Near-infrared Wavebands Research Articles

Colored Radiative Cooling and Flame-Retardant Polyurethane-Based Coatings: Selective Absorption/Reflection in Solar Waveband.

The aesthetic demand has become an imperative challenge to advance the practical and commercial application of daytime radiative cooling technology toward mitigating climate change. Meanwhile, the application of radiative cooling materials usually focuses on the building surface, related tightly to fire safety. Herein, the absorption and reflection spectra of organic and inorganic colorants are first compared in solar waveband, finding that iron oxides have higher reflectivity in NIR region. Second, three kinds of iron oxides-based colorants are selected to combine porous structure and silicon-modified ammonium polyphosphate (Si-APP) to engineer colored polyurethane-based (PU) coating, thus enhancing the reflectivity and flame retardancy. Together with reflectivity of more than 90% in near-infrared waveband and infrared emissivity of ≈91%, average temperature drops of ≈5.7, ≈7.9, and ≈3.8°C are achieved in porous PU/Fe2O3/Si-APP, porous PU/Fe2O3·H2O/Si-APP, and porous PU/Fe3O4·H2O/Si-APP, compared with dense control samples. The catalysis effect of iron oxides in the cross-linking reaction of pyrolysis products and dehydration mechanism of Si-APP enable PU coating to produce an intumescent and protective char residue. Consequently, PU composite coatings demonstrate desirable fire safety. The ingenious choice of colorants effectively minimizes the solar heating effect and trades off the daytime radiative cooling and aesthetic appearance requirement.

Spectral imaging based on custom-made multi-strip filter arrays

Spectral imaging technology based on on-chip spectroscopy can find applications in areas including aerospace, industrial and consumer electronics, and so on. Since each application normally requires a different set and number of spectral bands, the development of customized spectroscopy solutions with more compact size and lower cost becomes quite important. In this paper, we demonstrate a compact, highly customizable imaging spectrometer scheme based on custom-made multi-strip filter arrays, which maintains an average high transmission of ∼85%, narrow bandwidth of ∼30nm, and high optical density of ∼OD2 in the blocking regions across the visible to near-infrared waveband. Spectral imaging experiments are conducted, and the accurate reconstruction of sparse spectral image data is demonstrated as well to prove the validity of the proposed scheme. As a result, the work reported in this paper allows researchers to develop customized spectral imaging equipment in a relatively easy way and also has a great potential to be engineered further for scalable production with a quite low cost.

Modelling and design of human eye inspired concentric cylindrical metalens

Metalenses can potentially reduce the size and complexity of existing cameras, displays, and other optical devices, owing to their capability of flexible manipulation of the polarization, amplitude, and phase of light. However, a high meta-atom aspect ratio is still a drawback as it causes difficulty in fabrication of metalens. In this paper, we present the first demonstration of a human-eye inspired metalens with a much lower and constant meta-atom aspect ratio while maintaining the polarization under an arbitrarily polarized excitation in the near-infrared waveband.

Significant enhancement in the photoelectronic properties of SnI4 via pressure-tailored phase engineering

Constantly developing superior functional materials with fascinating photoelectric properties is crucial for advancing the optoelectronics industry. Here, we demonstrate a feasible strategy to significantly improve the photoelectric properties of SnI4 through employing high pressure phase engineering. With increasing pressure, the photocurrent response of SnI4 undergoes sequential enhancement and the maximum photocurrent of SnI4 is five orders of magnitude higher than that observed at initial pressure under Xe-lamp illumination. Surprisingly, the photoelectric spectral response range was expanded from visible region to near-infrared waveband (1650 nm) upon successive compression. These remarkable enhancements in photoelectric activities are attributed to the pressure-induced semiconducting cubic phase to metal triclinic phase transition in SnI4. The occurrence of the metal phase improves the charge carriers transfer ability and photon absorption in a wider spectral range. Our findings provide some valuable insights and an avenue for regulating the photoelectric properties of materials by manipulating the phase composition with pressure.

Comparing the Structural Parameters of the Milky Way to Other Spiral Galaxies

The structural parameters of a galaxy can be used to gain insight into its formation and evolution history. In this paper, we strive to compare the Milky Way’s structural parameters to other, primarily edge-on, spiral galaxies in order to determine how our Galaxy measures up to the Local Universe. For our comparison, we use the galaxy structural parameters gathered from a variety of literature sources in the optical and near-infrared wave bands. We compare the scale length, scale height, and disk flatness for both the thin and thick disks, the thick-to-thin disk mass ratio, the bulge-to-total luminosity ratio, and the mean pitch angle of the Milky Way’s spiral arms to those in other galaxies. We conclude that many of the Milky Way’s structural parameters are largely ordinary and typical of spiral galaxies in the Local Universe, though the Galaxy’s thick disk appears to be appreciably thinner and less extended than expected from zoom-in cosmological simulations of Milky Way-mass galaxies with a significant contribution of galaxy mergers involving satellite galaxies.

PbS/CsPbBr3 Heterojunction for Broadband Neuromorphic Vision Sensing.

Neuromorphic light sensors with analogue-domain image processing capability hold promise for overcoming the energy efficiency limitations and latency of von Neumann architecture-based vision chips. Recently, metal halide perovskites, with strong light-matter interaction, long carrier diffusion length, and exceptional photoelectric conversion efficiencies, exhibit reconfigurable photoresponsivity due to their intrinsic ion migration effect, which is expected to advance the development of visual sensors. However, suffering from a large bandgap, it is challenging to achieve highly tunable responsivity simultaneously with a wide-spectrum response in perovskites, which will significantly enhance the image recognition accuracy through the machine learning algorithm. Herein, we demonstrate a broadband neuromorphic visual sensor from visible (Vis) to near-infrared (NIR) by coupling all-inorganic metal halide perovskites (CsPbBr3) with narrow-bandgap lead sulfide (PbS). The PbS/CsPbBr3 heterostructure is composed of high-quality single crystals of PbS and CsPbBr3. Interestingly, the ion migration of CsPbBr3 with the implementation of an electric field induces the energy band dynamic bending at the interface of the PbS/CsPbBr3 heterojunction, leading to reversible, multilevel, and linearly tunable photoresponsivity. Furthermore, the reconfigurable and broadband photoresponse in the PbS/CsPbBr3 heterojunction allows convolutional neuronal network processing for pattern recognition and edge enhancements from the Vis to the NIR waveband, suggesting the great potential of the PbS/CsPbBr3 heterostructure in artificial intelligent vision sensing.

Analysis Of The Mangrove Structure In The Dong Rui Commune Based On Multispectral Unmanned Aerial Vehicle Image Data

Mangroves are one of the most important types of wetlands in coastal areas and perform many different functions. Assessing the structure and function of mangroves is a premise for the management, monitoring and development of this most diverse and vulnerable ecosystem. In this study, the unmanned aerial vehicle (UAV) Phantom 4 Multispectral was used to analyse the structure of a mangrove forest area of approximately 50 hectares in Dong Rui commune, Tien Yen district, Quang Ninh Province – one of the most diverse wetland ecosystems in northern Vietnam. Based on the visual classification method combined with the results of field taxonomic sampling, a mangrove tree classification map was established for UAV with three species, Bruguiera gymnorrhiza, Rhizophora stylosa, and Kandelia obovata, achieving an overall accuracy = 86.28%, corresponding to a Kappa coefficient =0.84. From the images obtained from the UAV, we estimated and developed maps and assessed the difference in tree height and four vegetation indices, including the normalized difference vegetation index (NDVI), green normalized difference vegetation index (GNDVI), enhanced vegetation index (EVI), and green chlorophyll index (GCI), for three mangrove plant species in the flying area. Bruguiera gymnorrhiza and Rhizophora stylosa reach an average height of 4 to 5 m and are distributed mainly in high tide areas. Meanwhile, Kandelia obovata has a lower height (ranging from 2 to 4 m), distributed in low-tide areas, near frequent flows. This study confirms the superiority of UAV with red edge and near-infrared wave bands in classifying and studying mangrove structures in small-scale areas.

Intracavity frequency-doubled external-cavity surface-emitting blue laser with high conversion efficiency

Blue laser with high power and high beam quality has many applications such as in laser display, underwater communication and imaging, and non-ferrous metal processing. Optically pumped external-cavity surface-emitting laser combines the advantages of both surface-emitting semiconductor lasers and solid-state disk lasers, and can produce high output power and good beam quality simultaneously. Its high intracavity circulating power is more conducive to intracavity frequency doubling, achieving high-power and high beam quality blue light through fundamental laser in the near-infrared waveband. This paper reports an efficient intracavity frequency doubled 490 nm high power blue light by using a 980 nm fundamental laser in an external-cavity surface-emitting laser. The V-type resonant cavity is formed by the high reflectivity distributed Bragg reflector (DBR) at the bottom of gain chip, a folded flat concave mirror (high reflectivity coated for 980 nm and anti-reflectivity coated for 490 nm), and a flat concave end mirror (high reflectivity coated for 980 nm and 490 nm). By inserting a nonlinear crystal LBO into the cavity at the beam waist formed by the folded mirror and end mirror, and employing a birefringent filter (BRF) to polarize the fundamental laser and narrow the linewidth of the laser, a high power and high beam quality blue laser with high conversion efficiency is obtained. The effects of different factors including the length of nonlinear crystal, the linewidth of fundamental laser, and the compensation of walk off angle on the output power of the blue laser are studied experimentally. The length of the nonlinear crystal is optimized based on the size of the fundamental laser beam waist at the position of the crystal in the resonant cavity. Under the type-I phase matching condition of LBO, over 6 W output power at 491 nm wavelength is obtained when the crystal length is 5 mm and the BRF thickness is 1 mm. The beam quality <i>M</i><sup>2</sup> factor in the <i>x</i> direction and the <i>y</i> direction are both 1.08, and the conversion efficiency of frequency doubling is 63%. The experimental results also show that symmetrically placed nonlinear crystals can compensate for the walk-off angle during frequency doubling to a certain extent, thereby clearly improving the conversion efficiency of the frequency doubled blue laser.

Growth of Snapdragon Under Simulated Transparent Photovoltaic Panels for Greenhouse Applications

Abstract Transparent photovoltaic (PV) materials can be used as greenhouse coverings that selectively transmit photosynthetically active radiation (PAR). Despite the economic importance of the floriculture industry, research on floriculture crops has been limited in these dual-purpose, agrivoltaic greenhouses. We grew snapdragon under simulated photoselective and neutral-density panels with transmissions ranging from ∼30 to 90%, and absorption edges in the green (G; 500–599 nm), red (R; 600–699 nm), far-red (FR, 700–750 nm), and near-infrared (NIR) wavebands. We hypothesized that snapdragon could tolerate some degree of PV shading without reducing growth and flower number or delaying flowering time. Biomass accumulation, compactness, time to flower, and crop quality under 1) a clear acrylic control, 2) a FR-absorbing, and 3) a NIR-absorbing PV panel were not statistically different when the average daily light integral was between 17 and 20 mol·m−2·d−1. Crop quality progressively diminished below 17 mol·m−2·d−1. These results indicate that snapdragon tolerated ∼15% PV shading during summer months without reduced growth or quality. Species used in the study: Snapdragon (Antirrhinum majus L.).

YOLACTFusion: An instance segmentation method for RGB-NIR multimodal image fusion based on an attention mechanism

The tomato plant’s main-stem is a feasible lead for robotic searching the grows discretely-growing targets of harvesting, pruning or pollinating. Owing to the highlighted reflection characteristics of the main-stem in the near-infrared (NIR) waveband, this study proposes a multimodal hierarchical fusion method (YOLACTFusion) based on the attention mechanism, to achieve an instance segmentation of the main-stem from similar-colored differentiation (i.e., green leaf and green fruit) in robotic vision systems. The model inputs RGB images and 900–1100 nm NIR images into two ResNet50 backbone networks and uses a parallel attention mechanism to fuse feature maps of various scales together into the head network, to improve the segmentation performance of the main-stem of RGB images. The loss function for the multimodal image weights the original loss on the RGB image and the position offset loss and classification loss on the NIR image. Furthermore, the local depthwise separable convolution is used for the backbone network, and Conv-BN layers are merged to reduce the computational complexity. The results show that the precision and recall of YOLACTFusion of the main-stem detection, respectively reached 93.90 % and 62.60 %; and the precision and recall of instance segmentation reached 95.12 % and 63.41 %, respectively. Compared to YOLACT, the mean average precision (mAP) of YOLACTFusion is increased from 39.20 % to 46.29 %, the model size is reduced from 199.03 MB to 165.52 MB, while the image processing efficiency remains similar. The overall results show that the multimodal instance segmentation method proposed in this study significantly improves the detection and segmentation of tomato main-stems under a similar-colored background, which would be a potential method for improving agricultural robot’s visual perception.

Integrated Encapsulation and Implementation of a Linear-Mode APD Detector for Single-Pixel Imaging Lidar

Single-pixel imaging lidar is a novel technology that leverages single-pixel detectors without spatial resolution and spatial light modulators to capture images by reconstruction. This technique has potential imaging capability in non-visible wavelengths compared with surface array detectors. An avalanche photodiode (APD) is a device in which the internal photoelectric effect and the avalanche multiplication effect are exploited to detect and amplify optical signals. An encapsulated APD detector, with an APD device as the core, is the preferred photodetector for lidar due to its high quantum efficiency in the near-infrared waveband. However, research into APD detectors in China is still in the exploratory period, when most of the work focuses on theoretical analysis and experimental verification. This is a far cry from foreign research levels in key technologies, and the required near-infrared APD detectors with high sensitivity and low noise have to be imported at a high price. In this present study, an encapsulated APD detector was designed in a linear mode by integrating a bare APD tube, a bias power circuit, a temperature control circuit and a signal processing circuit, and the corresponding theoretical analysis, circuit design, circuit simulation and experimental tests were carried out. Then, the APD detector was applied in the single-pixel imaging lidar system. The study showed that the bias power circuit could provide the APD with an operating voltage of DC 1.6 V to 300 V and a ripple voltage of less than 4.2 mV. Not only that, the temperature control circuit quickly changed the operating state of the Thermo Electric Cooler (TEC) to stabilize the ambient temperature of the APD and maintain it at 25 ± 0.3 °C within 5 h. The signal processing circuit was designed with a multi-stage amplification cascade structure, effectively raising the gain of signal amplification. By comparison, the trial also suggested that the encapsulated APD detector and the commercial Licel detector had a good agreement on the scattered signal, such as a repetition rate and pulse width response under the same lidar environment. Therefore, target objects in real atmospheric environments could be imaged by applying the encapsulated APD detector to the near-infrared single-pixel imaging lidar system.

How similar is the zoning of different vegetation indices: Defining the optimal framework for monitoring grapevines’ growth within vigorous vineyards

The spatial patterns of grapevines’ vigor across vigorous vineyards were assessed through distinct vegetation indices (VI) for comparison purposes. This survey also aids the selection of the most sensitive VI to monitor the vineyards during a given optimal stage of the last period of the growing season. The canopy reflectance (ρ) of grapevines cv. Cabernet Franc and Cabernet Sauvignon (Vitis vinifera L.) was measured throughout their berry development and ripening stages. Georeferenced data of ρ at the red (680 nm), red edge (730 nm), and near-infrared (780 nm) wavebands were taken using a hand-held active sensor to compute the normalized difference red edge index (NDRE), and the normalized difference vegetation index (NDVI). Spatial predictions of both VI were estimated by geostatistical interpolation technique (kriging) and then categorized into homogeneous zones (HZ) through the Jenks natural breaks optimization method to delimit low and high vigor regions. Statistics of the inter-rater reliability between each VI zoning were calculated afterward. The relative dispersion of data around their mean was assumed as a criterion to assess the sensibility of each VI in detecting vigor variability under a given condition. The most appropriate stage to monitor the vineyards was determined based on the peak of vegetative growth inferred from the values of the VI we presumed as most sensitive. Differences between the HZ defined from the best combination of VI and stage in terms of yield, cane weight, and crop load were also examined. We demonstrated that NDRE was more sensitive than NDVI in detecting the vigor variability of high-density vegetation due to the relative dispersion of its datasets and the saturation effect of NDVI at the late stages of the grapevine growing season. The lowest and higher agreement between the VI zoning was usually detected at the berry touch and berry softening growth stages, respectively. Although there were distinct levels of concordance between NDRE and NDVI maps, their overall agreement was higher than 70% in most surveys. The disagreement in quantity or spatial allocation of vigor categories provided by each VI relied on the vineyard, growth stage, and growing season. Nevertheless, the early definition of HZ from NDRE monitoring carried out at veraison could be suitable to identify differences in cane weight at pruning, with a moderate effect size.

Dynamically Adjusting Borophene-Based Plasmon-Induced Transparency in a Polymer-Separated Hybrid System for Broadband-Tunable Sensing.

Borophene, an emerging two-dimensional (2D) material platform, is capable of supporting highly confined plasmonic modes in the visible and near-infrared wavebands. This provides a novel building block for light manipulation at the deep subwavelength scale, thus making it well-suited for designing ultracompact optical devices. Here, we theoretically explore a borophene-based plasmonic hybrid system comprising a continuous borophene monolayer (CBM) and sodium nanostrip gratings (SNGs), separated by a polymer spacer layer. In such a structure, a dynamically tunable plasmon-induced transparency (PIT) effect can be achieved by strongly coupling dark and bright plasmonic modes, while actively controlling borophene. Here, the bright mode is generated through the localized plasmon resonance of SNGs when directly excited by TM-polarized incident light. Meanwhile, the dark mode corresponds to a propagating borophene surface plasmon (BSP) mode in the CBM waveguide, which cannot be directly excited, but requires phase matching with the assistance of SNGs. The thickness of the polymer layer has a significant impact on the coupling strength of the two modes. Owing to the BSP mode, highly sensitive to variations in the ambient refractive index (RI), this borophene-based hybrid system exhibits a good RI-sensing performance (643.8 nm/RIU) associated with a wide range of dynamically adjustable wavebands (1420-2150 nm) by tuning the electron density of borophene. This work offers a novel concept for designing active plasmonic sensors dependent on electrically gating borophene, which has promising applications in next-generation point-of-care (PoC) biomedical diagnostic techniques.

A refractive index sensor based on the MIM waveguide with a semi-elliptical and a rectangular ring resonant cavity

In this paper, a novel nanosensor comprising the metal–insulator–metal (MIM) plasmonics waveguide with a semi-elliptical and rectangular ring resonant cavity is designed. In near-infrared waveband, the propagation properties of electromagnetic waves in the structure are studied using the finite-difference time-domain (FDTD) method. The results show that, based on the coupling between the semi-elliptical and the rectangular ring resonant cavity, the transmission spectrum of the structure exhibits a sharp Fano resonance shape. Next, the influence of the refractive index and sensor structure parameters on performance is systematically investigated. The simulation results show that the sensor structure has the best sensitivity of 1384[Formula: see text]nm/RIU (refractive index unit), and the figure of merit (FOM) is 28.4. The simple MIM structure could be applied to sensitive plasmonic sensors.

Transmittance correlated real-time resistivity modulation and insulator-metal transition of electrochromic WO3 thin films

A quantitative relationship between optical and electrical properties of crystalline WO3 films during Li + insertion has been established in this work. Detailly, the exponential relationship between transmittance and resistivity of crystalline WO3 thin films in visible and near-infrared wavebands is determined, and the real-time characterization of the resistivity of WO3 thin films during electrochromic process is realized. The experimental results show the resistivity of WO3 film can be regulated in the range of 0.078–0.98 Ω m with rapid response (∼6 s) when applied ±1 V voltage for 30 s. By analyzing the fitted resistivity-transmittance curve of WO3 thin film, a high-efficiency optical modulation of WO3 film in near-infrared 1000 nm can be achieved. Combining the IMT phenomenon during Li+ insertion, when x of LixWO3 changes from 0.08 to 0.13, the WO3 film is in a transition state from the insulating state to the metallic state with the tetragonal crystalline phase. The symmetry structural evolution of crystalline WO3 thin films during the Li + insertion is further demonstrated by transmission electron microscopy (TEM). These results contribute to the development of WO3 based optoelectronic bifunctional electrochromic devices.

Bilayered Dion–Jacobson Hybrid Perovskite Bulk Single Crystals Constructed with Aromatic Diammonium for Ultraviolet–Visible–Near-Infrared Photodetection

Two-dimensional (2D) Dion–Jacobson (DJ) hybrid perovskites (HPs) are widely used in the photodetection field due to their high stability and excellent carrier transport performance. However, current photodetectors based on DJ HPs can work only in the ultraviolet and visible wavebands, which limits their application in near-infrared (NIR) photodetection. Herein, for the first time, ultraviolet–visible–NIR photodetection is realized in the bulk single crystal of 2D bilayered DJ HP (3AMPY)(EA)Pb2Br7 (3AMPY = 3-(aminomethyl)pyridinium, EA = ethylamine). Structurally, the introduction of aromatic diammonium makes (3AMPY)(EA)Pb2Br7 have a shorter layer spacing (3.561 Å) and a more rigid structure. In addition, benefiting from the strong quantum and dielectric confinements of 2D HPs, (3AMPY)(EA)Pb2Br7 shows an excellent two-photon absorption (TPA) coefficient of up to 61.1 cm/MW under the excitation of an 800 nm femtosecond laser. Notably, (3AMPY)(EA)Pb2Br7 exhibits efficient photodetection from ultraviolet (377 nm) and visible (405 nm) to NIR (800 nm) wavebands, with on/off ratios of current higher than 103.

High-Density and Low-Crosstalk Multilayer Silicon Nitride Waveguide Superlattices With Air Gaps

High-density and low-crosstalk waveguide arrays are critical components in optical phased arrays (OPAs) and are widely used in solid-state light detection and ranging (LiDAR). In this work, silicon nitride waveguide superlattices with air gaps are proposed and analyzed theoretically. Mode analysis shows that the introduced air gaps beside the waveguide help shorten the skin depth of the evanescent field and increase the effective index range of the waveguide under single mode conditions. Lower crosstalk is demonstrated between a pair of waveguides with air gaps compared with those without air gaps. On this basis, a waveguide superlattice with air gaps is designed by combining the eigenmode expansion method and particle swarm optimization. The crosstalk of the waveguide superlattice is optimized to −24.3 dB when the waveguide pitch is 0.9 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu \text{m}$</tex-math></inline-formula> , the propagation length is 1 mm, and the wavelength is 905 nm. The crosstalk is still below −22.4 dB under the typical process variations and over the wavelength range of 890 nm–920 nm. Therefore, the proposed air-gap waveguide superlattices with high density and low crosstalk offer opportunities to improve the beam steering performance of OPA chips in the near-infrared (NIR) waveband.

Visualization and prediction of TVB-N content in chilled pork by hyperspectral imaging

TVB-N is one of the important indexes of the freshness of chilled pork. To present the freshness more accurately and completely, a visualization technology of chemical composition was applied and optimized for pork freshness detection. Two hyperspectral cameras covering visible and near-infrared wavebands were used to detect the TVB-N content. GA, SPA, iPLSR, and siPLSR were employed to select characteristic wavebands from the spectra data extracted from the hyperspectral images, and PLSR models were established on full wavebands and characteristic wavebands. According to the models established on the spectral data, TVB-N distribution maps were obtained, and URV and ICV were employed to evaluate the performance of distribution maps. The results showed that, both PLSR models established on the characteristic wavebands and full wavebands achieved good accuracy, and that the characteristic wavebands cluster was in the spectral range of 600-750 nm, 1020-1120 nm, and 1450-1570 nm. To optimize the performance of distribution maps, prediction models were reconstructed under the guidance of URV and ICV. This study provided a method of predicting TVB-N content of pork in both spectral and imaging aspect for online testing the freshness of chilled pork, which improves the efficiency and quality of testing.

Research on the detection of early caries based on hyperspectral imaging

Objective: We applied hyperspectral imaging (HSI) system to distinguish early caries from sound and pigmented areas. It will provide a theoretical basis and technical support, for research and development of an instrument that could be used for screening and detection of early dental caries. Methods: Eighteen extracted human teeth (molars and premolars), with varying degrees of natural pathology and no degree of decay involving dentin were obtained. HSI system with a wavelength range from 400 to 1000[Formula: see text]nm was used to obtain images of all 18 teeth containing sound, carious and pigmented areas. We compared the spectra of the wavebands at both 500[Formula: see text]nm and 780[Formula: see text]nm from the different tooth states, and the reflectance difference between sound versus carious lesions and sound versus pigmented areas, respectively. Results: There was a slight difference in reflectance between carious areas and pigmented areas at 500[Formula: see text]nm. A substantial difference was additionally noted in reflectance between carious areas and pigmented areas at 780[Formula: see text]nm. Conclusion: The results have shown that the interference of tooth surface pigment can be eliminated in the near-infrared (NIR) waveband, and the caries can be effectively identified from the pigmented areas. Thus, it could be used to detect carious areas of teeth in place of the traditional visual inspection method or white light endoscopy. Clinical significance: The NIR diffused light signal enables the identification of early caries from pigment and other interference, providing a reasonable detection tool for early detection and early treatment of teeth diseases.

Bionic coating imitating reflection characteristics of plant leaf in solar spectrum waveband for hyperspectral camouflage

Imitating the reflection characteristics of plant leaves in solar spectrum waveband with bionic coatings can effectively counter the hyperspectral detection technology in the vegetation background. Herein, inspired by the formation mechanisms of the reflection characteristics of plant leaves, a bionic coating containing chromium oxide (Cr2O3) particles and a hydrophilic inorganic salt dispersed in a polyvinyl alcohol (PVA) matrix is prepared on metal plates, and a theoretical model was constructed to clarify the bionic mechanisms. Results show that the Cr2O3 particles exhibit absorption characteristics similar to those of chlorophyll and scattering characteristics similar to those of multilayer cells in plant leaves, thereby enabling the bionic coating to imitate the reflection characteristics in the visible and near-infrared waveband. Meanwhile, the hydrophilic inorganic salt can absorb moisture molecules in the air when the bionic coating is placed in natural environments, thereby absorbing incident radiation and imitating the water absorption valleys. Moreover, it was found that there is only non-freezing bound water in the bionic coating with a differential scanning calorimeter, thus the moisture absorption processes associated with the hydrophilic hydroxyl groups in PVA matrix were considered when determining the mass transfer characteristics in a natural environment. In the whole solar spectrum, the bionic coating can significantly suppress the high reflectance of the metal plates when the volume fraction of Cr2O3 particles is 0.81 % and the coating thickness is greater than 0.2 mm. Increasing the surface roughness of the metal plates can further improve the suppressing performance in the range from 800 to 1300 nm, achieving a better similarity of the reflection curves between the bionic coating and the plant leaves. The outstanding imitation performance of the key reflection characteristics of the plant leaves confirms the remarkable application potential of the bionic coating in the field of hyperspectral camouflage.