A combination weighting method for debris flow risk assessment based on t-distribution and linear programming optimization algorithm.

Debris flow risk assessment can provide some reference for debris flow prevention and control projects. In risk assessment, researchers often only focus on the impact of objective or subjective indicators. For this purpose, this paper proposed a weight calculation method based on t-distribution and linear programming optimization algorithm (LPOA). Taking 72 mudslides in Beichuan County as an example, this paper used analytic hierarchy process (AHP), entropy weight method (EWM) and variation coefficient method (VCM) to obtain the initial weights. Based on the initial weights, weight intervals with different confidence levels were obtained by t-distribution. Subsequently, the final weights were obtained by LOPA in the 90% confidence interval. Finally, the final weights were used to calculate the risk score for each debris flow, thus delineating the level of risk for each debris flow. The results showed that this paper's method can avoid overemphasizing the importance of a particular indicator compared to EWM and VCM. In contrast, EWM and VCM ignored the effect of debris flow frequency on debris flow risk. The assessment results showed that the 72 debris flows in Beichuan County were mainly dominated by moderate and light risks. Of these, there were 8 high risk debris flows, 24 medium risk debris flows, and 40 light risk debris flows. The excellent triggering conditions provide favorable conditions for the formation of high-risk debris flows. Slightly and moderate risk debris flows are mainly located on both sides of highways and rivers, still posing a minor threat to Beichuan County. The proposed fusion weighting method effectively avoids the limitations of single weight calculating method. Through comparison and data analysis, the rationality of the proposed method is verified, which can provide some reference for combination weighting method and debris flow risk assessment.

Schistosoma japonicum cystatin alleviates paraquat poisoning caused acute lung injury in mice through activating regulatory macrophages.

BACKGROUND: Paraquat (PQ) is a widely used herbicide that poisons human by accident or intentional ingestion. PQ poisoning causes systemic inflammatory response syndrome (SIRS) resulting in acute lung injury (ALI) with an extremely high mortality rate. Blood trematode Schistosoma japonicum-produced cystatin (Sj-Cys) is a strong immunomodulatory protein that has been experimentally used to treat inflammation related diseases. In this study, Sj-Cys recombinant protein (rSj-Cys) was used to treat PQ-induced lung injury and the immunological mechanism underlying the therapeutic effect was investigated. METHODS: PQ-induced acute lung injury mouse model was established by intraperitoneally injection of 20 mg/kg of paraquat. The poisoned mice were treated with rSj-Cys and the survival rate was observed up to 7 days compared with the group without treatment. The pathological changes of PQ-induced lung injury were observed by examining the histochemical sections of affected lung tissue and the wet to dry ratio of lung as a parameter for inflammation and edema. The levels of the inflammation related cytokines IL-6 and TNF-α and regulatory cytokines IL-10 and TGF-ß were measured in sera and in affected lung tissue using ELISA and their mRNA levels in lung tissue using RT-PCR. The macrophages expressing iNOS were determined as M1 and those expressing Arg-1 as M2 macrophages. The effect of rSj-Cys on the transformation of inflammatory M1 to regulatory M2 macrophages was measured in affected lung tissue in vivo (EKISA and RT-PCR) and in MH-S cell line in vitro (flow cytometry). The expression levels of TLR2 and MyD88 in affected lung tissue were also measured to determine their role in the therapy of rSj-Cys on PQ-induced lung injury. RESULT: We identified that treatment with rSj-Cys significantly improved the survival rate of mice with PQ-induced lung injury from 30 % (untreated) to 80 %, reduced the pathological damage of poisoning lung tissue, associated with significantly reduced levels of proinflammatory cytokines (IL-6 from 1490 to 590 pg/ml, TNF-α from 260 to 150 pg/ml) and increased regulatory cytokines (IL-10 from360 to 550 pg/ml, and TGF-ß from 220 to 410 pg/ml) in both sera (proteins) and affected lung tissue (proteins and mRNAs). The polarization of macrophages from M1to M2 type was found to be involved in the therapeutic effect of rSj-Cys on the PQ-induced acute lung injury, possibly through inhibiting TLR2/MyD88 signaling pathway. CONCLUSIONS: Our study demonstrated the therapeutic effect of rSj-Cys on PQ poisoning caused acute lung injury by inducing M2 macrophage polarization through inhibiting TLR2/MyD88 signaling pathway. The finding in this study provides an alternative approach for the treatment of PQ poisoning and other inflammatory diseases.

Out-of-plane Emission Dipole of Second Harmonic Generation in Odd- and Even-layered vdWs Janus Nb3SeI7.

Integration of atomically thin nonlinear optical (NLO) devices demands an out-of-plane (OP) emission dipole of second harmonic generation (SHG) to enhance the spontaneous emission for nanophotonics. However, the research on van der Waals (vdWs) materials with an OP emission dipole of SHG is still in its infancy. Here, by coupling back focal plane (BFP) imaging with numerical simulations and density functional theory (DFT) calculations, we demonstrate that vdWs Janus Nb3SeI7, ranging from bulk to the monolayer limit, exhibits a dominant OP emission dipole of SHG owing to the breaking of the OP symmetry. Explicitly, even-layered Nb3SeI7 with C6v symmetry is predicted to exhibit a pure OP emission dipole attributed to the only second-order susceptibility coefficient χzxx. Meanwhile, although odd-layered Nb3SeI7 with C3v symmetry has both OP and IP dipole components (χzxx and χyyy), the value of χzxx is 1 order of magnitude greater than that of χyyy, leading to an approximate OP emission dipole of SHG. Moreover, the crystal symmetry and OP emission dipole can be preserved under hydrostatic pressure, accompanied by the enhanced χzxx and the resulting 3-fold increase in SHG intensity. The reported stable OP dipole in 2D vdWs Nb3SeI7 can facilitate the rapid development of chip-integrated NLO devices.

Stability analysis of rainfall-induced landslide considering air resistance delay effect and lateral seepage.

Accumulation landslides are prone to occur during the continuous infiltration of heavy rainfall, which seriously threatens the lives and property safety of local residents. In this paper, based on the Green-Ampt (GA) infiltration model, a new slope rainfall infiltration function is derived by combining the effect of air resistance and lateral seepage of saturated zone. Considering that when the soil layer continues to infiltrate after the saturation zone is formed, the air involvement cannot be discharged in time, which delays the infiltration process. Therefore, the influence of air resistance factor in soil pores is added. According to the infiltration characteristics of finite long slope, the lateral seepage of saturated zone is introduced, which makes up for the deficiency that GA model is only applicable to infinite long slope. Finally, based on the seepage characteristics of the previous analysis, the overall shear strength criterion is used to evaluate the stability of the slope. The results show that the safety factor decreases slowly with the increase of size and is inversely correlated with the slope angle and initial moisture content. The time of infiltration at the same depth increases with the increase of size and slope angle, and is inversely correlated with the initial moisture content, but is less affected by rainfall intensity. By comparing with the results of experimental data and other methods, the results of the proposed method are more consistent with the experimental results than other methods.

Angular dependent terahertz emission from the interplay between nanocrystal diamond film and plasmonic metasurface.

Composite structures integrated with metasurfaces and nonlinear films have emerged as alternative candidates to enhance nonlinear response. The cooperative interaction between the two components is complicated. Herein, a split-ring resonator (SRR)-type metasurface was fabricated on a free-standing nanocrystal diamond (NCD) film utilizing electron beam lithography, electron beam evaporation, and a lift-off process. The terahertz (THz) radiation from the SRR-NCD under normal incidence originates from the high-order magnetic resonance of SRR because the NCD film cannot produce detectable THz radiation at this incident angle. As increasing the incident angle, the contribution of the THz radiation from the NCD film gradually increases until reaching 40° incident angle limitation. The results indicate that this angular-dependent THz radiation is induced by the interplay between the NCD film and SRR. This study offers a new approach to investigate nonlinear processes in composite structures.

High-Performance Flexible Broadband Photoelectrochemical Photodetector Based on Molybdenum Telluride.

Flexible broadband photodetectors are desired but challenging to be fabricated for next-generation wearable intelligent optoelectronic devices. Considering the narrow bandgap and strong light absorption, molybdenum telluride (MoTe2) based photoelectrochemical photodetectors are successfully assembled by liquid phase exfoliation accompanied with the electrophoretic deposited method. This MoTe2-based photodetector shows a broadband detection in ultraviolet-near-infrared band, long-term stability within 18000 s, and fast response in millisecond-level (response time≈19 ms, recovery time≈26 ms). More importantly, even though the MoTe2 photodetector is bent and twisted at a high degree for several hundred times, it still shows excellent flexibility with stable on-off switching characteristics. Additionally, this photodetector displays a good response for rotation angles in the range from 0° to 360°, and the extracted Iph maintain almost the same value approximately 0.97 µA cm-2, suggesting an omnidirectional detection capability. This work demonstrates the proposed flexible photoanode shows a great potential in future broadband omnidirectional detection systems.

Direct Z-Scheme Heterostructure of Vertically Oriented SnS2 Nanosheet on BiVO4 Nanoflower for Self-Powered Photodetectors and Water Splitting.

The construction of nanostructured Z-scheme heterostructure is a powerful strategy for realizing high-performance photoelectrochemical (PEC) devices such as self-powered photodetectors and water splitting. Considering the band structure and internal electric field direction, BiVO4 is a promising candidate to construct SnS2 -based heterostructure. Herein, the direct Z-scheme heterostructure of vertically oriented SnS2 nanosheet on BiVO4 nanoflower is rationally fabricated for efficient self-powered PEC photodetectors. The Z-scheme heterostructure is identified by ultraviolet photoelectron spectroscopy, photoluminescence spectroscopy, PEC measurement, and water splitting. The SnS2 /BiVO4 heterostructure shows a superior photodetection performance such as excellent photoresponsivity (10.43 mA W-1 ), fast response time (6 ms), and long-term stability. Additionally, by virtue of efficient Z-scheme charge transfer and unique light-trapping nanostructure, the SnS2 /BiVO4 heterostructure also displays a remarkable photocatalytic hydrogen production rate of 54.3 µmol cm-2 h-1 in Na2 SO3 electrolyte. Furthermore, the synergistic effect between photo-activation and bias voltage further improves the PEC hydrogen production rate of 360 µmol cm-2 h-1 at 0.8 V, which is an order of magnitude above the BiVO4 . The results provide useful inspiration for designing direct Z-scheme heterostructures with special nanostructured morphology to signally promote the performance of PEC devices.

Broadband All-Optical THz Modulator Based on Bi2Te3/Si Heterostructure Driven by UV-Visible Light.

All-optical terahertz (THz) modulators have received tremendous attention due to their significant role in developing future sixth-generation technology and all-optical networks. Herein, the THz modulation performance of the Bi2Te3/Si heterostructure is investigated via THz time-domain spectroscopy under the control of continuous wave lasers at 532 nm and 405 nm. Broadband-sensitive modulation is observed at 532 nm and 405 nm within the experimental frequency range from 0.8 to 2.4 THz. The modulation depth reaches 80% under the 532 nm laser illumination with a maximum power of 250 mW and 96% under 405 nm illumination with a high power of 550 mW. The mechanism of the largely enhanced modulation depth is attributed to the construction of a type-II Bi2Te3/Si heterostructure, which could promote photogenerated electron and hole separation and increase carrier density dramatically. This work proves that a high photon energy laser can also achieve high-efficiency modulation based on the Bi2Te3/Si heterostructure, and the UV-Visible control laser may be more suitable for designing advanced all-optical THz modulators with micro-level sizes.

A missense mutation of the WNK1 gene affects cold tolerance in Chinese domestic cattle.

Inclement weather conditions, especially cold stress, have threatened the cattle industry. Cattle exposed to cold environments for a longer time suffer developmental delay, immunity decline, and eventually death. WNK1 is a member of With-no-lysine kinases (WNKs), widely expressed in animal organs and tissues. WNK1 and WNK4 are expressed in adipose tissue, and WNK4 promotes adipogenesis. WNK1 does not directly affect adipogenesis but has been shown to promote WNK4 expression in several tissues or organs. One missense mutation NC_037346.1:g.107692244, A > G, rs208265410 in the WNK1 gene was detected from the database of bovine genomic variation (BGVD). Here, we collected 328 individuals of 17 breeds representing four groups of Chinese cattle, northern group cattle, southern group cattle, central group cattle, and special group cattle (Tibetan cattle). We also collected the temperature and humidity data records from their relative locations. The frequencies of the G allele in Chinese breeds increased from northern China to southern China, and the frequencies of the A allele showed an opposite trend. Our results indicate that the WNK1 gene might be a candidate gene marker associated with cold tolerance.

Direct Z-Scheme SnSe2/SnSe Heterostructure Passivated by Al2O3 for Highly Stable and Sensitive Photoelectrochemical Photodetectors.

To mimic the natural photosynthesis system, a Z-scheme heterostructure is proposed as a viable and effective strategy for efficient solar energy utilization such as photocatalysis and photoelectrochemical (PEC) water splitting due to the high carrier separation efficiency, fast charge transport, strong redox, and wide light absorption. However, it remains a huge challenge to form a direct Z-scheme heterostructure due to the internal electric-field restriction and vital band-alignment at the interface. Herein, the van der Waals heterostructure based on the allotrope SnSe2 and SnSe is designed and synthesized by a two-step vapor phase deposition method to overcome the limitation in the formation of the Z-scheme heterostructure for the first time. The Z-scheme heterostructure of SnSe2/SnSe is confirmed by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, PEC measurement, density functional theory calculations, and water splitting. Strikingly, the PEC photodetectors based on the Z-scheme heterostructure show a synergistic effect of superior stability from SnSe and fast photoresponse from SnSe2. As such, the SnSe2/SnSe Z-scheme heterostructure shows a good photodetection performance in the ultraviolet to visible wavelength range. Furthermore, the photodetector shows a faster response/recovery time of 13/14 ms, a higher photosensitivity of 529.13 µA/W, and a higher detectivity of 4.94 × 109 Jones at 475 nm compared with those of single components. Furthermore, the photodetection stability of the SnSe2/SnSe is also greatly improved by a-thin-Al2O3-layer passivation. The results imply the promising rational design of a direct Z-scheme heterostructure with efficient charge transfer for high performance of optoelectronic devices.

Bi2 Te3 /Bi2 Se3 /Bi2 S3 Cascade Heterostructure for Fast-Response and High-Photoresponsivity Photodetector and High-Efficiency Water Splitting with a Small Bias Voltage.

Large-scale multi-heterostructure and optimal band alignment are significantly challenging but vital for photoelectrochemical (PEC)-type photodetector and water splitting. Herein, the centimeter-scale bismuth chalcogenides-based cascade heterostructure is successfully synthesized by a sequential vapor phase deposition method. The multi-staggered band alignment of Bi2 Te3 /Bi2 Se3 /Bi2 S3 is optimized and verified by X-ray photoelectron spectroscopy. The PEC photodetectors based on these cascade heterostructures demonstrate the highest photoresponsivity (103 mA W-1 at -0.1 V and 3.5 mAW-1 at 0 V under 475 nm light excitation) among the previous reports based on two-dimensional materials and related heterostructures. Furthermore, the photodetectors display a fast response (≈8 ms), a high detectivity (8.96 × 109 Jones), a high external quantum efficiency (26.17%), and a high incident photon-to-current efficiency (27.04%) at 475 nm. Due to the rapid charge transport and efficient light absorption, the Bi2 Te3 /Bi2 Se3 /Bi2 S3 cascade heterostructure demonstrates a highly efficient hydrogen production rate (≈0.416 mmol cm-2  h-1 and ≈14.320 µmol cm-2  h-1 with or without sacrificial agent, respectively), which is far superior to those of pure bismuth chalcogenides and its type-II heterostructures. The large-scale cascade heterostructure offers an innovative method to improve the performance of optoelectronic devices in the future.

Vertically oriented SnS2 on MoS2 nanosheets for high-photoresponsivity and fast-response self-powered photoelectrochemical photodetectors.

Van der Waals heterostructures have great potential for the emerging self-powered photoelectrochemical photodetectors due to their outstanding photoelectric conversion capability and efficient interfacial carrier transportation. By considering the band alignment, structural design, and growth optimization, the heterostructures of vertically oriented SnS2 with different densities on MoS2 nanosheets are designed and fabricated using a two-step epitaxial growth method. Compared with SnS2, MoS2, and low density-vertical SnS2/MoS2 heterostructure, the high density-vertical SnS2/MoS2 heterostructure exhibits largely enhanced self-powered photodetection performances, such as a giant photocurrent density (∼932.8 µA cm-2), an excellent photoresponsivity (4.66 mA W-1), and an ultrafast response/recovery time (3.6/6.4 ms) in the ultraviolet-visible range. This impressive enhancement of high density-vertical SnS2/MoS2 photodetectors is mainly ascribed to the essentially improved charge transfer and carrier transport of type-II band alignment heterostructures and the efficient light absorption from the unique light-trapping structure. In addition, the photoelectrocatalytic water splitting performance of the high density-vertical SnS2/MoS2 heterostructure also benefits from the type-II band alignment and the light-trapping structure. This work provides valuable inspiration for the design of two-dimensional optoelectronic and photoelectrochemical devices with improved performance by the morphology and heterostructure design.

Sb2S3/Sb2Se3 heterojunction for high-performance photodetection and hydrogen production.

Photoelectrochemical (PEC)-type devices provide promising ways for harvesting solar energy and converting it to electric and chemical energy with a low-cost and simple manufacturing process. However, the high light absorption, fast carrier separation, and low carrier recombination are still great challenges in reaching high performance for PEC devices. As emergent two-dimensional (2D) materials, Sb2Se3 and Sb2S3 exhibit desirable photoelectric properties due to the narrow bandgap, large optical absorption, and high carrier mobility. Herein, Sb2S3/Sb2Se3 heterojunction is synthesized by a two-step physical vapor deposition method. The type-II Sb2S3/Sb2Se3 heterojunction displays excellentphotoelectric properties such as a high photocurrent density (Iph âˆ¼ 162 µA cm-2), a high photoresponsivity (Rph âˆ¼ 3700 µA W-1), and a fast time response speed (rising time ∼ 2 ms and falling time ∼ 4.5 ms) even in harsh environment (H2SO4 electrolyte). Especially, the Sb2S3/Sb2Se3 shows an excellent self-powered photoresponse (Iph âˆ¼ 40 µA cm-2, Rph âˆ¼ 850 µA W-1). This increment is attributed to the improvement in light absorption, charge separation, and charge transfer efficiency. Taking these advantages, the Sb2S3/Sb2Se3 heterojunction also exhibits higher PEC water splitting synergically, which is approximately 3 times larger than that of Sb2Se3 and Sb2S3. These results pave the way for high-performance PEC devices by integrating 2D narrow bandgap semiconductors.

Enhanced UV-Vis photodetector performance by optimizing interfacial charge transportation in the heterostructure by SnS and SnSe2.

Optimizing interfacial charge transfer in type-II heterostructures, is one promising solution to improve efficiency of the solar energy conversion in photodetectors and solar cells. Herein, the SnS/SnSe2/ITO and SnSe2/SnS/ITO heterostructures are prepared by two-step physical vapor epitaxial growth. X-ray photoelectron spectroscopy confirms the SnS/SnSe2 heterostructure belongs to type-II band-alignment. The SnS/SnSe2 based photodetector shows higher photoresponsivity, which is approximately 2, 9, and 14 times larger than that of SnSe2/SnS, SnSe2, and SnS, respectively. The improvement of SnS/SnSe2 in photoelectric response mainly comes from high light harvesting and efficient charge transportation than individual SnSe2 and SnS, which is verified by UV-Vis absorption spectra. Electrochemical impedance spectroscopy, open circuit potentials, and Mott-Schottky characterization results further confirm that the better photodetection performance of SnS/SnSe2/ITO than that of SnSe2/SnS/ITO heterostructure is from the appropriate energy level cascade facilitating electron transport. These results provide an effective way to further improve the performance of heterostructure-based optoelectronic devices by an appropriate interface design.

Coherent injection photocurrent in bismuth sulfide film induced by one-plus-two photon absorption quantum interference.

Quantum interference (QuI) effect is a powerful method to generate and control the ultrafast photocurrent in semiconductors. We utilize two-color pulsed light excitation in bismuth sulfide (Bi2S3) film to induce the photocurrent through the QuI effect. Experimentally, the photocurrent is indirectly monitored using a standard terahertz (THz) time-domain spectroscopic system. Due to the QuI, an asymmetric photon injection occurs in Bi2S3 film, resulting in coherent injection current and subsequently THz wave generation. Our results on the pump pulse energy dependence of the THz electric field suggests that the THz wave generation process follows the third-order nonlinear optical process.

Azimuth-Resolved Circular Dichroism of Metamaterials.

Chiral optical metamaterials have attracted a great deal of attention due to their intriguing properties with respect to fundamental research and practical applications. For metamaterials with achiral structures, the system composed of metamaterials and obliquely incident light has extrinsic chirality and can produce circular dichroism (CD) effect. However, there have been few studies on the azimuth-dependent CD spectra of achiral metamaterials that have greatly improved our understanding of optical phenomena caused by external chirality. In this work, we experimentally studied the azimuth-dependent CD that originated from the extrinsic chirality of the metamaterials in an asymmetric-U shape and a U-bar-shape gold unit structure, separately. We explain the origin of the CD in the coupling of the macro-electric dipole and magnetic dipole, and the simulation results are in good agreement with the experiment. Our results provide a possible way to build an on-chip azimuth sensor based on azimuth-dependent CD spectra of metamaterials.

Anomalous polarization pattern evolution of Raman modes in few-layer ReS2 by angle-resolved polarized Raman spectroscopy.

Low-symmetry of ReS2 has not only in-plane but also out-of-plane anisotropic light scattering, which is complicated, yet interesting with intrinsic strong electron-phonon coupling. In such a case, the Raman tensor also gets sophisticated with nine non-zero elements, which is layer-dependent for different Raman modes. Herein, we systematically investigated the polarization pattern evolution of both in-plane and out-of-plane Raman modes of few-layer ReS2 by angle-resolved polarized Raman spectroscopy. We found that in-plane Raman modes with less layer-dependence could be used to determine the crystal orientation (Re-chain direction) due to the weak electron-phonon interaction between layers. However, the out-of-plane and mixed vibration Raman modes demonstrate much evident layer-dependence due to the obvious electron-phonon interaction between layers. As such, the polarization patterns for the out-of-plane vibration Raman modes are distorted with layers in not only petal types but also maximum Raman intensity directions. This distortion is mainly due to the phase difference between Raman elements, which are complex values due to the near bandgap excitation laser. The results reveal that deep insights into anisotropy in low-symmetry two-dimensional materials could afford not only rich physics but also potential polarized optoelectronic devices.

Anisotropic Second-Harmonic Generation Induced by Reduction of In-Plane Symmetry in 2D Materials with Strain Engineering.

Strain engineering is an attractive method to induce and control anisotropy for polarized optoelectronic applications with two-dimensional (2D) materials. Herein, we have investigated the nonlinear optical coefficient dispersion relationship and the second-harmonic generation (SHG) pattern evolution under the uniaxial strains for graphene, WS2, GaSe, and In2Se3 monolayers. The uniaxial strain can break the in-plane symmetry of 2D materials, leading to both trade-off breaking of the nonlinear coefficient and new emergent nonlinear coefficients. In such a case, a classical sixfold ϕ-dependent SHG pattern is transformed into a distorted sixfold SHG pattern under the strain. Due to the lattice symmetry breaking and the uneven charge density distribution in strained 2D materials, the SHG patterns also depend on the excitation photon energy. The results could give a guide for the SHG pattern analysis in experiments, suggesting strain engineering on 2D materials for the tunable anisotropy in polarized and flexible nonlinear optical devices.

Layer-Dependent Nonlinear Optical Properties of WS2, MoS2, and Bi2S3 Films Synthesized by Chemical Vapor Deposition.

Two-dimensional (2D) layered materials have shown layer-dependent optical properties in both linear optical and nonlinear optical (NLO) regimes due to prominent interlayer coupling and quantum confinement in an atomic scale. However, the NLO properties become more complicated as both saturable absorption (SA) and reverse saturable absorption (RSA) easily happen in 2D materials, which results in a significant challenge to understand the evolution of nonlinear absorption with layers. Motivated by this, chemical vapor-deposited chalcogenide compounds (WS2, MoS2, and Bi2S3) are used to investigate the pump intensity and layer number-dependent NLO properties. The values of nonlinear absorption coefficients of these chalcogenide compounds increase with the pump intensity by an 800 nm femtosecond laser, which can be described by an empirical power law function. The SA process due to the large transition probability of the ground state readily takes place in thick samples, while RSA occurs easily in thin samples due to the two-photon absorption (TPA). The transition from TPA to SA is deduced to occur at 13L-WS2, 15L-MoS2, and 5L-Bi2S3, which is related to the layer-dependent band gaps. Our results provide an efficient way to tune optical nonlinearities with a controlled layer number and to design corresponding NLO devices such as optical switches and saturable absorbers.

Large In-Plane Anisotropic Terahertz Emission Induced by Asymmetric Polarization in Low-Symmetric PdSe2.

Palladium diselenide (PdSe2) exhibits air stability, low symmetry, and high carrier mobility, resulting in unique in-plane anisotropy for polarized optoelectronic devices. However, the relationship of the symmetry and the terahertz (THz) radiation remains elusive yet significant for both the THz source in technology and nonlinear optical physics in science. Herein, we observed large in-plane anisotropic THz radiation from multilayer PdSe2 under femtosecond laser excitation. The THz emission demonstrates 2α dependence on the optical polarization angle from the resonant optical rectification combined with a background from the photocarrier acceleration under the surface depletion field. Interestingly, the in-plane THz emission along and perpendicular to the puckered direction demonstrates large anisotropy. Furthermore, the THz time-domain signals exhibit reversed polarities along the positive and negative puckered directions. This asymmetric polarization could relate to the bonding of Pd-Se, resulting in the unidirectional photon-induced current. Our results bridge the gap between the low-symmetry two-dimensional materials and the THz technology, which could promote the development of THz-polarized devices based on low-symmetry layered materials.