weakest, and the highest line intensity could be obtained. In addition, there was a positive correlation between the increased plasma temperature and the degree of shielding effect. When the focal point was above the target surface, the high-irradiance pulse directly broke down the free air and produced a shock wave. Under the high pressure of the over-heated shock wave, the line intensity, plasma temperature and electron density increased again. This study provides an important insight into the experiments and applications of picosecond LIBS.

The terahertz detection sensitivity interaction with other substances has received extensive attention and remains challenging due to the loss of terahertz waves. Herein, a terahertz, multiband sensor with high sensitivity based on the local enhancement effect of an electric field at terahertz frequency is proposed. The designed terahertz sensor is based on a common metamaterial sandwich absorber of electromagnetic energy that only employs an asymmetric metallic strip, which produces three resonance peaks with an absorption coefficient of greater than 95% in the 0.2-1.5 THz range and a maximum -factor of 46 at 1.397. Based on the principle of impedance matching, the high absorption of electromagnetic wave energy is reasonably analyzed. Furthermore, the influence of the top metal structure on the absorptivity and resonance frequency is analyzed by simulation optimization, and the optimum structural parameters are obtained. In addition, the influence of the refractive index (RI) change of the sample and dielectric layer on the resonance characteristics of the sensor is analyzed. When the thickness of the surface sample of the terahertz sensor metamaterial-based absorber is changed so that the RI changes in 1-2, the RI sensitivity increases up to 208 GHz/RIU, which can be used as an RI sensor. Furthermore, the influence of the sensor sensitivity and reflection coefficient on the thickness parameters of the sample covered by the sensor surface is analyzed, and the optimal sample thickness is 6 μm. The influence of the incident angle of the THz wave on the detection performance in the TM polarization mode is analyzed, and it is determined that the change in the incident angle in the range of 0-40 degrees has minimal influence on the resonance peak and absorption rate of multiband detection. Therefore, we suggest that our proposed sensor has considerable potential in biomedicine and trace detection applications.

In this study, we have theoretically designed a micro-structured photoconductive antenna based on the combination of split-ring resonator (SRR) and dipole photoconductive antenna. Further, we have used numerical simulations to prove that the proposed antenna can enhance the terahertz radiation characteristics. According to the equivalent circuit model analysis, the terahertz radiation power and the resonance frequency of this antenna were found to be closely related with the structural parameters of the SRR. A numerical model was established to simulate the effect of the structural parameters of SRR on the terahertz radiation characteristics of the proposed antenna. The results indicated that an increase in the width and length of SRR caused an increase in the peak power intensity and a decrease in the resonance frequency. Moreover, as the gap width of SRR was increased, the peak power intensity was decreased and the resonance frequency was increased. Furthermore, the terahertz radiation power increased with increase in the gap width of antennas. Overall, we show that high-power terahertz radiation sources with specific frequency can be obtained by optimizing the structural parameters of SRR, and such sources are potentially useful in a variety of fields such as biomedicine, material analysis, and detection systems.

The dynamic process of polymorphic transformations provides deep insights into the understanding of both the performance of explosives and its packing sensitivity.