Low-noise mode-locking in a GHz repetition rate Tm3+-doped fiber laser.

We demonstrate a GHz repetition rate mode-locked Tm3+-doped fiber laser with low noise. Based on a home-made Tm3+-doped barium gallo-germanate fiber with reduced dispersion, a broad optical spectrum of mode-locking is achieved, and its amplified spontaneous emission quantum-limited timing jitter is largely suppressed. Besides, we carefully investigate the influence of the intracavity pump strength on the noise performance of the mode-locked pulses and find that manipulating the intracavity pump power can be an effective method for optimizing the timing jitter and relative intensity noise (RIN). Particularly, RIN, which originated from the relaxation oscillation, can be effectively suppressed by 33 dB at offset frequencies of >1 MHz. The integrated timing jitter and RIN are only 7.9 fs (10 kHz-10 MHz) and 0.05% (10 Hz-10 MHz), respectively.

Robust fatigue markers obtained from muscle synergy analysis.

This study aimed to utilize the nonnegative matrix factorization (NNMF) algorithm for muscle synergy analysis, extracting synergy structures and muscle weightings and mining biomarkers reflecting changes in muscle fatigue from these synergy structures. A leg press exercise to induce fatigue was performed by 11 participants. Surface electromyography (sEMG) data from seven muscles, electrocardiography (ECG) data, Borg CR-10 scale scores, and the z-axis acceleration of the weight block were simultaneously collected. Three indices were derived from the synergy structures: activation phase difference, coactivation area, and coactivation time. The indicators were further validated for single-leg landing. Differences in heart rate (HR) and heart rate variability (HRV) were observed across different fatigue levels, with varying degrees of disparity. The median frequency (MDF) exhibited a consistent decline in the primary working muscle groups. Significant differences were noted in activation phase difference, coactivation area, and coactivation time before and after fatigue onset. Moreover, a significant correlation was found between the activation phase difference and the coactivation area with fatigue intensity. The further application of single-leg landing demonstrated the effectiveness of the coactivation area. These indices can serve as biomarkers reflecting simultaneous alterations in the central nervous system and muscle activity post-exertion.

Transcranial ultrasound with electrical stimulation of the cerebello-parietal nucleus on cerebral blood flow and limb function in patients with stroke.

OBJECTIVE: To investigate the efficacy of transcranial ultrasound stimulation (TUS) combined with Fastigial nucleus stimulation (FNS) on cerebral blood flow and limb function in patients in the acute phase of ischemic stroke. METHODS: A total of 90 patients in the acute phase of ischemic stroke were randomly divided into an FNS, TUS, and TUS + FNS group (30 patients each), and all patients also received conventional treatment. The FNS group was treated with FNS alone. The TUS group was treated with TUS alone. The TUS + FNS group was treated with both TUS and FNS. The three groups were treated once a day for 6 days a week. RESULTS: The simplified Fugl-Meyer Assessment (FMA) and Barthel index scores (BI), and the peak systolic blood flow velocity (Vs) and the mean blood flow velocity (Vm) of the anterior cerebral artery, middle cerebral artery, and posterior cerebral artery, were significantly higher in all three groups compared with before treatment (P < 0.05). The scores for the TUS group were higher than for the FNS group (P < 0.05), and the scores of the TUS + FNS group were higher than the TUS and FNS groups, respectively (P < 0.05). The total effective rate was 63.3%, 70.0%, and 90.0% in the FNS, TUS, and TUS + FNS groups, respectively, and the difference between the three groups was statistically significant (P < 0.05). CONCLUSION: The FNS and TUS treatments improved the function of and accelerated cerebral blood flow in patients with acute ischemic stroke to different degrees, and the combined use of both treatment types was overall more effective.

Two Birds One Stone: Graphene Assisted Reaction Kinetics and Ionic Conductivity in Phthalocyanine-Based Covalent Organic Framework Anodes for Lithium-ion Batteries.

This work reports a covalent organic framework composite structure (PMDA-NiPc-G), incorporating multiple-active carbonyls and graphene on the basis of the combination of phthalocyanine (NiPc(NH2 )4 ) containing a large π-conjugated system and pyromellitic dianhydride (PMDA) as the anode of lithium-ion batteries. Meanwhile, graphene is used as a dispersion medium to reduce the accumulation of bulk covalent organic frameworks (COFs) to obtain COFs with small-volume and few-layers, shortening the ion migration path and improving the diffusion rate of lithium ions in the two dimensional (2D) grid layered structure. PMDA-NiPc-G showed a lithium-ion diffusion coefficient (DLi + ) of 3.04 × 10-10 cm2 s-1 which is 3.6 times to that of its bulk form (0.84 × 10-10 cm2 s-1 ). Remarkably, this enables a large reversible capacity of 1290 mAh g-1 can be achieved after 300 cycles and almost no capacity fading in the next 300 cycles at 100 mA g-1 . At a high areal capacity loading of ≈3 mAh cm-2 , full batteries assembled with LiNi0.8 Co0.1 Mn0.1 O2 (NCM-811) and LiFePO4 (LFP) cathodes showed 60.2% and 74.7% capacity retention at 1 C for 200 cycles. Astonishingly, the PMDA-NiPc-G/NCM-811 full battery exhibits ≈100% capacity retention after cycling at 0.2 C. Aided by the analysis of kinetic behavior of lithium storage and theoretical calculations, the capacity-enhancing mechanism and lithium storage mechanism of covalent organic frameworks are revealed. This work may lead to more research on designable, multifunctional COFs for electrochemical energy storage.

Enhanced measurement of tiny rotational angles using conjugate orbital angular momentum modes.

We report the enhanced experimental measurement of tiny rotational angles using two conjugate OAM modes upon rotation of a Dove prism. The two conjugate OAM modes interfere in a petal-like pattern and the orientation of the pattern depends on the phase difference between the two modes. We propose an accurate method of digital image processing to measure the tiny rotational angles of the Dove prism. In the presence of an imperfect pattern and light path, the measurement precision was enhanced by a factor of l. This scheme has potential applications in high-precision sensing and monitoring of tiny rotation angles.

Phenediamine bridging phthalocyanine-based covalent organic framework polymers used as anode materials for lithium-ion batteries.

In this study, phenediamine bridging phthalocyanine-based covalent organic framework materials (CoTAPc-PDA, CoTAPc-BDA and CoTAPc-TDA) with increasingly-widening pore sizes are prepared by reacting cobalt octacarboxylate phthalocyanine with p-phenylenediamine (PDA), benzidine (BDA) and 4,4''-diamino-p-terphenyl (TDA), respectively. The effects of frame size on the morphology structure and its electrochemical properties were explored. X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) and transmission electron microscopy (TEM) images show that the pore sizes of the CoTAPc-PDA, CoTAPc-BDA and CoTAPc-TDA are about 1.7 nm, 2.0 nm and 2.3 nm, respectively, which are close to the simulated results after geometric conformation optimization using Material Studio software. In addition, the specific surface areas of CoTAPc-PDA, CoTAPc-BDA and CoTAPc-TDA are 62, 81 and 137 m2 g-1, respectively. With increase in the frame size, the specific surface area of the corresponding material increases, which is bound to produce different electrochemical behaviors. Consequently, the initial capacities of the CoTAPc-PDA, CoTAPc-BDA and CoTAPc-TDA electrodes in lithium-ion batteries (LIBs) are 204, 251 and 382 mA h g-1, respectively. As the charge and discharge processes continue, the active points in the electrode material are continuously activated, leading to a continuous increase in charge and discharge capacities. After 300 cycles, the CoTAPc-PDA, CoTAPc-BDA and CoTAPc-TDA electrodes exhibit capacities of 519, 680 and 826 mA h g-1, respectively, and after 600 cycles, the capacities are maintained at 602, 701 and 865 mA h g-1, respectively, with a stable capacity retention rate at a current density of 100 mA g-1. The results show that the large-size frame structure materials have a larger specific surface area and more favorable lithium ion transmission channels, which produce greater active point utilization and smaller charge transmission impedance, thus showing larger charge and discharge capacity and superior rate capability. This study fully confirms that frame size is a key factor affecting the properties of organic frame electrodes, providing design ideas for the development of high-performance organic frame electrode materials.

The Validity and Reliability of a New Intelligent Three-Dimensional Gait Analysis System in Healthy Subjects and Patients with Post-Stroke.

Odonate is a new, intelligent three-dimensional gait analysis system based on binocular depth cameras and neural networks, but its accuracy has not been validated. Twenty-six healthy subjects and sixteen patients with post-stroke were recruited to investigate the validity and reliability of Odonate for gait analysis and examine its ability to discriminate abnormal gait patterns. The repeatability tests of different raters and different days showed great consistency. Compared with the results measured by Vicon, gait velocity, cadence, step length, cycle time, and sagittal hip and knee joint angles measured by Odonate showed high consistency, while the consistency of the gait phase division and the sagittal ankle joint angle was slightly lower. In addition, the stages with statistical differences between healthy subjects and patients during a gait cycle measured by the two systems were consistent. In conclusion, Odonate has excellent inter/intra-rater reliability, and has strong validity in measuring some spatiotemporal parameters and the sagittal joint angles, except the gait phase division and the ankle joint angle. Odonate is comparable to Vicon in its ability to identify abnormal gait patterns in patients with post-stroke. Therefore, Odonate has the potential to provide accessible and objective measurements for clinical gait assessment.

Heating rates are more strongly influenced by near-infrared than visible reflectance in beetles.

Adaptations to control heat transfer through the integument are a key component of temperature regulation in animals. However, there remain significant gaps in our understanding of how different optical and morphological properties of the integument affect heating rates. To address these gaps, we examined the effect of reflectivity in both ultraviolet-visible and near-infrared wavelengths, surface rugosity (roughness), effective area (area subjected to illumination) and cuticle thickness on radiative heat gain in jewel beetles (Buprestidae). We measured heating rate using a solar simulator to mimic natural sunlight, a thermal chamber to control the effects of conduction and convection, and optical filters to isolate different wavelengths. We found that effective area and reflectivity predicted heating rate. The thermal effect of reflectivity was driven by variation in near-infrared rather than ultraviolet-visible reflectivity. By contrast, cuticle thickness and surface rugosity had no detectable effect. Our results provide empirical evidence that near-infrared reflectivity has an important effect on radiative heat gain. Modulating reflectance of near-infrared wavelengths of light may be a more widespread adaptation to control heat gain than previously appreciated.

An Insight into Diversity and Functionalities of Gut Microbiota in Insects.

The gut microbiota has long been of research interests due to its nutritional importance for many insects. It has been demonstrated that diversity of gut microbiota in insects can be modulated by many factors, including habitats, feeding preference, etc. Besides, the community structure of gut microbiota could also be altered during the different life stages of host insects. With development of conventional culture-dependent technologies and advanced culture-independent technologies, comprehensive and deep understanding of the functions of gut microbiota and their relationship with host insects were achieved, especially for the nutrient metabolic process mediated by them. In this review, we summarized the gut microbiota composition, major methods for gut microbiota characterization, and vital nutrient metabolic process mediated by gut microbiota in different insects. The increasing knowledge on the modulation of gut microbiota will help us for the comprehension of the contribution of gut microbiota to the nutritional metabolism of insects, prompting their growth and health.

Endocuticle sclerotisation increases the mechanical stability of cuticle.

The cuticle plays an important role in the evolutionary success of insects. Many studies on insect cuticles have reported a soft, resilin-rich endocuticle. However, a recent study indicated the presence of a sclerotised endocuticle in the weevil Pachyrhynchus sarcitis kotoensis, which contradicts former knowledge. To understand the degree of sclerotisation in the endocuticle of the weevil and its potential function, we first examined the endocuticle by microscopic and staining techniques. We next performed mechanical tests to measure the material properties of the endocuticle, and numerical simulations to predict the structural effect of the sclerotisation. Our results provide the first evidence of the existence of a sclerotised endocuticle and its remarkable function in improving the mechanical stability of the cuticle. This study highlights the finding of a high degree of sclerotisation in the stiff endocuticle of the weevil, especially the matrix surrounding the fibres. This novel case brings new understanding of cuticle properties and gives promising insights into biomaterial design.

Too hard to swallow: a secret secondary defence of an aposematic insect.

Anti-predator strategies are significant components of adaptation in prey species. Aposematic prey are expected to possess effective defences that have evolved simultaneously with their warning colours. This study tested the hypothesis of the defensive function and ecological significance of the hard body in aposematic Pachyrhynchus weevils pioneered by Alfred Russel Wallace nearly 150 years ago. We used predation trials with Japalura tree lizards to assess the survivorship of 'hard' (mature) versus 'soft' (teneral) and 'clawed' (intact) versus 'clawless' (surgically removed) weevils. The ecological significance of the weevil's hard body was evaluated by assessing the hardness of the weevils, the local prey insects, and the bite forces of the lizard populations. The existence of toxins or deterrents in the weevil was examined by gas chromatography-mass spectrometry (GC-MS). All 'hard' weevils were instantly spat out after being bitten once and survived attacks by the lizards. In contrast, the 'soft' weevils were chewed and subsequently swallowed. The results were the same regardless of the presence or absence of the weevil's tarsal claws. The hardness of 'hard' Pachyrhynchus weevils was significantly higher than the average hardness of other prey insects in the same habitat and the mean bite forces of the local lizards. The four candidate compounds of the weevil identified by GC-MS had no known toxic or repellent functions against vertebrates. These results reveal that the hardness of aposematic prey functions as an effective secondary defence, and they provide a framework for understanding the spatio-temporal interactions between vertebrate predators and aposematic insect prey.

A strong and tough interpenetrating network hydrogel with ultrahigh compression resistance.

A novel interpenetrating network (IPN) hydrogel with ultrahigh compressive strength and fracture strain has been prepared using the copolymer of 2-acrylamide-2-methylpropane sulfonic acid (AMPS) and acrylamide (AM) [P(AMPS-co-AM)] or N-isopropylacrylamide (NIPAM) [P(AMPS-co-NIPAM)] as the primary network and polyacrylamide (PAM) as the secondary network. The as-prepared IPN hydrogel of P(AMPS-co-AM)/PAM has a significantly high compressive strength (91.8 MPa), which is 4 times greater than that of the common PAMPS/PAM IPN hydrogel as well as the compressively strongest hydrogel reported in the literature. The P(AMPS-co-AM)/PAM IPN hydrogel is tough enough not to fracture even when the compressive strain reaches 98%. Synchrotron radiation small-angle X-ray scattering (SAXS) analysis has indicated that the presence of an AM comonomer changes the size of the physically cross-linked domains in the IPN hydrogel, which may partially account for its unique mechanical properties. This study has presented the compressively strongest hydrogel reported to date and also provided a novel and feasible method to prepare the highly strong and tough hydrogel.

The prognostic significance of lymph nodes in patients with pT1c33N0M0 non-small cell lung cancer: a retrospective study.

Objective: The objective of this study was to appraise the prognostic impact of lymph nodes in patients diagnosed with pT1c33N0M0 non-small cell lung cancer (NSCLC) and to delve into the prognostic significance of lymph nodes located at the N1 lymph node station in this patient cohort. Methods: A retrospective analysis of clinical data was conducted for 255 patients diagnosed with pT1c33N0M0 NSCLC. Lymph nodes were tabulated and categorized into three groups (0-10 nodes, 11-16 nodes, >16 nodes). Clinical data among these three groups of pT1c33N0M0 NSCLC patients were compared. We conducted both univariate and multivariate analyses to pinpoint the factors that impact the prognosis of patients with pT1c33N0M0 non-small cell lung cancer (NSCLC). Additionally, we employed receiver operating characteristic (ROC) curve analysis to pinpoint the optimal lymph node criteria at the N1 station for prognostic prediction in pT1c33N0M0 NSCLC patients. Results: Within the cohort of 255 individuals afflicted with pT1c33N0M0 non-small cell lung cancer (NSCLC), a comprehensive tally of 3,902 lymph nodes was diligently established, yielding an average of 15.3 nodes for each patient. Multivariate analysis demonstrated that tumor size, T stage, and lymph nodes were independent factors significantly impacting the prognosis of pT1c33N0M0 NSCLC patients (P < 0.05). ROC curve analysis revealed an area under the curve of 0.6982 for predicting prognosis using N1 station in pT1c33N0M0 NSCLC patients. The maximum Youden index was observed at an N1 station of 2.7 nodes. Patients with N1 station ≥ three nodes had significantly better prognoses compared to those with < 3 nodes (both P < 0.05). Conclusion: Lymph nodes serve as an independent prognostic factor for pT1c33N0M0 NSCLC patients. Detecting at least three or more lymph nodes at the N1 station is associated with a more favourable prognosis in pT1c33N0M0 NSCLC patients.

Low-loss and polarization insensitive 32 × 4 optical switch for ROADM applications.

Integrated switches play a crucial role in the development of reconfigurable optical add-drop multiplexers (ROADMs) that have greater flexibility and compactness, ultimately leading to robust single-chip solutions. Despite decades of research on switches with various structures and platforms, achieving a balance between dense integration, low insertion loss (IL), and polarization-dependent loss (PDL) remains a significant challenge. In this paper, we propose and demonstrate a 32 × 4 optical switch using high-index doped silica glass (HDSG) for ROADM applications. This switch is designed to route any of the 32 inputs to the express ports or drop any channels from 32 inputs to the target 4 drop ports or add any of the 4 ports to any of the 32 express channels. The switch comprises 188 Mach-Zehnder Interferometer (MZI) type switch elements, 88 optical vias for the 44 optical bridges, and 618 waveguide-waveguide crossings with three-dimensional (3D) structures. At 1550 nm, the fiber-to-fiber loss for each express channel is below 2 dB, and across the C and L bands, below 3 dB. For each input channel to all 4 drop/add channels at 1550 nm, the loss is less than 3.5 dB and less than 5 dB across the C and L bands. The PDLs for all express and input channels to the 4 drop/add channels are below 0.3 dB over the C band, and the crosstalk is under -50 dB for both the C and L bands.

Effects of trigger laser pulse width on the jitter time of GaAs photoconductive semiconductor switch.

The effects of trigger laser pulse width on the jitter time of a GaAs photoconductive semiconductor switch (PCSS) is investigated in the experiment. The laser is split into two optical beams by a cross grating to excite two 3 mm gap GaAs PCSSs in parallel at the same time. This work reveals that the jitter time of the GaAs PCSS is reduced as the trigger laser pulse width decreases. Our results overcome a significant obstacle that hinders the testing and theory of GaAs PCSSs in high-time-precision synchronous control.

Influence of the incident laser pulse energy on jitter time of GaAs photoconductive semiconductor switches.

We have experimentally investigated the jitter time of a GaAs photoconductive switch (PCSS) when it is triggered by a laser pulse with 30 ns pulse width and 1064 nm wavelength. It is found that the jitter time decreases as the incident laser pulse energy increases from 0.40 to 1.6 mJ. In addition, a theoretical analysis indicates that the jitter time is proportional to relative deviation of the laser pulse energy. This work provides a path to improve the performance of the PCSS, which is used in applications such as a high time precision synchronous control system and ultrawide-band radiation source.

Strong attachment as an adaptation of flightless weevils on windy oceanic islands.

Enhanced attachment ability is common in plants on islands to avoid potential fatal passive dispersal. However, whether island insects also have increased attachment ability remains unclear. Here we measured the attachment of a flightless weevil, Pachyrhynchus sarcitis kotoensis, from tropical islands, and compared it with documented arthropods from the mainland. We examined the morphology and material gradient of its attachment devices to identify the specific adaptive modifications for attachment. We find that the weevil has much stronger attachment force and higher safety factor than previously studied arthropods, regardless of body size and substrate roughness. This probably results from the specific flexible bases of the adhesive setae on the third footpad of the legs. This softer material on the setal base has not been reported hitherto and we suggest that it acts as a flexible hinge to form intimate contact to substrate more effectively. By contrast, no morphological difference in tarsomeres and setae between the weevil and other beetles is observed. Our results show the remarkably strong attachment of an island insect and highlights the potential adaptive benefits of strong attachment in windy island environment. The unique soft bases of the adhesive hairs may inspire the development of strong biomimetic adhesives.

Mechanism analysis of hydroxypropyl guar gum degradation in fracture flowback fluid by homogeneous sono-Fenton process.

An effective hybrid system was applied as the first report for the successful treatment of key pollutants (hydroxypropyl guar gum, HPG) in fracturing flowback fluid, and the synergistic index of the hybrid system was 20.45. In this regard, chemical oxygen demand (COD) removal ratio was evaluated with various influencing operating factors including reaction time, H2O2 concentration, Fe2+ concentration, ultrasonic power, initial pH, and temperature. The optimal operating parameters by single-factor analysis method were: the pH of 3.0, the H2O2 concentration of 80 mM, the Fe2+ concentration of 5 mM, the ultrasonic power of 180 W, the ultrasonic frequency of 20-25 kHz, the temperature of 39 â„ƒ, the reaction time of 30 min, and the COD removal rate reached 81.15 %, which was permissible to discharge surface water sources based on the environmental standards. A possible mechanism for HPG degradation and the generation of reactive species was proposed. Results of quenching tests showed that various impacts of the decomposition rate by addition of scavengers had followed the order of EDTA-2Na < BQ < t-BuOH, therefore OH radicals had a dominant role in destructing the HPG. Based on the kinetic study, it was concluded that Chan Kinetic Model was more appropriate to describe the degradation of HPG. Identification of intermediates by GC-MS showed that a wide range of recalcitrant compounds was removed and/or degraded into small molecular compounds effectively after treatment. Under the optimal conditions, the sono-Fenton system was used to treat the fracturing flowback fluid with the initial COD value of 675.21 mg/L, and the COD value decreased to 80.83 mg/L after 60 min treatment, which was in line with the marine sewage discharge standard. In conclusion, sono-Fenton system can be introduced as a successful advanced treatment process for the efficient remediation of fracture flowback fluid.

Carbon skeleton materials derived from rare earth phthalocyanines (MPcs) (M = Yb, La) used as high performance anode materials for lithium-ion batteries.

In this work, novel carbon skeleton materials were prepared by high-temperature carbonization of rare earth phthalocyanines (MPcs) (M = Yb, La) under a nitrogen atmosphere. The resulting carbon materials of YbPc-900 (carbonisation temperature of 900 °C for 2 h) and LaPc-1000 (carbonization temperature of 1000 °C for 2 h) have a graphite-layered structure in predominantly ordered states, with a smaller particle size, a larger specific surface area and a higher degree of hard carbonization compared to those of the uncarbonized sample. As a result, the batteries using the YbPc-900 and LaPc-1000 carbon skeleton materials as electrodes display excellent energy storage behaviors. The initial capacities of the YbPc-900 and LaPc-1000 electrodes at 0.05 A g-1 were 1100 and 850 mA h g-1, respectively. After 245 cycles and 223 cycles, the capacities remain at 780 and 716 mA h g-1 with retention ratios of 71% and 84%. At a high rate of 1.0 A g-1, the initial capacities of the YbPc-900 and LaPc-1000 electrodes were 400 and 520 mA h g-1, respectively, and after 300 cycles, the capacities can still remain at 526 and 587 mA h g-1 with retention ratios of 131.5% and 112.8%, respectively, which were much higher than those of the pristine rare earth phthalocyanine (MPc) (M = Yb, La) electrodes. Moreover, better rate capabilities were also observed during the YbPc-900 and LaPc-1000 electrode tests. The capacities of the YbPc-900 electrode at 0.05, 0.1, 0.2, 0.5, 1 and 2C were 520, 450, 407, 350, 300 and 260 mA h g-1 respectively, which were higher than those of the YbPc electrode (550, 450, 330, 150, 90 and 40 mA h g-1). Similarly, the rate performance of the LaPc-1000 electrode at different rates was also significantly improved compared to that of the pristine LaPc electrode. In addition, the initial Coulomb efficiencies of the YbPc-900 and LaPc-1000 electrodes were also greatly improved compared to those of pristine YbPc and LaPc electrodes. After carbonization, the YbPc-900 and LaPc-1000 carbon skeleton materials derived from rare earth phthalocyanines (MPcs) (M = Yb, La) exhibit improved energy storage behaviors, which would provide new ideas for developing novel organic carbon skeleton negative materials for lithium ion batteries.

Recent Advances in Reconfigurable Metasurfaces: Principle and Applications.

Metasurfaces have shown their great capability to manipulate electromagnetic waves. As a new concept, reconfigurable metasurfaces attract researchers' attention. There are many kinds of reconfigurable components, devices and materials that can be loaded on metasurfaces. When cooperating with reconfigurable structures, dynamic control of the responses of metasurfaces are realized under external excitations, offering new opportunities to manipulate electromagnetic waves dynamically. This review introduces some common methods to design reconfigurable metasurfaces classified by the techniques they use, such as special materials, semiconductor components and mechanical devices. Specifically, this review provides a comparison among all the methods mentioned and discusses their pros and cons. Finally, based on the unsolved problems in the designs and applications, the challenges and possible developments in the future are discussed.