Nonlinear vectorial holography with quad-atom metasurfaces.

Vectorial optical holography represents a solution to control the polarization and amplitude distribution of light in the Fourier space. While vectorial optical holography has been experimentally demonstrated in the linear optical regime, its nonlinear counterpart, which can provide extra degrees of freedom of light-field manipulation through the frequency conversion processes, remains unexplored. Here, we experimentally demonstrate the nonlinear vectorial holography through the second harmonic generation process on a quad-atom plasmonic metasurface. The quad-atom metasurface consists of gold meta-atoms with threefold rotational symmetry. Based on the concept of nonlinear geometric phase, we can simultaneously manipulate the phase and amplitude of the left and right circularly polarized second harmonic waves generated from the quad-atom metasurface. By superposing the two orthogonal polarization components, the quad-atom metasurface can produce nonlinear holographic images with vectorial polarization distributions. The proposed metasurface platform may have important applications in vectorial polarization nonlinear optical source, high-capacity optical information storage, and optical encryption.

Twist-Angle-Controlled Nonlinear Circular Dichroism on Gold-Crystal Hybrid Metasurfaces.

Optical chirality, which plays important roles in liquid crystal display and biological and chemical detection, has been attracting scientists' attention due to its potential applications in optical information processing. Usually, the chiral optical response of natural molecules is very weak. However, the emergence of metasurfaces offers a promising solution to solve this issue. By judiciously designing the geometry of meta-atoms, we have realized strong optical circular dichroism (CD) in both linear and nonlinear optical regimes. However, tuning of the CD with a metasurface remains challenging. Here, we propose the twist-angle-controlled nonlinear CD effect by using the second-harmonic generation process on a gold-crystal hybrid metasurface. The CD effect of the second-harmonic waves can be tuned well by controlling the twist angle between the two constituent materials. The proposed hybrid metasurface may open new avenues for developing ultracompact and multifunctional nonlinear optical devices.

Time-Dependent Ultrafast Quadratic Nonlinearity in an Epsilon-Near-Zero Platform.

Ultrafast nonlinearity, which results in modulation of the linear optical response, is a basis for the development of time-varying media, in particular those operating in the epsilon-near-zero (ENZ) regime. Here, we demonstrate that the intraband excitation of hot electrons in the ENZ film results in a second-harmonic resonance shift of ∼10 THz (40 nm) and second-harmonic generation (SHG) intensity changes of >100% with only minor (<1%) changes in linear transmission. The modulation is 10-fold enhanced by a plasmonic metasurface coupled to a film, allowing for ultrafast modulation of circularly polarized SHG. The effect is described by the plasma frequency renormalization in the ENZ material and the modification of the electron damping, with a possible influence of the hot-electron dynamics on the quadratic susceptibility. The results elucidate the nature of the second-order nonlinearity in ENZ materials and pave the way to the rational engineering of active nonlinear metamaterials and metasurfaces for time-varying applications.

Spin Unlocked Vortex Beam Generation on Nonlinear Chiroptical Metasurfaces.

Optical vortices with spin and orbital angular momentum (SAM and OAM) states offer multiple degrees of freedom for manipulating optical fields and thus enable great potentials in optical information processing. Recently, the optical metasurface has become an important platform for vortex beam generation and steering. However, the strong spin-orbit interaction on such metasurfaces usually leads to spin locked OAM generation, which limits the complete control of the angular momentum state of light. Here, we propose to solve this constraint using geometric phase controlled nonlinear chiroptical metasurfaces. The metasurface consists of two types of plasmonic meta-atoms which have opposite handedness and exhibit a strong spin-dependent circular dichroism effect. By encoding specific phase singularities and phase gradients to different channels, we experimentally demonstrate the spin unlocked second harmonic beam steering. The proposed nonlinear chiroptical metasurfaces may have important applications in developing multifunctional nonlinear optical devices.

A Centimeter-Scale Dielectric Metasurface for the Generation of Cold Atoms.

The single-beam magneto-optical trap (MOT) based on the diffractive optical element offers a new route to develop compact cold atom sources. However, the optical efficiency in the previous single-beam MOT systems is usually low and unbalanced, which will affect the quality of the trapped atoms. To solve this issue, we developed a centimeter-scale dielectric metasurface optical chip with dynamic phase distributions, which was used to split a single incident laser beam into five separate ones with well-defined polarization states and uniform energy distributions. The measured diffraction efficiency of the metasurface is up to 47%. A single-beam MOT integrated with the metasurface optical chip was then used to trap the 87Rb atoms with numbers ∼1.4 × 108 and temperatures ∼7.0 µK. The proposed concept in this work may provide a promising solution for developing ultracompact cold atom sources.

Microbiome predators in changing soils.

Microbiome predators shape the soil microbiome and thereby soil functions. However, this knowledge has been obtained from small-scale observations in fundamental rather than applied settings and has focused on a few species under ambient conditions. Therefore, there are several unaddressed questions on soil microbiome predators: (1) What is the role of microbiome predators in soil functioning? (2) How does global change affect microbiome predators and their functions? (3) How can microbiome predators be applied in agriculture? We show that there is sufficient evidence for the vital role of microbiome predators in soils and stress that global changes impact their functions, something that urgently needs to be addressed to better understand soil functioning as a whole. We are convinced that there is a potential for the application of microbiome predators in agricultural settings, as they may help to sustainably increase plant growth. Therefore, we plea for more applied research on microbiome predators.

The A1762T/G1764A mutations enhance HBV replication by alternating viral transcriptome.

The A1762T/G1764A mutations, one of the most common mutations in the hepatitis B virus basal core promoter, are associated with the progression of chronic HBV infection. However, effects of these mutations on HBV replication remains controversial. This study aimed to systematically investigate the effect of the mutations on HBV replication and its underlying mechanisms. Using the prcccDNA/pCMV-Cre recombinant plasmid system, a prcccDNA-A1762T/G1764A mutant plasmid was constructed. Compared with wild-type HBV, A1762T/G1764A mutant HBV showed enhanced replication ability with higher secreted HBV DNA and RNA levels, while Southern and Northern blot indicated higher intracellular levels of relaxed circular DNA, single-stranded DNA, and 3.5 kb RNA. Meanwhile, the mutations increased expression of intracellular core protein and decreased the production of HBeAg and HBsAg. In vitro infection based on HepG2-NTCP cells and mice hydrodynamic injection experiment also proved that these mutations promote HBV replication. 5'-RACE assays showed that these mutations upregulated transcription of pregenomic RNA (pgRNA) while downregulating that of preC RNA, which was further confirmed by full-length transcriptome sequencing. Moreover, a proportion of sub-pgRNAs with the potential to express polymerase were also upregulated by these mutations. The ChIP-qPCR assay showed that A1762T/G1764A mutations created a functional HNF1α binding site in the BCP region, and its overexpression enhanced the effect of A1762T/G1764A mutations on HBV. Our findings revealed the mechanism and importance of A1762T/G1764A mutations as an indicator for management of CHB patients, and provided HNF1α as a new target for curing HBV-infected patients.

Nematode biomass changes along an elevational gradient are trophic group dependent but independent of body size.

Aboveground, large and higher trophic-level organisms often respond more strongly to environmental changes than small and lower trophic-level organisms. However, whether this trophic or size-dependent sensitivity also applies to the most abundant animals, microscopic soil-borne nematodes, remains largely unknown. Here, we sampled an altitudinal transect across the Tibetan Plateau and applied a community-weighted mean (CWM) approach to test how differences in climatic and edaphic properties affect nematode CWM biomass at the level of community, trophic group and taxon mean biomass within trophic groups. We found that climatic and edaphic properties, particularly soil water-related properties, positively affected nematode CWM biomass, with no overall impact of altitude on nematode CWM biomass. Higher trophic-level omnivorous and predatory nematodes responded more strongly to climatic and edaphic properties, particularly to temperature, soil pH, and soil water content than lower trophic-level bacterivorous and fungivorous nematodes. However, these differences were likely not (only) driven by size, as we did not observe significant interactions between climatic and edaphic properties and mean biomasses within trophic groups. Together, our research implies a stronger, size-independent trophic sensitivity of higher trophic-level nematodes compared with lower trophic-level ones. Therefore, our findings provide new insights into the mechanisms underlying nematode body size structure in alpine grasslands and highlight that traits independent of size need to be found to explain increased sensitivity of higher trophic-level nematodes to climatic and edaphic properties, which might affect soil functioning.

Cationic Gelatin Cross-Linked with Transglutaminase and Its Electrospinning in Aqueous Solution.

Gelatin (GE) is a renewable biopolymer with abundant active groups that are beneficial for manufacturing functional biomaterials via GE modification. An antibacterial fibrous GE film was prepared by electrospinning the modified GE in an aqueous solution. The original GE was modified by reacting it with N,N-dimethyl epoxypropyl octadecyl ammonium chloride (QAS), and then it was cross-linked with transglutaminase (TGase). FTIR analysis illustrated that QAS was grafted onto GE through the epoxy ring-opening reaction, and the modification did not influence the main GE skeleton structure. The investigation of the solution properties showed that the grafted cationic QAS group was the main factor that decreased the surface tension of the solution, increased the electrical conductivity of the solution, and endowed GE with antibacterial activity. TGase cross-linking clearly influenced the rheological properties such that the flow pattern of the spinning solution varied from Newton-type to shear thinning, and the aqueous solution of GE-QAS-TGs transformed from liquid-like to solid-like and even induced gelatinization with increasing TGase content. A satisfactory fibrous morphology of 200-500 nm diameter was obtained using a homemade instrument under the optimized electrospinning conditions of a temperature of 35 °C, a distance between electrodes of 12 cm, and a voltage of 15 kV. The study of film properties showed that the antibacterial activity of the fibrous GE film depended only on the grafted quaternary ammonium, whereas the thermostability, wettability, and permeability were greatly influenced by both the TGase cross-linking and film-forming methods. Cytotoxicity was tested using the CCK-8 and live/dead kit staining methods in vitro, which showed that the modified GE had good biocompatibility.

Terahertz Metagrating Emitters with Beam Steering and Full Linear Polarization Control.

We report the realization of broadband THz plasmonic metagrating emitters for simultaneous beam steering and all-optical linear polarization control. Two types of metagratings are designed and experimentally demonstrated. First, the plasmonic meta-atoms are arranged in a metagrating with a binary phase modulation which results in the nonlinear generation of THz waves to the ±1 diffraction orders, with complete suppression of the zeroth order. Complete tunability of the diffracted THz linear polarization direction is demonstrated through simple rotation of the pump polarization. Then, the concept of lateral phase shift is introduced into the design of the metagratings using interlaced phase gradients. By controlling the spatial shift of the submetagrating, we are able to continuously control the linear polarization states of the generated THz waves. This method results in a higher nonlinear diffraction efficiency relative to binary phase modulation. These functional THz metagratings show exciting promise to meet the challenges associated with the current diverse array of applications utilizing THz technology.

Double exceptional points in grating coupled metal-insulator-metal heterostructure.

In this work we theoretically study the exceptional points and reflection spectra characteristics of a grating coupled metal-insulator-metal heterostructure, which is a non-Hermitian system. Our results show that by selecting suitable geometrical parameters with grating periodicity @150 nm, that satisfy zero reflection condition, double exceptional points appear in a mode bifurcation regime. Furthermore, the thickness of partition metal layer between two cavities plays an important role in controlling the reflection properties of the heterostructure. There is a clear mode splitting when the partition layer allows strong coupling between the two cavity modes. Conversely, in weak coupling regime the mode splitting becomes too close to be distinguished. Moreover, the vanishing of reflection leads to unidirectional reflectionless propagation, which is also known as unidirectional invisibility. With grating periodicity ≥400nm, the transmissions for forward and backward incident directions are no longer the same due to the generation of diffraction. High contrast ratio (≈1) between the two incident directions leads to asymmetric transmission. This work lays the basis for designing double exceptional points and asymmetric transmission in coupled non-Hermitian photonics system. The proposed heterostructure can be a good candidate for new generation optical communications, optical sensing, photo-detection, and nano-photonic devices.

Simple construction of a two-component fluorescent sensor for turn-on detection of Hg2+ in human serum.

The simply constructed fluorescent sensor with inexpensive reagents and low toxicity has attracted increasing attention contributing to its practical application. However, the common construction methods usually required a few building blocks and complex procedures, which is inconvenient for their further application. Herein, a simply constructed fluorescent Hg2+ sensor has been developed based on the intrinsic fluorescence quenching power of G-quadruplex. Two components, AGRO 100 and AMT, were used to construct the sensor. AMT was selected as the fluorescent probe because of its distinct merits. The free AMT emits strongly. However, the fluorescence of AMT could be quenched by G-quadruplex DNA. Additionally, AMT is less toxic and inexpensive. AGRO 100 acts as both the quencher and the capture sequence because it consists of G-rich sequences and T-T mismatched base pairs. The fluorescence of AMT could be quenched by the formed G-quadruplex structure of AGRO 100 in the presence of K+. In the presence of Hg2+, G-quadruplex structure of AGRO 100 was switched to hairpin DNA structure because T-T mismatched base pairs in AGRO 100 could specifically recognize and capture Hg2+ with high affinity. Thus, AMT was released and the fluorescence of AMT was recovered. The developed sensing system was successfully applied to detect Hg2+ in human serum with good recovery and reproducibility.

Blending Technology Based on HPLC Fingerprint and Nonlinear Programming to Control the Quality of Ginkgo Leaves.

The breadth and depth of traditional Chinese medicine (TCM) applications have been expanding in recent years, yet the problem of quality control has arisen in the application process. It is essential to design an algorithm to provide blending ratios that ensure a high overall product similarity to the target with controlled deviations in individual ingredient content. We developed a new blending algorithm and scheme by comparing different samples of ginkgo leaves. High-consistency samples were used to establish the blending target, and qualified samples were used for blending. Principal component analysis (PCA) was used as the sample screening method. A nonlinear programming algorithm was applied to calculate the blending ratio under different blending constraints. In one set of calculation experiments, the result was blended by the same samples under different conditions. Its relative deviation coefficients (RDCs) were controlled within ±10%. In another set of calculations, the RDCs of more component blending by different samples were controlled within ±20%. Finally, the near-critical calculation ratio was used for the actual experiments. The experimental results met the initial setting requirements. The results show that our algorithm can flexibly control the content of TCMs. The quality control of the production process of TCMs was achieved by improving the content stability of raw materials using blending. The algorithm provides a groundbreaking idea for quality control of TCMs.

High-Order Nonlinear Spin-Orbit Interaction on Plasmonic Metasurfaces.

In linear optics, the angular momentum of light can be easily manipulated through the optical spin-orbit interaction (SOI) in structured media such as liquid crystals, metasurfaces, and forked gratings. Similarly, metasurfaces can be used to generate nonlinear optical beams with both custom-defined spin angular momentum (SAM) and orbital angular momentum (OAM) states. However, it has been limited to a low-order process in which only a Gaussian-shaped fundamental wave is used. In this work, the high-order nonlinear optical SOI effect on metasurfaces is demonstrated through the generation of multiple angular momentum states in nonlinear waves. This is achieved by exploiting the degrees of freedom provided by both the SAM and the OAM states of the fundamental wave (FW) and the topological charges of the plasmonic metasurfaces. The mechanism of both intrinsic and extrinsic contributions to the OAM of the nonlinear waves is revealed. High-order nonlinear SOI on metasurfaces offers new opportunities for realizing ultracompact nonlinear vortex beams.

3D Full-Color Image Projection Based on Reflective Metasurfaces under Incoherent Illumination.

Metasurfaces provide an efficient approach to control light wavefronts and have emerged at the forefront of digital holography. Nevertheless, full-color image projection remains challenging. Using a combination of specular and diffuse reflections from a metasurface, in analogy to the normal mapping technique, we designed a reflective metasurface performing in the whole visible spectral range to demonstrate 2D images with shading effects of 3D objects. The noninterleaved metasurface is based on aluminum nanostructures with high and relatively uniform efficiency across the visible spectrum. It operates under incoherent illumination and does not require polarizing optics to observe images. The integration of the metasurface behind pre-existing transparent color images is also demonstrated for introduction of 3D effects. Emulating color 3D images with flat metasurfaces can be useful for security applications and decorative purposes. The design of broadband metasurface diffusers is also interesting for flat optical diffusing elements with engineered properties and display technology.

Giant Enhancement of Second-Order Nonlinearity of Epsilon-near- Zero Medium by a Plasmonic Metasurface.

Nonlinear frequency conversion at the nanoscale is important for many applications in free space and integrated photonics. In epsilon-near-zero (ENZ) materials, second-harmonic generation (SHG) is significantly enhanced but the oblique incidence is required to address nonlinearity. To circumvent this constraint, we design a hybrid metasurface consisting of plasmonic nanostructures on an ENZ nanofilm generating strongly enhanced SHG at normal incidence in transmission. We show that the Au meta-atoms on an indium-tin-oxide (ITO) layer provide an approximately 104-fold experimentally measured SHG enhancement at normal incidence at the fundamental wavelength near the ENZ condition of ITO. This giant enhancement stems from reshaping the vectorial properties of the incident light near the Au nanostructures and its increased coupling to the ENZ film. The proposed hybrid ENZ metasurface offers a promising platform for developing ultracompact and efficient nonlinear optical sources at the nanoscale.

Nonlinear Diatomic Metasurface for Real and Fourier Space Image Encoding.

In linear optics, the metasurface represents an ideal platform for encoding optical information because of its unprecedented abilities of manipulating the intensity, polarization, and phase of light wave with subwavelength meta-atoms. However, controlling various degrees of freedom of light in nonlinear optics remains elusive. Here, we propose a nonlinear plasmonic metasurface working in the near-infrared regime that can simultaneously encode optical images in the real and Fourier spaces. This is achieved by designing a diatomic meta-molecule, which enables the independent control of the nonlinear geometric phase, polarization, and intensity of second harmonic waves. The proposed nonlinear diatomic metasurface provides an ultracompact platform for implementing multidimensional optical information encoding and may hold great potential in optical information security and optical anticounterfeiting.

Moiré Fringe Induced Gauge Field in Photonics.

We realize moiré fringe induced gauge field in a double-layer photonic honeycomb metacrystal with mismatched lattice constants. Benefitting from the generated strong effective gauge field, we report direct measurement of the band diagrams of both Landau level flat bands and intermagnetic-domain edge states. Importantly, we observe the correlation between the momentum and orbital position of the Landau modes, serving as an evidence of the noncommuteness between orthogonal components of the momentum. Without complicated time driving mechanics and careful site-by-site engineering, moiré superlattices could emerge as a powerful means to generate effective gauge fields for photonics benefiting from its simplicity and reconfigurability, which can be applied to nonlinearity enhancement and lasing applications at optical frequencies.

Strong Nonlinear Optical Activity Induced by Lattice Surface Modes on Plasmonic Metasurface.

Optical metasurfaces, consisting of spatially variant meta-atoms, represent a new kind of optical platform for controlling the wavefront of light, with which many interesting applications, such as metalens and optical holography, have been successfully demonstrated. Further extension of the optical functionalities of metasurfaces into the nonlinear optical regime has led to unprecedented control over the local optical nonlinear generation processes. It has been shown that the nonlinear optical metasurface with achiral geometries could exhibit intrinsic optical activity in second- and third- harmonic generations. In this work, we propose an alternative approach for achieving strong nonlinear optical activity in achiral plasmonic metasurfaces by exploiting the lattice surface modes of plasmonic metasurfaces. Specifically, we theoretically and experimentally demonstrate the strong circular dichroism for second harmonic generation (SHG) on plasmonic metasurfaces consisting of split-ring resonator meta-atoms. The strong nonlinear circular dichroism is attributed to the contribution from lattice surface modes at fundamental wavelengths. Our findings may open new routes to design novel nonlinear optical devices with strong optical activity.

Reversible Image Contrast Manipulation with Thermally Tunable Dielectric Metasurfaces.

Increasing demand for higher resolution of miniaturized displays requires techniques achieving high contrast tunability of the images. Employing metasurfaces for image contrast manipulation is a new and rapidly growing field of research aiming to address this need. Here, a new technique to achieve image tuning in a reversible fashion is demonstrated by dielectric metasurfaces composed of subwavelength resonators. It is demonstrated that by controlling the temperature of a metasurface the encoded transmission pattern can be tuned. To this end, two sets of nanoresonators composed of nonconcentric silicon disks with a hole that exhibit spectrally sharp Fano resonances and forming a Yin-Yang pattern are designed and fabricated. Through exploitation of the thermo-optical properties of silicon, full control of the contrast of the Yin-Yang image is demonstrated by altering the metasurface temperature by ΔT ≈ 100 °C. This is the first demonstrated technique to control an image contrast by temperature. Importantly, the turning technique does not require manipulating the external stimulus, such as polarization or angle of the illumination and/or the refractive index of this environment. These results open many opportunities for transparent displays, optical switches, and tunable illumination systems.