Yuye Wang

Low-frequency lattice vibrations are sensitive to intermolecular interactions and crystal packing, but reliable interpretation requires more than matching Raman and terahertz (THz) peak positions. Here, low-wavenumber Raman spectroscopy, THz absorption spectroscopy, and periodic density functional theory calculations were combined to investigate L-alanine, L-glutamic acid, and L-glutamine, which share a common α-amino-acid backbone but differ in side-chain chemistry and crystal packing. The three crystals exhibited distinct low-frequency spectral profiles. Periodic PBE-D3 calculations indicated that the lower-frequency modes are dominated by collective rigid-molecule librations, whereas higher-frequency modes increasingly involve coupling between lattice motion and internal or side-chain coordinates. A selection-rule-aware comparison based on frequency proximity, calculated Raman activities and infrared intensities, and atomic displacement patterns distinguished Raman-selective, IR-selective, cross-technique-compatible, and near-coincident but vibrationally non-equivalent features. These findings demonstrate that frequency coincidence alone is insufficient for establishing Raman–THz mode correspondence. The study provides a mode-resolved approach for interpreting complementary low-frequency Raman and THz spectra of molecular crystals.

This paper demonstrates the designs and implements of an FPGA-based LiDAR target simulator (LTS) for pulsed-coded LiDAR testing. The simulator employs a hybrid delay architecture integrating multi-phase clock sampling and FIFO buffering. By combining coarse and fine timing modules, the system achieves a temporal resolution of 0.5 ns, corresponding to a spatial resolution of 7.5 cm. It also supports an ultra-long maximum delay time up to 83.886 ms (12582 km range), covering simulation scenarios involving long-range targets. In addition, the system supports pulse width modulation, allowing it to simulate the temporal broadening of laser pulses caused by environmental factors such as atmospheric turbulence and target scattering, with resolution of 0.5 ns. Experimental results demonstrate that the proposed approach effectively overcomes the limitations of traditional methods in terms of precision, range, stability, and compatibility with various coding schemes. This work provides a reliable technical foundation for the development and evaluation of pulse-coded laser ranging systems.

高重频宽调谐太赫兹辐射源在物质光谱检测、快速成像等领域有着迫切需求．本文理论分析了太赫兹参量过程净增益的影响因素，明确了泵浦光能量密度对太赫兹波输出功率与调谐范围的影响．实验中，基于高重频电光调Q 1064 nm激光器泵浦氧化镁掺杂的铌酸锂晶体(MgO:LiNbO₃,MgO:LN)，实现了重复频率4kHz、调谐范围为1.05-4.96THz的太赫兹波输出，实验与理论计算能够很好地吻合。且当泵浦功率为28W时，在1.7THz处获得的太赫兹波最大平均功率为120µW，对应的能量转换效率为4.3×10^-6，峰值功率约为10.3W．

The diagnosis and treatment of gliomas depend greatly on the precise delineation of tumor boundaries and the rapid extraction of molecular pathological features. The development of high-resolution and high-sensitivity terahertz (THz) attenuated total reflection (ATR) imaging technology can greatly expand its application in the clinical medical field. In this study, we demonstrated a THz ATR imaging system based on a solid immersion lens (SIL). The resolution improvement mechanism by a solid immersion lens in the THz ATR imaging system has been studied theoretically and experimentally. According to the theoretical analysis results, the optimal parameters of the system have been selected. The spatial resolution of the THz imaging system was up to 120μm × 140μm. On this basis, the THz reflectivity of fresh normal brain tissue and glioma tissue in a mouse model was studied. Compared with the visible, MR, and H&E-stained images, the accurate identification of the glioma region boundary and microscopic structures in brain tissues was realized. The glioma regions in H&E-stained and THz ATR images were segmented automatically based on the Chan-Vese active contour model, where the performance evaluation rates were all above 95%. These promising results suggest that THz ATR imaging based on SIL could be used as a tool for label-free, high-sensitivity, and real-time imaging of brain gliomas.

A highly sensitive terahertz (THz) wave parametric upconversion detector was demonstrated based on KTiOPO ₄ (KTP) crystal pumped by a nanosecond 1064-nm laser. The effects of pump energy on the detection sensitivity were studied in this experiment, where the upconversion signal and spontaneous noise were both amplified linearly with the increase of pump energy. The minimum detectable THz wave energy of 57.6 aJ was realized at 5.74 THz under higher pump energy, which gives the dynamic range of 104 dB and noise equivalent power (NEP) of 147.8 fW/Hz ^1/2 . It was three orders of magnitude higher than that of bolometer. Moreover, a rapid-response detection in the THz frequency range of 5.24-6.14 THz was achieved. This THz parametric upconversion detector based on KTP crystal can provide a technical solution to achieve high-sensitive THz detection in a wide THz region.

Terahertz (THz) waves have been widely investigated recently due to their ability to reflect the fingerprint characteristics of samples. As a promising method, THz technology has aroused great interest in various applications, especially biological imaging, environmental monitoring, non-destructive evaluation, spectroscopy and molecular analysis. In order to reveal the intramolecular vibration/rotation information of various compounds, the linewidths of their absorption lines are usually in a range of GHz or even MHz, and THz waves with wide tunability, narrow linewidth, high frequency accuracy, and high power stability are required. Currently, the linewidth with GHz level and low SNR at higher frequency still limit its further applications in reveal intramolecular information. In this work, the thermal distribution characteristics of DAST crystals based on diamond substrates under continuous laser pumping conditions are theoretically studied by COMSOL Multiphysics, and the effectiveness of diamond substrates in dissipating heat from DAST crystals is experimentally verified. Then, a narrow-linewidth and tunable organic-crystal continuous-wave terahertz source is demonstrated. Two narrow-linewidth continuous-wave (CW) fiber lasers are used as the pump sources for generating difference frequency. The terahertz wave is continuously tunable in a range of 1.1–3 THz. The maximum output power of 3.39 nW is obtained at 2.493 THz. The power fluctuation in 30 min is measured to be 2.19%. In addition, the generated THz wave has a high polarization extinction ratio of 9.44 dB. Using this CW-THz source for high-precision spectral detection of air with different humidity, the results correspond well with the gas absorption spectral lines in the Hitran database, proving that the CW-THz source has narrow linewidth, high frequency accuracy and stability. Therefore, it can promote the practical application of tunable CW-THz source, thus having good potential in THz high-precision spectroscopic detection and multispectral imaging.

A novel fiber sensor for the refractive index sensing of seawater based on a Mach–Zehnder interferometer has been demonstrated. The sensor consisted of a single-mode fiber (SMF)–no-core fiber (NCF)–single-mode fiber structure (shortened to an SNS structure) with a large lateral offset spliced between the two sections of a multimode fiber (MMF). Optimization studies of the multimode fiber length, offset SNS length, and vertical axial offset distance were performed to improve the coupling efficiency of interference light and achieve the best extinction ratio. In the experiment, a large lateral offset sensor was prepared to detect the refractive index of various ratios of saltwater, which were used to simulate seawater environments. The sensor’s sensitivity was up to −13,703.63 nm/RIU and −13,160 nm/RIU in the refractive index range of 1.3370 to 1.3410 based on the shift of the interference spectrum. Moreover, the sensor showed a good linear response and high stability, with an RSD of only 0.0089% for the trough of the interference in air over 1 h.

The fast label-free detection of the extent and degree of cerebral ischemia has been the difficulty and hotspot for precise and accurate neurosurgery. We experimentally demonstrated that the fresh cerebral tissues at different ischemic stages within 24 hours can be well distinguished from the normal tissues using terahertz (THz) attenuated total reflection (ATR) imaging system. It was indicated that the total reflectivity of THz wave for ischemic cerebral tissues was lower than that for normal tissues. Especially, compared to the images stained with 2,3,5-triphenyl tetrazolium chloride (TTC), the ischemic tissues can be detected using THz wave with high sensitivity as early as the ischemic time of 2.5 hours, where THz images showed the ischemic areas became larger and diffused as the ischemic time increasing. Furthermore, the THz spectroscopy of cerebral ischemic tissues at different ischemic times was obtained in the range of 0.5-2.0 THz. The absorption coefficient of ischemic tissue increased with the increase of ischemic time, whereas the refractive index decreased with prolonging the ischemic time. Additionally, it was found from hematoxylin and eosin (H&E) staining microscopic images that, with the ischemic time increasing, the cell size and cell density of the ischemic tissues decreased, whereas the intercellular substance of the ischemic tissues increased. The result showed that THz recognition mechanism of the ischemia is mainly based on the increase of intercellular substance, especially water content, which has a stronger impact on absorption of THz wave than that of cell density. Thus, THz imaging has great potential for recognition of cerebral ischemia and it may become a new method for intraoperative real-time guidance, recognition in situ, and precise excision.