An ultraviolet organic–inorganic nonlinear optical material benefited from organic π-conjugated [C ₃ N ₆ ] groups and inorganic [ZnNCl ₃ ] groups, revealing excellent optical performance and moderate thermal stability.

Recently researchers have found that the non-resonant second-harmonic generation (SHG) intensity of the GaSe monolayer (ML) is the strongest among all two-dimensional (2D) atomic layered crystals. Here we perform a systematic first-principles study of the SHG coefficient of GaX (X = S, Se, Te) monolayers (MLs) and few-layers. We find that the non-resonant SHG intensity of the GaS ML can be comparable with that of the GaSe ML, while the non-resonant SHG intensity of the GaTe ML is much stronger than that of the GaSe ML. Furthermore, the magnitudes of SHG coefficients of the few-layers exfoliated from bulk ε-GaSe and newly constructed N₁-GaSe crystals are very close to that of the GaSe ML, showing no dependence on the layer number. The magnitude of the SHG coefficient of the trilayer exfoliated from the bulk β-GaSe crystals is around 1/3 of that of the GaSe ML, decreasing rapidly with the layer number. This study indicates that a strong SHG response can be obtained in a wide range of monolayers and few-layers. Moreover, we point out that one can identify the layer number and the stacking sequence of 2D nanosheets by measuring their elastic constants and SHG coefficients.

Water and its distribution and transport dynamics in green plant leaves are vital to the growth of plants. Owing to the high sensitivity of terahertz (THz) wave to water, THz spectroscopy has great advantages in analyzing the water status of plant leaves. This paper presents a new approach to estimate the water status of plant leaves by the THz time-domain spectroscopy (THz-TDS) technique. Spatial distribution of THz transmission amplitudes located in vein xylem and mesophyll of all three kinds of leaves including wintersweet, ginkgo and bamboo is detected by THz-TDS measurements. Based on the transmission amplitude, the reconstructed THz images show that the water loss in the basal leaf region is more than that in the distal region during the natural drying process for all three plants. A good agreement is reached between the THz imaging method and the direct water weight measurement. To illustrate the accuracy and the sensitivity of the THz technique, the temporal and spatial variations of the water content in the damaged ginkgo leaf with a wound by cutting are also investigated for comparison. The water flow from the basal region to the distal region of the leaf is inferred according to the variation of THz transmission amplitude with the leaf region in different dehydration periods, which is consistent with the string-of-lakes model prediction. This work shows the feasibility of using THz technology to monitor the temporal and spatial variability of the water status in plant leaves.

Terahertz (THz) time-domain spectroscopy (TDS) imaging is considered a nondestructive evaluation method for composite materials used for examining various defects of carbon fiber reinforced polymer (CFRP) composites and fire-retardant coatings in the reflective imaging modality. We demonstrate that hidden defects simulated by Teflon artificial inserts are imaged clearly in the perpendicular polarization mode. The THz TDS technique is also used to measure the thickness of thin fire-retardant coatings on CFRP composites with a typical accuracy of about 10 micrometers. In addition, coating debonding is successfully imaged based on the time-delay difference of the time-domain waveforms between closely adhered and debonded sample locations.

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.