Nonreciprocal toroidal dipole resonance and one-way quasi-bound state in the continuum.

Bound states in the continuum (BICs) provide an alternative way of trapping light at nanoscale. Although the last 10 years have witnessed tremendous progress on BICs from fundamentals to applications, nonreciprocal BICs and their potential applications have not been fully exploited yet. In this study, we demonstrated a one-way quasi-BIC by leveraging an all-dielectric magneto-optical (MO) metasurface. We show that the key point for achieving a one-way quasi-BIC is to excite a magnetization-induced leaky resonance. Here we adopt the longitudinal toroidal dipole (TD) resonance characterized by a vortex distribution of head-to-tail magnetic dipoles parallel to the plane of the MO metasurface. We show that, by breaking the time-reversal symmetry, at critical conditions, the TD resonance can be enhanced in the forward channel and perfectly canceled in the time-reversed channel, resulting in a one-way quasi-BIC. The demonstrated phenomena hold significant promise for practical applications such as magnetic field optical sensing, nonreciprocal optical switching, isolation, and modulation.

Real-time wavefront correction using diffractive optical networks.

Real-time wavefront correction is a challenging problem to present for conventional adaptive optics systems. Here, we present an all-optical system to realize real-time wavefront correction. Using deep learning, the system, which contains only multiple transmissive diffractive layers, is trained to realize high-quality imaging for unknown, random, distorted wavefronts. Once physically fabricated, this passive optical system is physically positioned between the imaging lens and the image plane to all-optically correct unknown, new wavefronts whose wavefront errors are within the training range. Simulated experiments showed that the system designed for the on-axis field of view increases the average imaging Strehl Ratio from 0.32 to 0.94, and the other system intended for multiple fields of view increases the resolvable probability of binary stars from 30.5% to 69.5%. Results suggested that DAOS performed well when performing wavefront correction at the speed of light. The solution of real-time wavefront correction can be applied to other wavelengths and has great application potential in astronomical observation, laser communication, and other fields.

Curvature wavefront sensing based on a single defocused image and intensity compensation.

Curvature wavefront sensing usually requires the measurement of two defocused images at equal distances before and after the focus. In this paper, a new wavefront recovery algorithm based on only one defocused image is proposed. This algorithm contains the following four steps: response matrix calculation, establishment of intensity distribution equations, Zernike coefficients solution derived from the least squares method, and defocused image compensation with the solved Zernike coefficients. The performance of the algorithm in a large obscuration ratio and fast focal ratio optical system on axis and the edge of the field of view (FOV) is examined. Two optical systems of the Hubble telescope and a modified Paul-Baker telescope are employed to test the algorithm. The simulations show that the proposed algorithm outperforms in structural simplicity, and applications are expected in the wavefront recovery of the extreme environment (i.e., in space and the Antarctic).

Thermal stratification controls taste and odour compounds by regulating the phytoplankton community in a large subtropical water source reservoir (Xin'anjiang Reservoir).

2-Methylisoborneol (2-MIB) and geosmin are compounds released by algae that significantly degrade reservoir water quality, posing a threat to both the safety of drinking water and the quality of aquatic products sourced from these environments. However, few studies have explored how enhanced thermal stratification affects the occurrence and regulation of odorants in large drinking water reservoirs. Through systematic monitoring and investigation of Xin'anjiang Reservoir, we found that enhanced thermal stratification promotes filamentous cyanobacteria, particularly Leptolyngbya sp., as the primary contributor to 2-MIB production within the 1-10 m layer of the water column. The highest 2-MIB concentration, 92.5 ng/L, was recorded in the riverine region, which was 2.54 and 14.52 times higher than that in the transitional and central parts of the reservoir, respectively. Temperature indirectly impacted algal growth and odorant production by modulating TN/TP ratios. Geosmin concentration responded rapidly to relatively low TN/TP ratios (< 25). Our findings suggest that phosphorus control in estuaries should be enhanced during thermal stratification period. In summary, our study provides valuable insights to inform pragmatic water intake strategies and the distribution and release of odorants caused by thermal stratification. This is particularly relevant in the context of future global warming and extremely high temperatures during the warm season.

High-Performance and Stable Sb2S3 Thin-Film Photodetectors for Potential Application in Visible Light Communication.

Photodetectors (PDs) are critical parts of visible light communication (VLC) systems for achieving efficient photoelectronic conversion and high-fidelity transmission of signals. Antimony sulfide (Sb2S3) as a nontoxic, high optical absorption coefficient, and low-cost semiconductor becomes a promising candidate for applications in VLC systems. Particularly, Sb2S3 PDs were verified to have significantly weak light detection ability in the visible region. However, the response speed of Sb2S3 PDs with existing device structures is still relatively slow. Herein, through optimizing the device structure for the p-i-n type PDs, a p-type Sb2Se3 hole transport layer (HTL) is designed to enhance the built-in electric field and to accelerate the migration of photogenerated carriers for the high responsivity and fast response speed. The optimal thickness of the structure is obtained through the simulation of SCAPS-1D software, and the optimized devices show high-performance parameters, including a responsivity of 0.34 A W-1, a specific detectivity (D*) of 2.20 × 1012 Jones, the -3 dB bandwidth of 440 kHz, high stability, and the value of the Sb2S3 PDs can reach 60% in the range of 360-600 nm, which indicates that the device is very suitable for working in the visible light band. In addition, the resulting Sb2S3 PD is successfully integrated into VLC systems by designing a matched light detection circuit. The results suggest that the Sb2S3 PDs are expected to provide an alternative to future VLC system applications.

Self-Powered Sb2S3 Thin-Film Photodetectors with High Detectivity for Weak Light Signal Detection.

Photodetectors (PDs) for weak light signal detection have wide applications for optical communication and imaging. Antimony sulfide (Sb2S3) as a nontoxic and stable light-sensitive material becomes a promising candidate for weak light PDs, which are developing in the direction of high response, high speed, and low cost. Herein, a self-powered Sb2S3 PD with the structure of FTO/TiO2/Sb2S3/Au is developed to achieve weak light detection for 300-750 nm visible light. We control the Sb2S3 thickness with about 460 nm to match depletion region width (438 nm) and obtain an excellent photoresponsivity and 3 dB bandwidth. Furtherly, we prepare pyramid structure polydimethylsiloxane (PDMS) on the illuminating surface to enhance the performance of weak light detection by light-trapping effects. The photocurrent of Sb2S3 PD with 20 µm-sized PDMS texture achieves 13.6% improvement compared with the control one. Under weak 530 nm light illumination of 1 µW cm-2, the self-powered Sb2S3 PD with PDMS achieves high responsivity (3.41 A W-1), large detectivity (2.84 × 1013 Jones), and ultrafast speed (15 µs). The present Sb2S3 PD and light-trapping strategy are expected to provide an alternative to future commercial weak light detection applications.

Warming affects crustacean grazing pressure on phytoplankton by altering the vertical distribution in a stratified lake.

Zooplankton could efficiently graze algae and thus improve water quality. However, as thermal stratification commonly occurs in deep lakes, the effect of warming on the trophic interactions of plankton in depth profiles is still not clear. To explore the pressure of crustacean grazing on phytoplanktonic responses to enhanced thermal stratification under warming, we evaluated the monthly changes in the Secchi depth (SD), thermocline depth (TD), and the mean residence depth of zooplankton (zp MRD) and chlorophyll a (Chla MRD) in Lake Qiandaohu from January 2015 to December 2018. The thermal-stratification cycle was divided into weakness (from March to June) and formation periods (from July to February). Linear regression analyses showed that during both periods, the zp MRD was more sensitive to Chla MRD than to TD, and TD was negatively related to the difference between the zp MRD and Chla MRD. Structural equation model (SEM) analyses showed that the TD could be decreased by the direct effect of warming and the indirect effect of the decreased SD during weakness periods. A 0.95 °C increase in air temperature and a 0.85 m decrease in the SD between 1987 and 2018 corresponded to a decrease in the TD. Therefore, decreasing TD would weaken the top-down control on phytoplankton by moving phytoplankton far from zooplankton. Future decreasing TDs under climate warming may decouple crustacean grazing pressure on phytoplankton, which may further deteriorate the water quality.

[High-Frequency Dynamics of Water Quality and Phytoplankton Community in Inflowing River Mouth of Xin'anjiang Reservoir, China].

The tail of the reservoir is the unstable zone regarding water quality and phytoplankton community. Therefore, it is the crucial zone in aquatic ecosystem transitions. To understand the transition characteristics and driving mechanisms of water environment dynamics, high-frequency monitoring of the water environment and phytoplankton community in the tail of a deep and large reservoir, the Xin'anjiang Reservoir in southeast of China, was conducted using a water quality monitoring buoy and three-day interval water sampling during 18 months. Results show clear seasonal thermal and oxygen stratification in the river mouth of the reservoir. The nutrient and chlorophyll-a concentrations also show stratifying phenomena during the thermal stratification period. Heavy rain and inflow quickly consume the stratification. Nutrient concentrations were highly dynamic in the river mouth. The total phosphorus ranges from 0.011 mg·L-1 to 0.188 mg·L-1, and total nitrogen ranges from 0.75 mg·L-1 to 2.76 mg·L-1. Dissolved phosphorus comprised 56% of total phosphorus, and dissolved nitrogen occupied 88% of total nitrogen, respectively. Nutrient concentrations were influenced strongly by rainfall intensity and inflow rate. Total phosphorus and nitrogen concentrations were significantly related to the three-day accumulated rainfall. Nutrient concentrations in the flood season (March to June) were significantly higher than in the non-flood season (P<0.001). Seasonal phytoplankton proliferation also significantly influenced by total phosphorus concentration. The phytoplankton community changes significantly with seasons and flood events. Bacillariophytea was generally dominant throughout the year, with the predominant genus of Fragilaria spp., Cyclotella spp., Synedra spp., and Melosira spp. Cyanophyta biomass peaked in July, August, and September, with the dominant genus of Aphanizomenon spp., Microcystis spp., and Oscillatoria spp. Apart from the high temperature, storm inflow events also triggered Cyanophyta proliferation. The proliferation of Chlorophyta was similar to Cyanophyta, with the predominant genus of Pediastrum spp. and Closterium spp.. While the Cryptophyta biomass peaked during March to May, with the predominant genus of Cryptomonas spp.. Redundancy analysis shows that the influence factors of phytoplankton community dynamics include the inflow rate, temperature, water level, water transparency, total nitrogen, total phosphorus, and nitrogen to phosphorus ratio. The meteorological and hydrological factors were major factors for phytoplankton dynamics during later autumn and winter, while the nutrient will be the co-driving factors of phytoplankton community dynamics during summer and early autumn. The research confirmed the huge influence of the intensity rainfall event on the water environment in reservoirs and described the key environmental conditions for phytoplankton community dynamics. The research is useful for the design of the monitoring and forecasting system for water safety in drinking water source reservoirs.

[Spatial-temporal Distribution of Suspended Solids and Its Sedimentation Flux and Nutrients Effects in Xin'anjiang Reservoir, China].

To analyze the spatial-temporal distribution and sedimentation characteristics of suspended solids in reservoirs, high-frequency monitoring of a sediment trap and buoy, combined with three-dimensional water sampling, was conducted and analyzed in Xin'anjiang Reservoir for a year. The results showed that the turbidity data of the buoy has significant correlation with rainfall, inflow, and suspended solids (SS), particularly for SS (P<0.01, R2=0.86). There is an obvious spatial difference in SS between spring and summer, when the rainfall season occurs (river area > transition area > lake area). However, there is little difference in SS concentration between autumn and winter. There is a spatial trend of river area > transition area > lake area (with rates of 27.82, 4.34, and 0.26 g·(m2·d)-1, respectively), and a temporal trend of spring and summer > autumn and winter. The sedimentation flux of the whole lake is 2.57×106 t·a-1 combined with the investigation of the four-season SS at 60 points across the whole lake, and the settlement flux in spring and summer is higher than that in autumn and winter. The contents of particulate nitrogen (PN) in JK, XJS, and DB were 6812, 15886, and 21986 mg·kg-1, and the particulate phosphorus (PP) contents were 2545, 3269, and 3077 mg·kg-1, respectively. Statistical analysis shows that there is a good exponential relationship between moderate rainfall and turbidity growth rate in the river area of the reservoir (R2=0.81). Moreover, the continuous heavy rainfall affects turbidity in river area, but has little effect on the transition area. The concentration of SS has a good exponential decay with distance from the river to the dam (R2=0.84), especially in spring and summer. Research shows that the average annual deposition rate in Xin'anjiang Reservoir is 0.07%, lower than other large reservoirs in the country; however, there are certain risks in front of the dam because the nutrient sediments are high. The results suggest that reservoir managers should pay attention to water and soil conservation in the watershed to reduce the impact of rainfall on reservoir water quality. Meanwhile, the potential nutrient internal release risk in the downstream area before the dam should be considered.

Thermal stratification dynamics in a large and deep subtropical reservoir revealed by high-frequency buoy data.

We measure the thermal stratification dynamics in Lake Qiandaohu, China, a deep subtropical reservoir, to better understand the mixing mechanism and its response to lake warming. A high-frequency monitoring buoy dataset from February 2016 to October 2017 is used to evaluate variations in the water temperature profile, Schmidt stability (SS) and thermocline parameters, such as the thermocline depth (TD), bottom depth (TB), thickness (TT), and strength (TS), and elucidate the potential effects of thermal stratification on the lake's ecosystem. High-frequency observation data demonstrate that the lake's thermal-stratification cycle can be divided into three stages: formation, stationary and weakening periods. Consequently, a significant positive correlation between the TB and TT during the formation period and a significant negative correlation between the TD and TT are found during the stationary and weakening periods. Additionally, strong positive correlations exist among the TS, TT and SS for all the data. Our data indicated that an increase in the air temperature caused the surface water temperature, TT, TS and SS to increase. Furthermore, thermal stratification affected the vertical distribution of dissolved oxygen and expanded the area of the hypoxic-anoxic zone. The incomplete mixing of the water from December 2016 to February 2017 because of the high air temperature, which was 2.49 °C higher than the mean air temperature of 1966-2015 (6.44 °C), created the hypoxia hypolimnion from March to May 2017. Under the background of global warming, the thermal stratification of Lake Qiandaohu will likely intensify and further significantly affect the lake's ecosystem.

Seasonal-Spatial Distribution and Long-Term Variation of Transparency in Xin'anjiang Reservoir: Implications for Reservoir Management.

Water transparency is a useful indicator of water quality or productivity and is widely used to detect long-term changes in the water quality and eutrophication of lake ecosystems. Based on short-term spatial observations in the spring, summer, and winter and on long-term site-specific observation from 1988 to 2013, the spatial, seasonal, long-term variations, and the factors affecting transparency are presented for Xin'anjiang Reservoir (China). Spatially, transparency was high in the open water but low in the bays and the inflowing river mouths, reflecting the effect of river runoff. The seasonal effects were distinct, with lower values in the summer than in the winter, most likely due to river runoff and phytoplankton biomass increases. The transparency decreased significantly with a linear slope of 0.079 m/year, indicating a 2.05 m decrease and a marked decrease in water quality. A marked increase occurred in chlorophyll a (Chla) concentration, and a significant correlation was found between the transparency and Chla concentration, indicating that phytoplankton biomass can partially explain the long-term trend of transparency in Xin'anjiang Reservoir. The river input and phytoplankton biomass increase were associated with soil erosion and nutrient loss in the catchment. Our study will support future management of water quality in Xin'anjiang Reservoir.

Dissolved oxygen stratification and response to thermal structure and long-term climate change in a large and deep subtropical reservoir (Lake Qiandaohu, China).

From January 2010 to March 2014, detailed depth profiles of water temperature, dissolved oxygen (DO), and chromophoric dissolved organic matter (CDOM) were collected at three sites in Lake Qiandaohu, a large, deep subtropical reservoir in China. Additionally, we assessed the changes in DO stratification over the past 61 years (1953-2013) based on our empirical models and long-term air temperature and transparency data. The DO concentration never fell below 2 mg/L, the critical value for anoxia, and the DO depth profiles were closely linked to the water temperature depth profiles. In the stable stratification period in summer and autumn, the significant increase in CDOM in the metalimnion explained the decrease in DO due to the oxygen consumed by CDOM. Well-developed oxygen stratification was detected at the three sites in spring, summer and autumn and was associated with thermal stratification. Oxycline depth was significantly negatively correlated with daily air temperature and thermocline thickness but significantly positively correlated with thermocline depth during the stratification weakness period (July-February). However, there were no significant correlations among these parameters during the stratification formation period (March-June). The increase of 1.67 °C in yearly average daily air temperature between 1980 and 2013 and the decrease of 0.78 m in Secchi disk depth caused a decrease of 1.65 m and 2.78 m in oxycline depth, respectively, facilitating oxygen stratification and decreasing water quality. Therefore, climate warming has had a substantial effect on water quality through changing the DO regime in Lake Qiandaohu.

[Dynamic distributions of dissolved oxygen env in Lake Qiandaohu and its environmental influence factors].

Based on monthly in situ data collected at six sampling sites in Qiandaohu Lake between 2011 and 2012, the dynamic distributions of dissolved oxygen (DO) were analyzed and the relationships between DO and the environmental factors were investigated. The results showed that there were obviously vertical and temporal variations in the distributions of DO. In winter, the average values of DO were generally higher than those in other seasons, but no significant vertical distribution variation was found except Dabaqian. However, the vertical differences of DO in summer were larger than those in spring and autumn. Moreover, the maximum values of DO found in euphotic zone at the sites of Xiaojinshan, Santandao, Dabaqian in summer were 11.59, 12.52, 10.96 mg x L(-1), respectively. The maximum DO at surface layer was found in spring while the minimum value appeared in autumn. Seasonal differences in relationships between dissolved oxygen and water temperature, pH, and Chla concentration were discussed. In summer, highly significant linear correlation between DO and water temperature was found indicating that the temperature thermal stratification was the key factor to influence the vertical distribution of DO. The relative higher correlation coefficients between DO and pH, Chla concentration in spring and summer were due to the phytoplankton photosynthesis.

[Pollution characteristics and health risk assessment of organochlorine pesticides (OCPs) in the water of Lake Qiandao and its major input rivers].

Organochlorine pesticides (OCPs) were quantitatively determined by GC in several surface water samples collected in July 2011 and November 2011 from Lake Qiandao (Xin'an River Reservoir) and its major input rivers. Then the component characteristics and source apportionment of HCHs and DDTs were confirmed, and the health risk assessment was evaluated. The results showed that 8 OCPs were found to be in trace amount, and p,p'-DDT, a-HCH and p,p'-DDE were the highest frequently detected OCPs. The concentrations of total OCPs in surface water of the studied Lake ranged from 1.9 to 7.6 ng-L-1 , which were at lower pollution level, and 1.2-212 ng.L-1 in the samples from its three major input rivers. The spatial distribution of OCPs in the water of lake was varying, and Xin'an River, the mainstream of the lake, was the main input source of OCPs. Also, different contamination patterns among sampling seasons were found, the concentrations of OCPs in surface water collected in wet period were higher than those in dry season, which display the characteristics of nonpoint source pollution. According to the ratio of feature components, the OCPs in surface water from the Lake Qiandao originated largely from long distance transmission or degradation of technical HCHs, while additional sources of DDTs existed in the region. In addition, human health risk assessment of ingestion through the drinking water and skin contact absorption was performed using EPA recommends methods, the carcinogenic and non-carcinogenic risks caused by OCPs were 0. 06 x 10(-7)-23. 2 x 10(-7) and 3.43 x 10(-5) -6.01 x 10(-3), respectively. According to the acceptable risk level, the carcinogenic and non-carcinogenic risks of the chemicals investigated can be considered negligible in water body of Lake Qiandao.

[Study on seasonal characteristics of thermal stratification in lacustrine zone of Lake Qiandao].

Lake Qiandao is a typical subtropical man-made reservoir in China. The investigation on the seasonal and vertical dynamics of water temperature, dissolved oxygen (DO), pH value, turbidity, photosynthetic available radiation (PAR) and chlorophyll a was conducted in 2011 in order to find out the physical characteristics of Lake Qiandao. The average surface water temperature ranged from 10.4 to 32.7 degrees C. A monomictic thermal stratification was observed in Lake Qiandao, initiating in April and lasting until December. The results showed that thermal stratification had influences on vertical distribution of DO, pH value, turbidity, PAR and chlorophyll a. Very strong stratification of DO was found, inducing lower oxygen concentration in the thermocline layer and temporal hypoxia in the bottom water. The maximum turbidity was found in the thermocline layer and the precipitation affected the surface turbidity value. Moreover, the chlorophyll a concentration was higher in the surface water and lower in the bottom water as found in this study, implying that water quality was affected by stratification. Besides, the maximum photosynthesis rate and algal growth rate were found at the depth 5-10 m below the water surface. Therefore, the results can provide theoretical support for the sampling and analysis of algal blooms in Lake Qiandao.