A multi-dimensional photodetector based on an α-MoO3 grating and graphene.

Integration of multi-dimensional optical information enhances the recognition and anti-interference capabilities of the detection system, allowing for better adaptation to complex environments. Therefore, this technology represents a crucial developmental pathway for the future of infrared optical detectors. In this study, a dual-band polarization photodetector based on a two-dimensional α-MoO3 grating structure is proposed. The structure utilizes the special dispersion property of the α-MoO3 material to excite the localized plasmon resonance, which generates narrowband high absorption peaks with Q-factors as high as 110.24 and 92.65, with peaks close to 1 under TM and TE polarized waves, respectively. The direct measurement of multi-dimensional optical information including intensity, spectrum and polarization states is realized. By adjusting the structural parameters, polarization-dependent dual-band detection can be achieved. Meanwhile, the introduction of graphene material realizes the electronically tunable function of the device. This study provides unexplored strategies for realizing more efficient, flexible and versatile dual-band polarization wave detection.

A polarization-dependent perfect absorber with high Q-factors enabled by Tamm phonon polaritons in hyperbolic materials.

As a natural biaxial hyperbolic material, α-phase molybdenum trioxide (α-MoO3) is highly anisotropic, making it an ideal candidate for polarization-dependent devices. Herein, using a Tamm configuration where one-dimensional photonic crystal is coated on an α-MoO3 substrate separated by a dielectric interlayer, we demonstrate the perfect absorption effect in the mid-infrared band governed by Tamm phonon polaritons. The resultant absorption peak exhibits an ultra-narrow bandwidth due to the polaritonic resonance with a high quality factor of up to 181. By varying the thickness of the interlayer, we demonstrate that near-unity absorption resonances can be tuned to a wider range of wavelengths. In addition, due to the in-plane anisotropy of α-MoO3, the device exhibits an outstanding polarization-dependent absorption performance, rendering it highly useful for various applications. Also, we show that the electronic tunability of the device is through addition of a graphene monolayer. These excellent results suggest that the designed structure could be promising in applications such as infrared absorbers, polarization detectors, sensors and energy harvesting devices.

Enhanced chirality induced in a composite structure consisting of α-MoO3 film and a silver metasurface.

Chiral structures have been widely used in many fields, such as biosensing and analytical chemistry. In this paper, the chiral response of a composite structure consisting of α-M o O 3 film and a silver (Ag) metasurface is studied. First, the effect of the thickness of α-M o O 3 film on the circular dichroism (CD) is discussed, and it is found that CD can reach 0.93 at a wavelength of 9.6 µm when the thickness of α-M o O 3 film is 6.075 µm. To better understand the physical mechanism, we analyze the transverse electric and transverse magnetic wave components in the transmitted wave for the whole structure and each layer. One can see that the strong chirality of the structure is attributed to the polarization conversion of α-M o O 3 film and the selective transmissivity of Ag ribbons. In addition, the influence of the filling factor of the Ag ribbons on chirality is also studied. This work combines hyperbolic material α-M o O 3 with Ag ribbons to enhance CD. Also, it provides greater freedom in the tuning of chirality. We believe that this work not only deepens the understanding of the chiral response of anisotropic materials, but also gives promise for its applications in the fields of polarization optics and biosensing.

Gradient index effect assisted anisotropic broadband absorption in α-MoO3 metamaterial.

As an excellent natural hyperbolic material (HM), α-M o O 3 has a larger hyperbolic bandwidth and longer polariton lifetime than other HMs, which makes it an ideal candidate for broadband absorbers. In this work, we theoretically and numerically investigated the spectral absorption of an α-M o O 3 metamaterial using the gradient index effect. The results show that the absorber has an average spectral absorbance of 99.99% at 12.5-18 µm at transverse electric polarization. When the incident light is transverse magnetic polarization, the broadband absorption region of the absorber is blueshifted, and a similar strong absorption is achieved at 10.6-12.2 µm. By simplifying the geometric model of the absorber using equivalent medium theory, we find that the broadband absorption is caused by the refractive index matching of the metamaterial to the surrounding medium. The electric field and power dissipation density distributions of the metamaterial were calculated to clarify the location of the absorption. Moreover, the influence of geometric parameters of pyramid structure on broadband absorption performance was discussed. Finally, we investigated the effect of polarization angle on the spectral absorption of the α-M o O 3 metamaterial. This research contributes to developing broadband absorbers and related devices based on anisotropic materials, especially in solar thermal utilization and radiation cooling.

Transparent spacecraft smart thermal control device based on VO2 and hyperbolic metamaterials.

Effective spacecraft thermal control technologies are essential to avoid undesirable effects caused by extreme thermal conditions. In this paper, we demonstrate a transparent smart radiation device (TSRD) based on vanadium dioxide (VO2) and a hyperbolic metamaterial (HMM) structure. Using the topological transition property of HMM, high transmission in the visible band and high reflection in the infrared can be achieved simultaneously. The variable emission essentially originates from the phase change material VO2 film. Due to the high reflection of HMM in the infrared band, it can form Fabry-Pérot (FP) resonance with the VO2 film after adding the dielectric layer SiO2, which further enhances the emission modulation. Under optimized conditions, solar absorption can be reduced to 0.25, while emission modulation can reach 0.44 and visible transmission can be up to 0.7. It can be found that the TSRD can simultaneously achieve infrared variable emission, high visible transparency and low solar absorption. The HMM structure instead of traditional metal reflectors offers the possibility to achieve high transparency. In addition, the formation of FP resonance between the VO2 film and HMM structure is the key to achieving variable emission. We believe that this work can not only provide a new approach for the design of spacecraft smart thermal control devices, but also show great potential for application in spacecraft solar panels.

Comparative analysis of two models for the optical response of a thin slab: a film of finite thickness versus a surface current sheet.

An accurate description of the electromagnetic properties of materials is fundamental to optical and electric devices. As a current research hotspot, thin slabs generally are modeled as a film of finite thickness with a dielectric function. However, inspired by two-dimensional materials, thin slabs can be regarded as surface current sheets with conductivity. Due to the convenience of the latter in simplifying the calculations, it becomes increasingly significant to determine the equivalent conditions of the two models. In this work, we compare the differences between the thin film and surface current models in calculating the transmissivity, reflectivity, and absorptivity of a SiC film. For normal incidence, the difference between the calculations of the two models is only non-negligible when the thickness is large (500 nm), because of the invalidation of surface current models and the excitation of Fabry-Perot resonance. In particular, we derive analytical formulas for the relative error in transmittance versus phase difference, which can be used to predict the difference between the two models as a function of film thickness. For oblique incidence, the two models have significant differences in the vicinity of epsilon-near-zero (ENZ) frequency. The excitation of the Berreman leaky mode in a thin film model causes a narrow blank absorption peak close to the ENZ frequency. However, we found that the surface current model is unable to form this resonance mode and further demonstrate it theoretically. In addition, it is found that the two models are equivalent in the case of a transverse electric wave even though the incidence is oblique. This work can enhance the awareness of the light-matter interaction and open unprecedented avenues for designing ultrathin optical devices.

Spinning thermal radiation from twisted two different anisotropic materials.

Thermal radiation has applications in numerous fields, such as radiation cooling, thermal imaging, and thermal camouflage. Micro/nanostructures such as chiral metamaterials with polarization-dependent or symmetry-breaking properties can selectively emit circularly (spin) polarized polarization waves. In this paper, we propose and demonstrate the spinning thermal radiation from two twisted different anisotropic materials. Taking industrial polymer and biaxial hyperbolic material α-MoO3 as an example, it is found that broadband spinning thermal radiation can be obtained from 13 µm to 18 µm. The spin thermal radiation of the proposed twisted structure originates from the combined effect of polarization conversion of circularly polarized wave and selective absorption of linearly polarized wave by the top and bottom layers of anisotropic materials, respectively. Besides, the narrowband spinning thermal radiation with 0.9 circular dichroism is achieved at wavelength of 12.39 µm and 18.93 µm for finite thickness α-MoO3 due to the epsilon-near-zero mode, and the magnetic field distribution can confirm the phenomenon. This work achieves broadband and narrowband spin thermal radiation and significantly enhances circular dichroism, which may have applications in biological sensing and thermal detection.

Accurate Characterization of the Adhesive Layer Thickness of Ceramic Bonding Structures Using Terahertz Time-Domain Spectroscopy.

Ceramic adhesive structures have been increasingly used in aerospace applications. However, the peaks of the signal on the upper and lower surface of the adhesive layer are difficult to measure directly due to the thin thickness of the adhesive layer and the effect of the attenuation dispersion of the ceramic layer. Thus, the existing non-destructive testing techniques have been ineffective in detecting adhesive quality. In this paper, the thickness of the adhesive layer is measured using terahertz time-domain spectroscopy. A sparse deconvolution method is proposed for the terahertz time-domain spectral signal of ceramic adhesive structures with different adhesive layer thicknesses. The results show that the methods proposed in this paper can realize the separation of reflection signals for glue layers with a thickness of 0.20 mm. By comparing with a wavelet denoising method and a modified covariance method (AR/MCM), the effectiveness of the sparse deconvolution method in estimating the thickness of the glue layer is demonstrated. This work will provide the theoretical and experimental basis for using terahertz time-domain spectroscopy to detect the homogeneity of ceramic adhesive structures.

Metasurfaces Assisted Twisted α-MoO3 for Spinning Thermal Radiation.

Spinning thermal radiation has demonstrated applications in engineering, such as radiation detection and biosensing. In this paper, we propose a new spin thermal radiation emitter composed of the twisted bilayer α-MoO3 metasurface; in our study, it provided more degrees of freedom to control circular dichroism by artificially modifying the filling factor of the metasurface. In addition, circular dichroism was significantly enhanced by introducing a new degree of freedom (filling factor), with a value that could reach 0.9. Strong-spin thermal radiation resulted from the polarization conversion of circularly polarized waves using the α-MoO3 metasurface and selective transmission of linearly polarized waves by the substrate. This allowed for extra flexible control of spinning thermal radiation and significantly enhanced circular dichroism, which promises applications in biosensing and radiation detection. As a result of their unique properties, hyperbolic materials have applications not only in spin thermal radiation, but also in areas such as near-field thermal radiation. In this study, hyperbolic materials were combined with metasurfaces to offer a new idea regarding modulating near-field radiative heat transfer.

Tunable high-quality-factor absorption in a graphene monolayer based on quasi-bound states in the continuum.

A tunable graphene absorber, composed of a graphene monolayer and a substrate spaced by a subwavelength dielectric grating, is proposed and investigated. Strong light absorption in the graphene monolayer is achieved due to the formation of embedded optical quasi-bound states in the continuum in the subwavelength dielectric grating. The physical origin of the absorption with high quality factor is examined by investigating the electromagnetic field distributions. Interestingly, we found that the proposed absorber possesses high spatial directivity and performs similar to an antenna, which can also be utilized as a thermal emitter. Besides, the spectral position of the absorption peak can not only be adjusted by changing the geometrical parameters of dielectric grating, but it is also tunable by a small change in the Fermi level of the graphene sheet. This novel scheme to tune the absorption of graphene may find potential applications for the realization of ultrasensitive biosensors, photodetectors, and narrow-band filters.

Extremely broadband light absorption by bismuth-based metamaterials involving hybrid resonances.

An ultra-broadband polarization-insensitive perfect absorber for the 400-4000 nm spectral range is proposed and studied. The absorber is composed of a dielectric film and a phase change material film sandwiched between a bismuth square ring array and a continuous bismuth mirror. Greater than 94% absorptivity across the wavelength range of 400-4000 nm and an averaged absorptivity of about 97% can be achieved. The physical origin results from the mixed effect of guided mode resonance (GMR), cavity resonance (CR) and surface plasmon resonance (SPR). In addition, such a property is maintained excellent in a very large viewing angle range. Furthermore, the proposed scheme exhibits certain geometrical parameter tolerance, which is beneficial for practical fabrication with low cost. Finally, the potential application of the solar cells is investigated as an illustration. The designed metamaterial absorber will find promising applications in solar cells, thermo-photovoltaics and detection.

Super-resolution reconstruction of terahertz images based on a deep-learning network with a residual channel attention mechanism.

To date, the existing terahertz super-resolution reconstruction methods based on deep-learning networks have achieved noteworthy success. However, the terahertz image degradation process needs to fully consider the blur and noise of the high-frequency part of the image during the network training process, and cannot be replaced simply by interpolation, which has high complexity. The terahertz degradation model is systematically investigated, and effectively solves the above problems by introducing the remaining channel mechanism into the deep-learning network. On the one hand, an image degradation model suitable for the terahertz imaging process is adopted for the images in the training dataset, which improves the accuracy of network training. On the other hand, the residual channel attention mechanism is introduced to realize the adaptive adjustment of the dependence between network channels, which results in the network being more focused on the restoration of high-frequency information, thereby supporting the extraction of high-frequency edge details in the image. In addition, experimental results demonstrate that this method successfully improves the peak signal-to-noise ratios, and offers clearer edge details and a better overall reconstruction effect. We believe that this work may provide a new possibility to improve the resolution of terahertz images.

Defect Detection of Composite Material Terahertz Image Based on Faster Region-Convolutional Neural Networks.

Terahertz (THz) nondestructive testing (NDT) technology has been increasingly applied to the internal defect detection of composite materials. However, the THz image is affected by background noise and power limitation, leading to poor THz image quality. The recognition rate based on traditional machine vision algorithms is not high. The above methods are usually unable to determine surface defects in a timely and accurate manner. In this paper, we propose a method to detect the internal defects of composite materials by using terahertz images based on a faster region-convolutional neural networks (faster R-CNNs) algorithm. Terahertz images showing internal defects in composite materials are first acquired by a terahertz time-domain spectroscopy system. Then the terahertz images are filtered, the blurred images are removed, and the remaining images are enhanced with data and annotated with image defects to create a dataset consistent with the internal defects of the material. On the basis of the above work, an improved faster R-CNN algorithm is proposed in this paper. The network can detect various defects in THz images by changing the backbone network, optimising the training parameters, and improving the prior box algorithm to improve the detection accuracy and efficiency of the network. By taking the commonly used composite sandwich structure as a representative, a sample with typical defects is designed, and the image data are obtained through the test. Comparing the proposed method with other existing network methods, the former proves to have the advantages of a short training time and high detection accuracy. The results show that the mean average precision (mAP) without data enhancement reached 95.50%, and the mAP with data enhancement reached 98.35% and exceeded the error rate of human eye detection (5%). Compared with the original faster R-CNN algorithm of 84.39% and 85.12%, the improvement is 11.11% and 10.23%, respectively, which demonstrates superb feature extraction capability and reduces the occurrence of network errors and omissions.

Near-field radiative heat transfer between topological insulators via surface plasmon polaritons.

Recently, thanks to its excellent opto-electronic properties, two-dimension topological insulator not only has attracted broad interest in fields such as tunable detectors and nano-electronics but also shall yield more interesting prospect in thermal management, energy conversion, and so on. In this work, the excellent near-filed radiative heat transfer (NFRHT) resulting from monolayer topological insulator (Bi2Se3) is demonstrated. The NFRHT of this system is mainly dominated by the strong coupling effect of the surface plasmon polaritons (SPPs) between two Bi2Se3 sheets. Moreover, the non-monotonic dependence of the Fermi energy of Bi2Se3 on NFRHT is then discovered. It is indicated that the system can provide great thermal adjustability by controlling the Fermi energy, achieving a modulation factor of heat flux as high as 98.94%. Finally, the effect of substrate on the NFRHT is also explored. This work provides a promising pathway for the highly efficient thermal management.

Strong extrinsic chirality in biaxial hyperbolic material α-MoO3 with in-plane anisotropy.

Chirality has always been a hot research topic because it possesses potential applications in polarization optics, chemical and biosensing. In the previous works, intrinsic chirality has been extensively explored, but its development is limited due to the complexity in fabrication of chiral metamaterials. Therefore, there is an urgent need to simplify fabrication and design of compact devices with chiral response. Extrinsic chirality has shown great potential because it can be realized in nonchiral anisotropic planar structures with low-cost fabrication techniques. In this paper, the extrinsic chirality of biaxial hyperbolic material $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ with in-plane anisotropy has been investigated. By analyzing the effect of thickness of $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film, the angle of incidence, azimuth angle, and wavelength of incidence on the circular dichroism (CD), the maximum CD can reach 0.77. This strong extrinsic chirality of the $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film results from the mutual orientation of the $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film and the incident light. In addition, $\alpha {\text -}{\rm{Mo}}{{\rm{O}}_3}$ film can still maintain strong extrinsic chirality when the azimuthal angle ranges from approximately 20°-57° and the angle of incidence is from 55°-80°.

Virus-inspired surface-nanoengineered antimicrobial liposome: A potential system to simultaneously achieve high activity and selectivity.

Enveloped viruses such as SARS-CoV-2 frequently have a highly infectious nature and are considered effective natural delivery systems exhibiting high efficiency and specificity. Since simultaneously enhancing the activity and selectivity of lipopeptides is a seemingly unsolvable problem for conventional chemistry and pharmaceutical approaches, we present a biomimetic strategy to construct lipopeptide-based mimics of viral architectures and infections to enhance their antimicrobial efficacy while avoiding side effects. Herein, a surface-nanoengineered antimicrobial liposome (SNAL) is developed with the morphological features of enveloped viruses, including a moderate size range, lipid-based membrane structure, and highly lipopeptide-enriched bilayer surface. The SNAL possesses virus-like infection to bacterial cells, which can mediate high-efficiency and high-selectivity bacteria binding, rapidly attack and invade bacteria via plasma membrane fusion pathway, and induce a local "burst" release of lipopeptide to produce irreversible damage of cell membrane. Remarkably, viral mimics are effective against multiple pathogens with low minimum inhibitory concentrations (1.6-6.3 µg mL-1), high bactericidal efficiency of >99% within 2 h, >10-fold enhanced selectivity over free lipopeptide, 99.8% reduction in skin MRSA load after a single treatment, and negligible toxicity. This bioinspired design has significant potential to enhance the therapeutic efficacy of lipopeptides and may create new opportunities for designing next-generation antimicrobials.

Tailored core‒shell dual metal-organic frameworks as a versatile nanomotor for effective synergistic antitumor therapy.

Malignant tumor has become an urgent threat to global public healthcare. Because of the heterogeneity of tumor, single therapy presents great limitations while synergistic therapy is arousing much attention, which shows desperate need of intelligent carrier for co-delivery. A core‒shell dual metal-organic frameworks (MOFs) system was delicately designed in this study, which not only possessed the unique properties of both materials, but also provided two individual specific functional zones for co-drug delivery. Photosensitizer indocyanine green (ICG) and chemotherapeutic agent doxorubicin (DOX) were stepwisely encapsulated into the nanopores of MIL-88 core and ZIF-8 shell to construct a synergistic photothermal/photodynamic/chemotherapy nanoplatform. Except for efficient drug delivery, the MIL-88 could be functioned as a nanomotor to convert the excessive hydrogen peroxide at tumor microenvironment into adequate oxygen for photodynamic therapy. The DOX release from MIL-88-ICG@ZIF-8-DOX nanoparticles was triggered at tumor acidic microenvironment and further accelerated by near-infrared (NIR) light irradiation. The in vivo antitumor study showed superior synergistic antitumor effect by concentrating the nanoparticles into dissolving microneedles as compared to intravenous and intratumoral injection of nanoparticles, with a significantly higher inhibition rate. It is anticipated that the multi-model synergistic system based on dual-MOFs was promising for further biomedical application.

A homogenous nanoporous pulmonary drug delivery system based on metal-organic frameworks with fine aerosolization performance and good compatibility.

Pulmonary drug delivery has attracted increasing attention in biomedicine, and porous particles can effectively enhance the aerosolization performance and bioavailability of drugs. However, the existing methods for preparing porous particles using porogens have several drawbacks, such as the inhomogeneous and uncontrollable pores, drug leakage, and high risk of fragmentation. In this study, a series of cyclodextrin-based metal-organic framework (CD-MOF) particles containing homogenous nanopores were delicately engineered without porogens. Compared with commercial inhalation carrier, CD-MOF showed excellent aerosolization performance because of the homogenous nanoporous structure. The great biocompatibility of CD-MOF in pulmonary delivery was also confirmed by a series of experiments, including cytotoxicity assay, hemolysis ratio test, lung function evaluation, in vivo lung injury markers measurement, and histological analysis. The results of ex vivo fluorescence imaging showed the high deposition rate of CD-MOF in lungs. Therefore, all results demonstrated that CD-MOF was a promising carrier for pulmonary drug delivery. This study may throw light on the nanoporous particles for effective pulmonary administration.

Structural Superiority of Guanidinium-Rich, Four-Armed Copolypeptides: Role of Multiple Peptide-Membrane Interactions in Enhancing Bacterial Membrane Perturbation and Permeability.

The development of novel antimicrobials is a top priority to address the growing epidemic of multidrug-resistant pathogens. Since cationic nonamphiphilic star-shaped antimicrobials are promising molecular scaffolds that provide a high charge density in binding anionic bacterial bilayers, this research aimed to further increase their membrane perturbation capability by introducing guanidinium groups to the antimicrobials via enhancing membrane insertion. In particular, computational simulation and experimental investigations revealed that our designed guanidinium-rich alternating copolypeptide, four-armed poly(arginine-alt-glycine), can interact with both the headgroups and unsaturated tails of phospholipids in bacterial membranes through multiple interactions, including electrostatic, cation-π, and T-shaped π-π interactions, allowing it to penetrate deeper inside the biologically inaccessible high-energy barrier of the hydrophobic lipid bilayer interior to cause membrane permeabilization and precipitation of the bacterial cytoplasm. Furthermore, glycine was observed to have a unique effect in enhancing the performance of arginine-based copolypeptide. Four-armed poly(arginine-alt-glycine) exhibited broad-spectrum antimicrobial activity, high bactericidal efficiency, and negligible hemolysis. The in vivo antibacterial performance of the copolypeptide was superior to that of doxycycline in a mouse model of Pseudomonas aeruginosa skin infection, accompanied by negligible local and systemic toxicity. Our results demonstrate that this guanidinium-rich, nonamphiphilic, star-shaped structure may promote the development of next-generation antimicrobials.

Erratum: Wu, B.; et al. Metal-Organic Framework-Based Chemo-Photothermal Combinational System for Precise, Rapid, and Efficient Antibacterial Therapeutics. Pharmaceutics 2019, 11, 463.

The authors wish to make the following corrections to this paper [1]: the hematoxylin and eosin-stained images of kidney in the group of healthy tissue in Figure 8 of this work [1] inadvertently duplicated the kidney results of the PBS group.[...].