Objective Terahertz single-pixel imaging employs structured measurement matrices for compressive sampling, converting images into intensity signals. Consequently, the design of the measurement matrix critically impacts imaging efficiency and reconstruction quality. However, achieving high-quality reconstruction under low sampling rates (3 %-30 % ) remains challenging. Traditional Hadamard matrix ordering strategies (e.g., natural order, Walsh order, cake-cutting order) prioritize mathematical properties over image content, leading to impaired detail preservation and inefficient feature capture during reconstruction. This study proposes a multi-feature co-driven Hadamard matrix ordering optimization method, aiming to significantly enhance imaging quality and detail recovery capability, particularly at low sampling rates (5 %-10 % ). This advancement provides crucial technical support for the practical implementation of low-cost, high-efficiency terahertz imaging systems. Methods To address the limitations of conventional image quality assessment methods-specifically their global statistical bias and uniform weighting-we introduce a dynamic normalized mean squared error (EDNMSE) metric to establish a precise matching model between projection patterns and image features. Using the MNIST dataset, projection pattern sequence optimization is performed. The Laplacian operator is first applied to preprocess both the images and projection patterns for feature extraction. The proposed EDNMSE dynamically maps local feature gradients to error weights, improving sensitivity to errors in high-variation regions such as edges and textures. Optimization is guided by three distinctive features: local contrast (Eq. (7)), representing intensity variation within a sliding window; edge/texture intensity (Eq. (8)), derived from squared gradient magnitudes; and deviation from global image brightness (Eq. (9)). Lower EDNMSE values indicate reduced error and higher similarity. After optimization, the frequency and weights of projection patterns are statistically analyzed to derive an optimized ordering through descending arrangement. Reconstruction is conducted using the TVAL3 algorithm (Eq. (3)), and quality is evaluated using the peak signal-to-noise ratio (PPSNR, Eq. (11)) and structural similarity index (SSSIM, Eq. (12)). Results and Discussions Visual sequence analysis reveals that the optimized projection patterns exhibit faster pixel transitions and denser horizontal and vertical stripes compared to the natural order (low-frequency bias), Walsh order (vertical blurring), and cake-cutting order (centralized sampling), thereby enhancing spatial feature coverage (Fig. 4). In reconstructed images, the optimized ordering effectively eliminates ghosting artifacts present in the natural order and reduces the vertical smearing characteristic of Walsh ordering, while outperforming the cake-cutting method in preserving sharp corners and intersections at sampling rates below 10 %-20 % (Figs. 5, 6, 9). Quantitatively, the digit 9 achieved PPSNR/SSSIM values of 19.14 dB/ 0.7367, 19.50 dB/ 0.7707, and 23.68 dB /0.9174 at 3 %, 5 %, and 10 % sampling rates, respectively, using the optimized ordering (Table 1). For a triangle, SSSIM and PPSNR reached 0.9689 and 25.29 dB at 10 % sampling, surpassing cake-cutting (0.8794/19.44 dB) and Walsh (0.5100/12.01 dB) methods (Table 2). It is noteworthy that the special phenomenon where both the CC order and the optimized ordering achieve an SSSIM value of 1 for the triangular image at a 20 % sampling rate stems from the high compatibility between the triangle's low-complexity features and the centralized sampling pattern. Nevertheless, the optimized approach still achieves superior performance with a significantly higher PPSNR of 69.85 dB (Table 2). This demonstrates that its dynamic gradient screening mechanism enhances the resolution and matching precision for high-frequency details such as vertex sharpness. Terahertz single-pixel experiments with letters H, J, and U confirmed these findings: the optimized ordering consistently achieved the highest PPSNR and SSSIM across all sampling rates (Fig. 8). For instance, reconstruction of the letter U at 10 % sampling reached 31.55 dB and 0.929, reflecting 11.7 percentage points and 6.3 percentage points improvements over the cake-cutting method, respectively (Table 3, Fig. 9). The steeper PPSNR-sampling rate curve (Fig. 8) demonstrates prioritized capture of critical features. Even at similar metric levels, optimized reconstructions exhibited superior edge continuity and structural integrity. Particularly noteworthy is that although low-similarity features undergo delayed processing in the low-sampling phase, they still achieve effective reconstruction at high sampling rates. This fully validates the superior performance of our optimization strategy in both feature-information matching and resource utilization efficiency. Conclusions This study presents an image feature-driven Hadamard ordering optimization method that dynamically links projection patterns to local contrast, edge strength, and global brightness deviation through the EDNMSE metric. The optimized matrices exhibit rapid black-white transitions and increased horizontal/vertical stripe frequencies, enabling multi-directional feature sampling and improved resource utilization at low sampling rates. Experimental and simulation results demonstrate consistent superiority over traditional orderings across 3 %-30 % sampling: effective reconstruction at 5 % sampling, enhanced detail clarity and natural appearance at 10 % sampling, and significant improvements in PPSNR and SSSIM. These findings validate the robustness and practicality of the proposed approach and establish a foundation for extending lightweight deep-learning frameworks to further enhance generalization in terahertz single-pixel imaging.

Objective Vector vortex beams, characterized by helical phase fronts, complex spatial structures, and anisotropic polarization distributions, have broad applications in super-resolution imaging, long-distance transmission in atmospheric or underwater environments, and laser micro/nano-processing. Conventional vector vortex polarization converters are predominantly based on natural birefringent crystals. However, their bulky size, challenging fabrication and integration, and severe chromatic dispersion limit their applicability in integrated optical systems. To address these limitations, this study proposes a design method for an all-dielectric transmissive optical metasurface-based vector vortex polarization converter, leveraging the birefringent effects of locally engineered generalized waveplate (GWP) and quarter-wave plate (QWP). Methods The proposed approach employs meta-atom structures with waveplate-like functionalities to construct vector vortex beams via geometric phase modulation. The meta-atom unit consists of a silicon dioxide (SiO2) substrate and an amorphous silicon (alpha-Si) nanostructure. By optimizing the dimensions of the meta-atoms, GWP-and QWP-type units are designed to achieve desired polarization conversion. The metasurface is fabricated by arranging these meta-atoms on the substrate, and its polarization conversion properties are systematically investigated. The operational bandwidth and robustness of the device are further validated by varying the incident light source and introducing intentional structural deviations in the meta-atom dimensions. Results and Discussions Under left circularly polarized (LCP) illumination at lambda =632.8 nm, optimized GWP and QWP meta-atoms are designed (Fig. 4). Based on geometric phase principles, polarization converters for generating radial polarized vortex (RPV) and LCP beams are constructed (Fig. 7). Experimental results demonstrate that the generated RPV and LCP beams exhibit doughnut-shaped intensity profiles, uniform polarization states, and helical phase distributions ranging from-180 degrees to 180 degrees, consistent with the properties of vector vortex beams (Fig. 8). The polarization conversion efficiencies for RPV and azimuthally polarized vortex (APV) beams reach 94.13 degrees o and 91.41 degrees o for GWP-type devices, and 80.18 degrees o and 82.78 degrees o for QWP-type devices, respectively. Broadband characterization is conducted at wavelengths A1=600 nm, A2=610 nm, A3=620 nm, A4=640 nm, and A5=650 nm. The GWP-type device exhibits polarization angles with less than 17 degrees deviation from theoretical predictions at 632.8 nm and 640 nm, while demonstrating gradient variations at 620 nm. The QWP-type device maintains polarization angles within 17 degrees deviation across 620-650 nm, with consistent gradient behavior (Fig. 10). These results indicate operational bandwidths of 610-640 nm for GWP-type devices and 620-650 nm for QWP-type devices.Robustness analysis is performed by introducing +/- 5 nm deviations in the length (L), width (W), and height (H) of the meta-atoms (Fig. 11). The device maintains stable beam intensity, polarization, and phase distributions, confirming its tolerance to fabrication tolerances within +/- 5 nm. Conclusions To overcome the limitations of conventional bulky and complex vector vortex polarization converters, this study presents an all-dielectric metasurface-based design for transmissive vector vortex beam generation. Jones matrix analysis confirms the theoretical polarization conversion efficiencies of 99 degrees 0 and 98 degrees 0 for GWP and QWP-type devices, respectively. Finite-difference time-domain (FDTD) simulations demonstrate experimentally achievable efficiencies of 94.13 degrees 0 (GWP-type, operational bandwidth: 610- 640 nm) and 80.18 degrees 0 (QWP-type, operational bandwidth: 620-650 nm), with fabrication tolerances within +/- 5 nm. Although the nano-scale meta-atoms require high precision, their simplified structural constraints and strong robustness significantly reduce practical fabrication challenges. This metasurface-based approach offers a compact, high-performance solution for vector vortex beam generation, with promising applications in optical microscopy and biomedical imaging.

To address the high time cost and low efficiency of traditional terahertz (THz) metamaterial absorber (MMA) design, an adaptive method based on multi-objective particle swarm optimization (PSO) was proposed. First, we presented a symmetric absorber of a four-split-ring resonator with a cross-shaped top layer. The structural parameters were optimized by the PSO and multi-objective PSO (MOPSO) algorithms. The simulation results indicated that the MOPSO rapidly obtained geometric parameters that balance the high absorptivity and high quality factors. This method quickly identified a symmetric structure with absorptivity greater than 99% and quality factor of 377.45 at 1.584 THz. Additionally, the asymmetric structure achieved near-perfect absorption at 1.585 THz with a Q value of 281.32. By studying the electric field distribution and the surface current distribution, the physical mechanism of the absorber designed has been explained. This adaptive assisted method based on MOPSO can efficiently design MMAs and offer an artificial intelligence strategy for THz functional device design.

This paper addresses the issue of reduced measurement stability in low-coherence measurement techniques when applied to glass-based dispersive media. A high-precision, low-coherence interferometric measurement method is proposed based on synchrosqueezed Wavelet Transform (SSWT). First, the Schott dispersion formula is used to analyze the impact of glass-based dispersive media on the phase of the interference spectrum. Wavelet transform is then employed to extract the chirp information of the interference spectrum, enabling the separation of optical path lengths for different wavenumbers. Building upon this, SSWT is introduced to enhance time-frequency resolution further, improving the measurement performance of the system. An experimental setup is constructed to validate the effectiveness of the proposed method. Compared to traditional methods, SSWT optimizes the distribution of wavelet coefficients, concentrating signal energy and significantly improving instantaneous frequency capture accuracy. Experimental results show that, in addition to effectively capturing chirp characteristics and reducing phase noise, SSWT achieves a 3-fold and 7-fold improvement in peak full-width at half-maximum compared to conventional Fourier Transform (FT) and Continuous Wavelet Transform (CWT), respectively, and a 9.1-fold and 17.1-fold improvement in standard deviation over 20 measurements, demonstrating superior noise resistance and measurement precision. In conclusion, the SSWT-based white-light interferometric measurement method provides a high-precision, reliable solution for dimensional measurements in industrial applications.

Objective Bifocal devices can generate two focal points in the vertical or transverse direction, have the advantages of improving the depth and resolution of imaging as well as the transmission efficiency and quality of optical signals, and have a wide range of applications in optical communications, optical imaging, microscopy technology, and optical sensors. Traditional bifocal devices are limited by the Abbe diffraction limit, but recently optical super-oscillation provides a new technology for far-field super-resolution focusing, which can make the focused focal spot break through the diffraction limit. However, the existing bifocal super-oscillatory lenses rely on complex optimization algorithms, the device processing is difficult, and the device size is limited. Based on the principle of optical super-oscillation, combined with the binary particle swarm optimization (BPSO) algorithm and the angular spectrum diffraction theory, two single-focal focused binary phase super-oscillation masks with different focal lengths are designed, and the bifocal focused binary phase super-oscillation masks can be obtained by using the Boolean logic "AND" operation. A bifocal super-oscillation mask is loaded into a spatial light modulator (SLM) to generate a super-resolution bifocal focusing device. This method has the advantages of easy implementation and controllable design focal length, and has significant application potential in optical communication, biomedical imaging and other fields. Methods This paper consists of three core steps: first, for circularly polarized light (wavelength lambda=632.8 nm), two binary phase super-oscillation masks (S-SOM1 and S-SOM2) are designed and optimized by using the BPSO algorithm and the angular spectrum diffraction theory with the corresponding focal lengths of f(1)=280000 lambda (177184 mu m) and f(2)=300000 lambda (189840 mu m), respectively. Second, a Boolean "AND" operation is performed on the phase distributions of the two masks to synthesize a bifocal super-oscillation mask (B-SOM), and the bifocal synchronous modulation is achieved by preserving the overlapping region of the phases. Finally, a far-field super-resolution bifocal focusing and measurement platform based on a spatial light modulator is designed and constructed, the B-SOM is loaded into the SLM in the form of a grayscale map, the scanning shot of the optical field with a step size of 0.05 mm is taken through the electrically-controlled displacement stage to obtain the distribution of the optical field, and the key parameters are extracted to verify the performance. Results and Discussions The Boolean "AND" operation is performed on the phase distributions of the single-focal super-oscillation masks S-SOM1 and S-SOM2 obtained from the optimized design to obtain the phase distributions of the bifocal super-oscillation masks (Fig. 3). From the theoretical results, it can be seen that the transverse full widths at half maximum (FWHMs) of the two focal spots are 21.846 mu m and 21.114 mu m, respectively, which are lower than the diffraction limits of 22.277 mu m and 23.495 mu m, and super-resolution focusing has been realized in theoretical calculations (Fig. 5). The experimental results show that the designed bifocal device successfully realizes far-field super-resolution focusing. The longitudinal FWHMs of the bifocal are 4.523 mm and 4.198 mm, respectively (Fig. 7), and the transverse FWHMs are 20.333 mu m and 23.353 mu m, respectively (Fig. 8), which are lower than the Abbe diffraction limits of 22.225 mu m and 23.650 mu m. In addition, the bifocal sidelobe ratios are as low as 5.1% and 12.7%, respectively (Table 1), which effectively suppress the optical field crosstalk. The experimental focal spot positions (z=177.784 mm and z=189.184 mm) are in good agreement with the theoretical calculations, and the small deviations are due to the partial loss of the modulation phase after the logic "AND" operation. Conclusions In this paper, we propose a far-field super-resolution bifocal focusing method based on spatial light modulator, which is based on the principle of super-oscillation, and utilize the binary particle swarm optimization algorithm and the angular spectrum diffraction theory to design a single-focal binary-phase super-oscillation mask, and adopt the Boolean logic "AND" operation to generate a bifocal super-oscillation mask from two binary-phase super-oscillation masks with different focal lengths. Based on this method, the bifocal super-oscillation masks with working wavelength of 632.8 nm and focal lengths of 280000 lambda (177184 mu m) and 300000 lambda (189840 mu m) are designed and optimized, respectively. By loading the super-oscillation mask into a spatial light modulator without precision processing, far-field super-resolution bifocal focusing is experimentally demonstrated, with the peak intensity positions of the two focal spots located at 177784 mu m and 189184 mu m, respectively. The average FWHMs are 20.333 mu m and 23.353 mu m, respectively, which are lower than the Abbe diffraction limit. The low sidelobes are also maintained with ratios of 5.1% and 12.7%, respectively. The results significantly improve the control accuracy and resolution of far-field super-resolution bifocal imaging, which can be applied to the visible wavelength band and extended to other optical bands.

Terahertz metamaterial dual-band absorbers are used for multi-target detection and high-sensitivity sensing in complex environments by enhancing information that reflects differences in the measured substances. Traditional design processes are complex and time-consuming. Machine learning-based methods, such as neural networks and deep learning, require a large number of simulations to gather training samples. Existing design methods based on single-objective optimization often result in uneven multi-objective optimization, which restricts practical applications. In this study, we developed a metamaterial absorber featuring a circular split-ring resonator with four gaps nested in a "(sic)" structure and used the Multi-Objective Firefly Algorithm based on Multiple Cooperative Strategies to achieve fast optimization of the absorber's structural parameters. A comparison revealed that our approach requires fewer iterations than the Multi-Objective Particle Swarm Optimization and reduces design time by nearly half. The absorber designed using this method exhibited two resonant peaks at 0.607 THz and 0.936 THz, with absorptivity exceeding 99%, indicating near-perfect absorption and quality factors of 31.42 and 30.08, respectively. Additionally, we validated the absorber's wave-absorbing mechanism by applying impedance-matching theory. Finally, we elucidated the resonance-peak formation mechanism of the absorber based on the surface current and electric-field distribution at the resonance frequencies. These results confirmed that the proposed dual-band metamaterial absorber design is efficient, representing a significant step toward the development of metamaterial devices.

Metamaterials can freely control terahertz waves by designing the geometric shape and direction of the unit structure to obtain the desired electromagnetic characteristics, so they have been widely used in sensing, communication and radar stealth technology. The traditional design of terahertz metamaterial absorber usually requires continuous structural adjustment and a large number of simulations to meet the expected requirements. The process largely relies on the experience of researchers, and the physical modeling and simulation solution process is time-consuming and inefficient, greatly hindering the development of metamaterial absorbers. Therefore, due to its powerful learning ability, deep learning has been used to predict the structural parameters or spectra of metamaterial absorbers. However, when designing a new structure, it is necessary to prepare a large number of training samples again, which is both time-consuming and not universal. Particle swarm optimization algorithm can quickly converge to the optimal solution through the sharing and cooperation of individual information in the group, with no need for prior preparation. Therefore, a method of fast designing terahertz metamaterial absorber is proposed based on multi-objective particle swarm optimization algorithm in this work. Taking a new center symmetric absorber structure composed of four Ls for example, the structure parameters are optimized to achieve rapid and automatic design of metamaterial absorber. The multi-objective particle swarm optimization algorithm takes the absorptivity and quality factor as independent targets to design the structure parameters of the absorber, realizing the dual-objective optimization of the absorber, and overcoming the shortcoming of the multi-objective conflicts that cannot be solved by PSO. When used for refractive index sensing, the optimally-designed absorber achieves perfect absorption at 1.613 THz with a quality factor of 319.72 and a sensing sensitivity of 264.5 GHz/RIU. In addition, the reasons of absorption peaks are analyzed in detail through impedance matching, surface current, and electric field distribution. By studying the polarization characteristics of the absorber, it is found that the absorber is not sensitive to polarization, which is more stable in practical application. In summary, the multi-objective particle swarm optimization algorithm can realize the design according to the requirements, reduce the experience requirement of researchers in the design of metamaterial absorber, thereby improving design efficiency and performance, and has great potential for application in the design of terahertz functional devices.

Objective The traditional focusing device is restricted by the Abbe diffraction limit. This means that the spatial resolution cannot exceed its theoretical minimum value of 0.5 lambda/ NA, where lambda is the working wavelength and NA is the numerical aperture. Existing methods to break the diffraction limit require a near-field environment, which is insufficient for far- field super- resolution imaging in the optical sense. The principle of optical super-oscillation states that it is theoretically possible to produce a super-resolution spot of arbitrary smallness by rationally modulating the wavefront of incident light. Optical super- oscillation has been extensively studied by researchers in super- resolution optical lenses, and this principle enables the experimental realization of far-field super-resolution focusing. However, the optical field regulation of the superoscillation lens depends on precise nano-processing technology. Additionally, the fabrication cost and complexity limit the device to a small size. Thus, we propose a method to generate the far-field super-resolution optical field based on the spatial light modulator. The design of the far- field super-resolution focusing device is based on the super-oscillation principle, with the binary particle swarm optimization algorithm and the angular spectrum diffraction theory combined. The generated focal spot full width at half maximum ( FWHM) is smaller than the diffraction limit, which can be employed to construct the far-field super-resolution optical field. Methods The device is designed based on the super- oscillation principle and adopts eight-value phase control for circularly polarized light with a wavelength of 632.8 nm. The two-dimensional phase distribution of the device is optimized using the binary particle swarm optimization algorithm and angular spectrum diffraction theory. This optimization helps obtain the optimal phase of the mask and its corresponding characteristic parameters. The device is composed of a series of concentric ring belts, each with 8 mu m width, which is equal to the size of spatial light modulator (SLM) pixels adopted in subsequent experiments. To obtain an optimized phase mask, we calculate the phase of each ring belt and generate a grayscale image based on the SLM phase control characteristics. Additionally, to verify the focusing performance of the designed device, we design and build a construction and measurement system for the far-field super-resolution optical field. We measure the characteristic parameters of the super-resolution optical field using an objective lens combined with a complementary metal oxide semiconductor (CMOS) camera. The motorized linear translation stage is moved to obtain the two- dimensional optical field distribution at different positions. Finally, an image processing algorithm is then utilized to extract the key focusing parameters of the focal spot, leading to a three- dimensional intensity distribution of the optical field. Results and Discussions First, the corresponding grayscale images are generated based on the phase of each ring belt of the super-oscillatory mask obtained from the optimized design ( Fig. 3). Next, the design results of the optical field are calculated by adopting the angular spectrum diffraction theory (Fig. 4). An experimental platform is then set up, and the super- oscillatory mask is loaded onto the liquid crystal screen of the spatial light modulator. Finally, the optical field is scanned and tested within the range of Z= 185.00 mm to Z=195.00 mm. The scanning step Delta Z is 0.05 mm, and the intensity distribution of the optical field is obtained. Experiment and theoretical results demonstrate excellent agreement, and the transverse FWHM at the focal length of the focal spot is 22.384 mu m, which is below the diffraction limit (0.5 lambda/ NA, 23.732 mu m), with far-field super-resolution focusing achieved (Fig. 6). Along the propagation direction, the vertical FWHM is 6.029 mm, creating an optical needle (Fig. 7). The device is easy to operate and does not require complex processing. Conclusions To solve the problem of traditional focusing devices are constrained by the diffraction limit, we propose a method for constructing a far-field super- resolution optical field with eight-value phase control based on the optical super-oscillation principle. By adopting particle swarm optimization and angular spectrum diffraction theory, we design a far- field super- resolution focusing device for circularly polarized light with a wavelength of 632.8 nm. This is achieved by loading a super- oscillation phase mask onto the liquid crystal screen of a spatial optical modulator. By adjusting the phase of the incident optical field, the device generates an optical needle with the vertical FWHM of 6.029 mm. The FWHM at the focal length of the focal spot is lower than the diffraction limit, thus achieving far-field super-resolution focusing. This method can be applied to the visible bands and extended to other optical bands, providing core focusing devices for optical microscopy, optical imaging, and other optical applications.

Lenses are a fundamental component of optical systems. Bifoci and optical needle devices have excellent application potential in many optical systems. Conventional lenses are limited by their diffraction limits, and the spot size has a considerable influence on the resolution of optical and microscopic images. Optical super-oscillation is a novel technique, to the best of our knowledge, for far-field sub-diffraction focusing. In this study, we proposed a binary-amplitude super-oscillatory lens (SOL) approach for generating bifoci and optical needles (ON), and it was based on the angular spectrum method (ASM) and a binary-particle-swarm optimization (BPSO) algorithm. We reported a class of binary-amplitude-based Bifoci- and ON-SOLs that generated sub-diffraction bifoci and optical needles. Sub-diffraction bifoci with a transverse range of 0.401 λ −0.522 λ were recorded for a work wavelength of λ =632.8nm. The generated optical needle had a sub-diffraction length of 4.122 λ , and the super-oscillation region was 2.083 λ long. This provides potential applications for further super-resolution imaging, optical communication, and precision manufacturing.

One of the benefits of using terahertz(THz)spectra in characterizing weak interactions is that their anharmonicity can help us to understand the macroscopic properties of crystals.In this study,2,6-diamino-3,5-dinitropyrazine(ANPZ)was adopted to analyze the anharmonic mechanism of terahertz spectra.First,the temperature-induced anharmonicity was obtained from the THz spectral measurement under heating.Next,density functional theory was used to identify the vibration properties of each absorption.Vibration mode decomposition was then employed to deeply analyze the origins of these anharmonic differences.The results show that the softening of the special intermolecular hydrogen bonding is responsible for the strong anharmonicity.Furthermore,the displacement properties of the atomic temperature factor calculated based on phonon and quasi-simple harmonic approximations also verify the above conclusion.The present study demonstrates that THz spectroscopy can provide insight into the response of hydrogen bonding under heating and can be used as a scientific analysis method for understanding the macroscopic properties of crystals.

Cocrystals represent an effective method to manipulate the physicochemical properties of materials at a molecular level. However, understanding the relationship between their complex crystal structures and macroscopic properties is a challenge. In this paper, by using terahertz (THz) spectroscopy to characterize non-covalent interactions within crystals, the THz vibrational spectra of the CL-20/MTNP cocrystal are studied. Firstly, the THz spectra of CL-20, MTNP, and the CL-20/MTNP cocrystal are measured at room temperature. Both absorption positions and intensities of the cocrystals differ from those of their original components, confirming the unique advantage of terahertz spectroscopy in cocrystal identification. Secondly, the THz vibrational features of the three materials are calculated based on density functional theory (DFT). Then, the experimental absorptions are matched with the calculated vibrations. Furthermore, a vibrational decomposition method is employed to decompose the molecular vibrations into intermolecular and intramolecular vibrations. The vibrational variations of the cocrystal compared with its original components are analyzed. The results reveal that in the cocrystal, the intermolecular vibrational modes of both CL-20 and MTNP molecules have changed compared with their raw materials. This indicates that the non-covalent interactions in the cocrystal have changed the original intermolecular interactions of these molecules. Consequently, this enhancement promotes the heat transfer between MTNP and CL-20 molecules, thereby improving the thermal stability of the cocrystal. These findings in this study demonstrate that the THz vibrational spectroscopy technology helps establish a relationship between the molecular structure of cocrystal and its macroscopic properties. This research contributes to deepening our understanding of cocrystal systems and opens up a new way for designing and optimizing materials.

Understanding the phase transition mechanism of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is crucial for ensuring its safe applications. In this study, we observed the temperature-induced phase transition of CL-20 using terahertz spectroscopy. Subsequently, quantum chemical calculations were employed to assign the vibrations to experimental absorptions. Finally, the variations of intra- and intermolecular vibrations before and after phase transition were analyzed. The results indicated hydrogen bonds formed by the rotation of 5-nitro promoted hydrogen transfer, resulting in the decrease in thermal stability.

The far-field super-resolution focusing devices possess characteristics such as super-resolution focusing, achromatic, small size and easy machining, which make them highly promising in optical imaging, optical microscopy and lithography. In this work, we propose a binary-amplitude modulation-based method for generating far-field super-resolution achromatic focusing. By using the principles of optical super-oscillation, combined with angular spectral diffraction theory and binary particle swarm optimization (BPSO), we optimize the binary amplitude-type far-field super-resolution focusing devices, which have an identical radius of 100λ but different focal lengths: λ1 = 405 nm, λ2 = 532 nm and λ3 = 632.8 nm, respectively. Additionally, an achromatic metalens is integrated by using Boolean AND operation. To assess the feasibility of our proposed approach, numerical simulations are conducted via COMSOL Multiphysics employing FEM analysis. The simulation results demonstrate that the generated spots are located at 25.105λ, 25.106λ, and 25.105λ, respectively. The corresponding full width at half maximum (FWHM) values are 0.441λ1 (0.179 μm), 0.469λ2 (0.249 μm) and 0.427λ3 (0.270 μm), which are smaller than the Abbe diffraction limit, and the far-field super-resolution achromatic focusing is realized. The sidelobe ratios are at low levels, i.e. 12.5%, 12.6%, and 14.2%. The binary amplitude-type far-field super-resolution achromatic devices have the advantages of easy machining, achromatism and super-resolution, and are suitable for miniaturization and integration of optical systems.

Terahertz (THz) single-pixel imaging has received major research attention because of the lack of a suitable high-resolution array detector for THz imaging applications. Improving both imaging speed and quality has become a research hotspot for this field in recent years. In this study, a terahertz single-pixel imaging system with Hadamard spatial encoding was constructed by using optically induced semiconductor materials to perform THz wave modulation. Sparse coding was added to the system's reconstruction algorithm to enhance imaging quality. Numerous image patches were then collected from a natural image set to train an overcomplete dictionary and each patch in the measured image was reconstructed through sparse representation. To validate the effectiveness of the proposed algorithm, the reconstruction performances of different algorithms were compared under various conditions (i.e., with sampling rates varying from 5 to 100% and with noise levels within a signal-to-noise ratio range of 10-50 dB). The proposed algorithm, in combination with sparse representation of an overcomplete dictionary, showed a higher peak signal-to-noise ratio and a lower mean square error than both the inverse Hadamard transform (IHT) and TVAL3 algorithms. Finally, THz imaging experiments were performed to validate the algorithm's reconstruction performance at sub-Nyquist sampling rates. The experimental and simulation results coincided closely, thus indicating that the use of the proposed algorithm enhances the signal-to-noise ratio of the reconstructed image, reduces its mean square error, and retains greater image detail. The proposed algorithm was demonstrated to be the preferred choice for THz single-pixel imaging applications.

Aiming at the measurement of the temperature field during the ignition reaction of the elemental energetic material TKX-50, based on the principle of the calibration schlieren method, a transmission schlieren measurement system was built, and a one-to-one mapping model between the temperature field to be measured and the refractive index was established,combined with the Abel value calculation method to invert the distribution curve of the temperature field to be measured. The temperature field measurement experiment of TKX-50 ignition reaction under different experimental conditions was carried out,and the schlieren images of the whole process of ignition reaction were obtained. The results show that the maximum peak temperature of TKX-50 ignition reaction process is about 1 200K. Visual measurement of temperature field evolution during ignition reaction of TKX-50 is realized effectively by the method.

安定剂的检测是推进剂和发射药状态评估的重要环节.为实现安定剂的快速无损检测,研究了紫外-可见漫反射光谱法对安定剂进行定性定量检测的可行性.搭建了一套运用光纤传感的光谱测量系统,获取了 3 种安定剂样本的光谱.通过支持向量机(SVM)对光谱进行分类识别,运用主成分分析(PCA)可视化样本的聚类趋势;结合化学计量学方法,以N-甲基-4-硝基苯胺(MNA)470～500 nm波段的光谱强度作为特征数据通过偏最小二乘回归(PLSR)和主成分回归(PCR),建立了2 个MNA的定量预测模型.结果表明:运用SVM算法可以对具有浓度变化的 3 种安定剂样本实现分类识别,在测试集中分类的准确率达到100%;2 个定量预测模型的决定系数(R2)分别为0.993 9 和0.994 6,模型外部验证的均方根误差(RMSE)最大为0.000 23,可以对MNA进行定量表征.该方法在安定剂检测领域具有较大的应用潜力,并可扩展至其他检测领域.

In this study, terahertz time-domain spectroscopy (THz-TDS) was used to obtain the terahertz absorption spectra of three free anthraquinones (Chrysophanol, Emodin, Physcion) in the frequency range of 0.2-4.3 THz. The results show that terahertz spectroscopy is an effective detecting such compounds. Meanwhile, the theoretical spectrum using density functional theory calculations agrees well with the experimental spectrum. A modal decoupling method was used to identify each low-frequency vibrational mode and determine the average contribution of different atoms and groups. Modal decoupling provides a better understanding of molecules' mixed vibrational modes and enables quantifying the atoms' vibrational contributions. Results show that the substituent group facilitates the transition between the fundamental vibrational modes; subsequently, the substituent group shifts the vibrational centre of gravity of the three molecules and affects the vibrational contribution of hydrogen bonds. Furthermore, insignificant Emodin absorption is related to the nearly symmetrical structure formed by the substituents. The feasibility of terahertz analysis of differential molecular structures has also been confirmed.

Lenses with sub-diffraction focusing are extensively used in advanced optical imaging and microscope. Due to the longitudinal electric component, the radially polarized beam could offer sub-diffraction focusing by high-numerical-aperture (NA) lenses. However, slightly oblique incidence light could pose serious off-axis aberrations for high-NA metalens. Here, we demonstrate that a high NA (0.97) metalens design approach can simultaneously provide sub-diffraction focusing for oblique incidence and broadband operation. Simulation results demonstrate that the metalens with an aperture stop, R (stop), of 7 & mu;m have the operating angle range of [-15 & DEG;, +15 & DEG;] for sub-wavelength focusing. For the case of R (stop) = 9 & mu;m, sub-diffraction focusing is realized in the oblique angle range at & PLUSMN;6 & DEG;. Significantly, the metalens is able to realize a sub-diffraction focusing over the wavelength range 600-750 nm. These types of metalens have the important advantages of sub-diffraction focusing and broadband operation. They are also ultra-thin and easy to integrate, allowing such metalens could be used in miniaturized and integrated optical systems.

Objective The traditional design of metamaterial absorber depends on the experience of researchers to obtain excellent optical performance by modifying geometric parameters. The design pattern of trial and error leads to low efficiency but high cost. Therefore, deep learning is proposed as an inverse design method to improve design productivity and shorten the design circle of terahertz (THz) metamaterial absorber due to the powerful learning ability. It can map the relationship of structural parameters with its absorption performance to predict the optimum value of structural parameters. However, the response spectrum composed of multiple sampling points is employed as the input, which results in a complex network system with a large number of input nodes, output nodes, and hidden layers. Therefore, this paper puts forward a way to simplify the structure of the neural network and apply it to the design of a THz metamaterial absorber with a novel top pattern of the circular ring and double-opening resonance ring. Methods The whole design process is divided into four steps in Fig. 1: determining key structural parameters of the top layer, processing data sets, analyzing the structure of the neural network, and predicting structural parameters. Step 1 is determining key structural parameters. The absorber designed in this paper is composed of three layers. The copper with conductivity s= 5. 71x107 S/m, permeability mu= 4px10- 7 H/ m, and thickness of 0. 2 mu m is selected for the top layer and bottom layer. The intermediate medium layer is FR- 4 with the dielectric constant er = 4. 3 and thickness of 50 mu m. The pattern of the top layer is shown in the upper right of Fig. 1. According to the theory of LC electromagnetic resonance, the resonant characteristics of the unit are easily affected by the width of the circular ring (r(1)- r(2)), the width of the doubleopening resonant square ring (L 1-L2), and the opening width G. Step 2 is processing data sets. With the quality factor and absorptivity as inputs, and the structural parameters including the inner diameter of metal ring r(1), the inner side length of double-opening resonance ring L 1, and opening width G as outputs, 1000 sample data sets are calculated through CST simulation and divided into training sets and test sets according to the ratio of 7: 3. Step 3 is analyzing the structure of the neural network. The Sigmoid function is employed as the activation function of neurons. The error rate fluctuates with the changing number of hidden layers but reaches a minimum of 0. 9% at five layers. Thus, the hidden layer is set to five. The mean square error is smaller when the number of nodes m= 6, 9, and 12, and the error rate has an obvious minimum value when the number of nodes m = 6, 9, and 12. Therefore, the number of hidden layer nodes is set to be 6, 9, or 12. Step 4 is predicting structural parameters. When the demand performance is set as A= 100% and Q= 23, the structural parameters calculated by the neural network are r(1)=42. 5 mu m, L-1= 37 mu m, and G=19 mu m. The optical performance calculated by CST simulation is 99. 99% and the quality factor Q is 23. 2. Thus, the error of target absorption performance is 0. 9%. When the required performance is set as A= 85% and Q= 30, the structural parameters calculated by the neural network are r(1)= 45 mu m, L-1= 35 mu m, and G= 21 mu m. The optical performance by CST simulation is 85. 86% and the quality factor Q is 31. 7. Therefore, the error of the target absorption performance is 1. 05%. Results and Discussions This paper analyzes the influence of structural parameters r(1), L-1, and G on the absorption performance of the absorber. When r1=45 mu m, the change trend of absorbance and Q value with L-1 and G is shown in Fig. 3. The absorbance increases and the Q value gradually decreases as L-1 increases and G decreases. When L-1= 36 mu m and G= 25 mu m, the change trend of absorbance and Q value with r 1 is shown in Fig. 4. The absorption rate decreases and the Q value increases with the rising r1. Additionally, the electric field distribution and surface current distribution of the high absorption structure at the resonance frequency f0= 1. 192 THz are analyzed as shown in Fig. 5. The electric field is mainly distributed at the four parts of the circular ring and the double- opening resonant ring. For the double-opening resonant ring, the surface current flows down through the left and right sides respectively to generate electric dipole resonance. For the external ring, the current mainly converges at the four parts of the adjacent double-open resonant ring, as the upper and lower of the ring, thus producing electric dipole resonance. The two absorbers of Model A and Model B designed for the requirements of high absorptivity and high Q value respectively with the same top layer pattern can be produced by micro-nano fabrication. When the fabrication tolerance of Model A is - 2%-2%, the absorption rate fluctuates between 97. 40%-99. 99%, the absolute error is - 2. 6%- 0, and the maximum relative error is 2. 6%. The Q value fluctuates between 22. 5 and 24. 3, with an absolute error of - 0. 7-1. 1 and a maximum relative error of 4. 7%. Table 6 shows that when the fabrication tolerance of Model B is - 3%-3%, the absorption rate fluctuates between 84. 98%similar to 89. 10%, the absolute error is - 0. 88%-3. 24%, and the maximum relative error is 3%. The Q value fluctuates between 30. 8 and 31. 7, the absolute error is - 0. 9-0, and the maximum relative error is 2. 8%. This indicates that Model A is within the fabrication tolerance of - 2%-2%, and Model B is within the fabrication tolerance of - 3% -3%, with good fabrication tolerance. Conclusions In this paper, an absorber structure with a top pattern of the circular ring and double- opening resonant ring is proposed, and the reverse design of THz metamaterial absorber is realized through neural networks. The input and output nodes are simplified by electromagnetic resonance theory and absorption performance characterization to reduce the complexity of the neural network. The maximum absorption rate of metamaterial absorber designed by the proposed neural network can reach 99. 99% at the frequency of 1. 192 THz, which is close to perfect absorption. The maximum Q value can be 31. 7 at frequency of 1. 22 THz. The maximum relative error should not exceed 4. 7% within the fabrication tolerance of - 2%- 2%. Additionally, this paper analyzes the influence of three geometric parameters on the absorptivity and quality factor in detail and discusses the absorption mechanism of the absorber from three aspects of current, electric field distribution, and equivalent circuit. The proposed method can effectively improve the design efficiency of metamaterial absorber according to the performance requirements and has great application prospects in terahertz functional device design.