Showing papers by "Hengyang Normal University published in 2017"

Broadband Vortex Beam Generation Using Multimode Pancharatnam–Berry Metasurface

Abstract: Vortex beams have been extensively realized using different approaches. Typically, the efficiency and bandwidth of a vortex beam are limited by impure copolarized components and the intrinsic dispersion of passive resonant structures. Here, we propose a strategy to generate wideband vortex beams by using a Pancharatnam–Berry metasurface in which two orthogonal reflections exhibit a broadband out-of-phase difference. To achieve this, a broadband strategy based on multimode operation and dispersion engineering methods was established. A dual-layer meta-atom is proposed; each layer comprises of five metallic dipoles, and the geometrical parameters are carefully adjusted to tune the resonant frequencies. Because the dipole orientations in each layer are orthogonal, the reflection responses under the two orthogonal polarizations can be independently engineered. Both numerical and experimental results indicate that our method not only enables a high-efficiency spiral beam conversion over a broad range of 6.95–18 GHz (>82%) but also causes a polarization-insensitive effect; thus, it can be adapted for any linear or circular polarization.

Enhanced optical absorption via cation doping hybrid lead iodine perovskites

Abstract: The suitable band structure is vital for perovskite solar cells, which greatly affect the high photoelectric conversion efficiency. Cation substitution is an effective approach to tune the electric structure, carrier concentration, and optical absorption of hybrid lead iodine perovskites. In this work, the electronic structures and optical properties of cation (Bi, Sn, and TI) doped tetragonal formamidinium lead iodine CH(NH2)2PbI3 (FAPbI3) are studied by first-principles calculations. For comparison, the cation-doped tetragonal methylammonium lead iodine CH3NH3PbI3 (MAPbI3) are also considered. The calculated formation energies reveal that the Sn atom is easier to dope in the tetragonal MAPbI3/FAPbI3 structure due to the small formation energy of about 0.3 eV. Besides, the band gap of Sn-doped MAPbI3/FAPbI3 is 1.30/1.40 eV, which is considerably smaller than the un-doped tetragonal MAPbI3/FAPbI3. More importantly, compare with the un-doped tetragonal MAPbI3/FAPbI3, the Sn-doped MAPbI3 and FAPbI3 have the larger optical absorption coefficient and theoretical maximum efficiency, especially for Sn-doped FAPbI3. The lower formation energy, suitable band gap and outstanding optical absorption of the Sn-doped FAPbI3 make it promising candidates for high-efficient perovskite cells.

Graphene-Bi 2 Te 3 Heterostructure as Broadband Saturable Absorber for Ultra-Short Pulse Generation in Er-Doped and Yb-Doped Fiber Lasers

Abstract: A new type of broadband saturable absorber made of graphene-Bi2Te3 heterostructure was reported, which synergistically combines light–matter interaction in graphene with that in a small bandgap semiconductor material Bi2Te3 to achieve an improved broadband nonlinear optical response. The graphene-Bi2Te3 heterostructure films were grown by chemical vapor deposition with 15% ${\rm Bi}_{2}{\rm Te}_{3}$ coverage on graphene, in which most of the Bi2Te3 nanoplatelets are less than 30 nm thick. It is interesting to find that the heterostructure thin film shows broadband saturable absorption property. At the communication band (around 1560 nm), the saturable intensity and modulation depth are measured to be 4.95 MW/cm2 and 18.98%, respectively. While around 1067 nm, the corresponding saturable intensity and modulation depth are experimentally measured to be 2.61 MW/cm2 and 23.11%, respectively. By incorporating this optical saturable absorber inside either an Er-doped or Yb-doped fiber laser, we are able to generate ultra-short pulse with very stable operation at 1565.6 and 1049.1 nm. Our experimental results clearly demonstrate that the graphene-Bi 2Te3 heterostructure can be a promising broadband nonlinear optical material for broadband ultra-fast laser photonics.

Tripole-mode and quadrupole-mode solitons in (1 + 1)-dimensional nonlinear media with a spatial exponential-decay nonlocality

Abstract: The approximate analytical expressions of tripole-mode and quadrupole-mode solitons in (1 + 1)-dimensional nematic liquid crystals are obtained by applying the variational approach. It is found that the soliton powers for the two types of solitons are not equal with the same parameters, which is much different from their counterparts in the Snyder-Mitchell model (an ideal and typical strongly nolocal nonlinear model). The numerical simulations show that for the strongly nonlocal case, by expanding the response function to the second order, the approximate soliton solutions are in good agreement with the numerical results. Furthermore, by expanding the respond function to the higher orders, the accuracy and the validity range of the approximate soliton solutions increase. If the response function is expanded to the tenth order, the approximate solutions are still valid for the general nonlocal case.

Interaction between anomalous vortex beams in nonlocal media

Abstract: In this paper, the interactional behaviors of anomalous vortex beams in highly nonlocal media are investigated. An analytical expression describing the interaction is given, and some numerical simulations are carried out to illustrate the interactional characteristics. It is found that two interactional anomalous vortex beams always attract each other, and their evolution is periodical. The influence of different relative phases between two anomalous vortex beams on their interaction is presented in details.

Highly efficient flexible quantum-dot light emitting diodes with an ITO/Ag/ITO cathode

Abstract: Flexible quantum dot light emitting diodes (QLEDs) are highly desired due to their advantages of foldability, lightweight, and potential applications in lighting and displays. In this report, we successfully fabricated high performance red (R), green (G), and blue (B) three primary color QLEDs based on a poly(ethylene-terephthalate)/ITO/Ag/ITO (PET–IAI) cathode. The multilayer flexible IAI electrode shows outstanding stability even after bending over 2000 times with a critical bending radius of 5 mm; the sheet resistance of the IAI film only increases from 12.7 to 14.8 Ω □−1. The maximum current efficiencies are 16.3, 86.5, and 16.1 cd A−1 for RGB QLEDs, respectively, which is the best device performance for flexible RGB QLEDs reported to date. Moreover, to the best of our knowledge, these are also record efficiencies for the green and blue devices in all the reported QLEDs. Furthermore, all the devices show saturated electroluminescence (EL) with the corresponding emission peaks at 606, 530, and 478 nm for three primary color QLEDs. The superior performance is a result of high transmittance and stability of the PET–IAI film. These results signify remarkable progress in flexible QLEDs and suggest that the PET–IAI based flexible QLEDs can offer a practicable platform for foldable applications.

Tuning band gaps and optical absorption of BiOCl through doping and strain: insight form DFT calculations

Abstract: Bismuth oxyhalides (BiOX, X = Cl, Br, and I) are a new family of promising photocatalysts. BiOCl and BiOBr possess large band gaps and weak absorption in visible light regions, which limit their applications. Although the band gap of BiOI is suitable to absorb most of the visible light, its redox capability is very weak. In this work, the doping and strain effects on the electronic structures and optical properties of BiOCl are explored using first principle calculations. The results show that doping in BiOCl, especially co-doping of Sb and I atoms, can obviously decrease the band gaps along with enhancing the optical absorption coefficients of pristine BiOCl because of the electronegativity difference between Sb/I atoms and Bi/Cl atoms. Meanwhile the band gap of BiOCl can be tuned under strain. This work offers potential strategies to enhance BiOCl absorption coefficients in the visible light region and its photocatalyst activity.

A pseudo-ELISA based on molecularly imprinted nanoparticles for detection of gentamicin in real samples

Abstract: The enzyme-linked immunosorbent assay (ELISA) is one of the most widely employed tests in diagnostics, and it relies on the use of antibodies to quantify the molecule of interest. Molecularly imprinted nanoparticles (nanoMIPs), thanks to their stability, cost efficiency and easy production, are a promising alternative to antibodies in assays and sensors. In this work, nanoMIPs have been produced by means of a solid-phase approach and employed for the detection of gentamicin in real samples. The produced nanoMIPs were characterized using dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques. The determination of gentamicin in spiked milk was implemented through an assay similar to enzyme-linked immunosorbent assay, in which the nanoMIPs were used as a synthetic capture antibody (pseudo-ELISA). The detection of gentamicin was achieved in competitive binding experiments with a horseradish peroxidase–gentamicin conjugate. Gentamicin was determined in milk at clinically relevant concentrations with a mean accuracy of 94%. The cross-reactivity of such nanoparticles was investigated with streptomycin and ampicillin as control antibiotics, demonstrating excellent specificity.

Unimodal Stopping Model-Based Early SKIP Mode Decision for High-Efficiency Video Coding

Abstract: High-efficiency video coding (HEVC) can greatly improve coding efficiency compared with the prior video coding standard H.264/AVC by adopting advanced hierarchical coding structures such as coding unit (CU), prediction unit (PU), and transform unit. For each CU, an exhaustive mode decision strategy is adopted to achieve the best rate distortion (RD) cost, which simultaneously results in enormous computational complexity. In this paper, an early SKIP mode decision algorithm is proposed for the HEVC encoder to speed up the process of mode decision. Each CU size is categorized into either rare used or frequent used by exploiting the correlation of CU depth, which is estimated from the temporally colocated CUs. For the rare-used CU size, the SKIP mode is directly selected as the optimal mode and the remaining mode decision process is early terminated. For the frequent-used CU size, a unimodal stopping model is designed for its early SKIP mode decision by exploiting both hierarchical mode structure and RD cost property. Experimental results show that the proposed early SKIP mode decision method achieves average 58.5% and 54.8% encoding time savings, while the Bjontegaard Delta bit rate only increases average 0.8% and 0.8% for various test sequences under the random access and the low delay B conditions, respectively.

Molecularly imprinted fluorescent probe based on FRET for selective and sensitive detection of doxorubicin

Abstract: In this work, a new type of fluorescent probe for detection of doxorubicin has been constructed by the combined use of fluorescence resonance energy transfer (FRET) technology and molecular imprinting technique (MIT). Using doxorubicin as the template, the molecularly imprinted polymer thin layer was fabricated on the surfaces of carbon dot (CD) modified silica by sol-gel polymerization. The excitation energy of the fluorescent donor (CDs) could be transferred to the fluorescent acceptor (doxorubicin). The FRET based fluorescent probe demonstrated high sensitivity and selectivity for doxorubicin. The detection limit was 13.8 nM. The fluorescent probe was successfully applied for detecting doxorubicin in doxorubicin-spiked plasmas with a recovery of 96.8–103.8%, a relative standard deviation (RSD) of 1.3–2.8%. The strategy for construction of FRET-based molecularly imprinted materials developed in this work is very promising for analytical applications.

Rules to write mathematics to clarify metrics such as the land use dynamic degrees

Abstract: Scientists frequently describe temporal change among land categories by reporting the single land use dynamic degree and the comprehensive land use dynamic degree (CLUDD). The original intention of CLUDD was to compute the annual change percentage, which is the size of annual change expressed as a percentage of the size of the spatial extent. However, the literature’s mathematical descriptions of CLUDD have been unclear, thus readers have imagined various ways to compute CLUDD. Our manuscript clarifies the confusion and offers rules for mathematical notation so that authors can avoid future confusion. We examine the literature to see how authors have computed and interpreted the land use dynamic degrees. We illustrate deficiencies of one version of CLUDD. Then we propose equations for the components of annual change percentage. The literature shows three common misunderstandings. First, some authors add percentages of categories without accounting for the sizes of the categories. Second, other authors compute either double or half of the annual change percentage. Third, many authors interpret CLUDD as if CLUDD were the annual change percentage when they use a version of CLUDD that is not the annual change percentage. We recommend that the professional community use annual change percentage, its three components and Intensity Analysis to express temporal change among categories. Vague mathematical notation has created confusion concerning the land use dynamic degrees; therefore, we give rules for how to write mathematics clearly in a manner that applies to many professions.

Precise identification of graphene layers at the air-prism interface via a pseudo-Brewster angle

Abstract: We propose a simple method for the precise identification of graphene layers at the air-prism interface via a pseudo-Brewster angle, where the horizontally polarized reflection is close to zero. We find that the pseudo-Brewster angle is sensitive to the variation of graphene layers where the pseudo-Brewster angle is approximately linearly increased about 0.5 deg as the layer numbers increased. Furthermore, the sensitivity of the pseudo-Brewster angle can be greatly enhanced and reaches 0.04 deg by eliminating the influence of the cross-polarization effect. Our scheme can provide a simple and effective method to identify the layer numbers of graphene.

Efficient photocatalytic degradation of 4-nitrophenol over graphene modified TiO2

Abstract: BACKGROUND Photocatalytic degradation of organic compounds, including nitrophenol, is one of the most popular approaches to pollution control and disposal. Graphene-based nanocomposites have attracted considerable attention in photocatalytic research owing to their excellent electrical, thermal and mechanical properties. In this work, TiO2 material was modified with graphene by a hydrothermal method, the photocatalytic performance of the prepared catalyst was evaluated by degradation of 4-nitrophenol, which is a highly toxic, stable and carcinogenic organic pollutant. RESULTS The characterization results showed that the low amount of graphene disperses well over TiO2 and it does not influence the crystal phase, but makes the specific surface area of graphene/TiO2 greater than that of TiO2. Compared with TiO2, graphene/TiO2 exhibits enhanced photocatalytic activity for the degradation of 4-nitrophenol under simulated solar irradiation. CONCLUSION The enhanced activity of photo-degradation could be attributed to the involved graphene being beneficial to charge transportation and separation of photogenerated charge carriers, and providing more surface active sites for degradation reaction of the target pollutant. The present result highlights the important application of graphene as a novel carbon material in treating highly toxic pollutant through photocatalysis technology. © 2017 Society of Chemical Industry

Quantum non-Markovian reservoirs of atomic condensates engineered via dipolar interactions

Abstract: We investigate the quantum dephasing dynamics of an impurity qubit immersed in a quasi-two-dimensional dipolar Bose-Einstein condensate whose collective excitations act as a reservoir for the qubit. We show that the properties of the environment are highly engineerable through the relative strength of the dipolar and contact interactions such that qubit's dephasing dynamics could be Markovian, weak non-Markovian, or even highly non-Markovian. It is also revealed that the appearance of the roton excitation is responsible for the highly non-Markovian dephasing dynamics. Since rotonlike dispersions also appear in condensates placed in cavities or with spin-orbit couplings, our results pave the way for searching for systems that are suitable environment engineering.

Adaptive Inter CU Depth Decision for HEVC Using Optimal Selection Model and Encoding Parameters

Abstract: High efficiency video coding adopts a new hierarchical coding structure, including coding unit (CU), prediction unit (PU), and transform unit to achieve higher coding efficiency than its predecessor H.264/AVC high profile. However, its hierarchical unit partitioning strategy leads to huge computational complexity. In this paper, an adaptive inter CU depth decision algorithm is proposed, which exploits both temporal correlation of CU depth and available encoding parameters. An optimal selection model of CU depth is established to estimate the range of candidate CU depth by exploiting the temporal correlation of CU depth among current CU and temporally co-located CUs. To reduce the accumulated errors in the process of CU depth prediction, the maximum depth of the co-located CUs and the coded block flag (CBF) of the current CU are used. Moreover, PU size and CBF information are also used to decide the maximum depth for the current CU. Experimental results show that the proposed CU depth decision approach reduces 56.3% and 51.5% average encoding time, and the Bjontegaard delta bit rate increases only 1.3% and 1.1% for various test sequences under the random access and the low delay B conditions, respectively.

Measurements of Pancharatnam-Berry phase in mode transformations on hybrid-order Poincaré sphere.

Abstract: We report direct measurements of the Pancharatnam-Berry (PB) phase in mode transformations on a hybrid-order Poincare sphere. This geometric phase arises when the vector vortex states undergo a cyclic transformation over a closed circuit on a hybrid-order Poincare sphere. The measured PB phase is proportional to the solid angle of the closed circuit, as well as to the variation of the total angular momenta between north and south poles. More importantly, a zero PB phase has been demonstrated, despite the vector vortex states taken through a closed circuit on the hybrid-order Poincare sphere. This interesting phenomenon can be explained as being due to the zero Berry curvature.

Graphene decorated MoS2 for eosin Y-sensitized hydrogen evolution from water under visible light

Abstract: Some two-dimensional nanomaterials, such as graphene and molybdenum disulfide, are presently being intensively investigated due to their excellent and unique performances. In the field of photocatalysis, MoS2 is considered as a promising alternative to noble metal Pt for the hydrogen evolution reaction (HER). However, its poor electrical conductivity restricts its catalytic activity in the HER. In this work, MoS2 was modified with graphene (G) by a simple hydrothermal method. The prepared G/MoS2 composite was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results show that graphene modification does not influence the crystal phase of MoS2, but makes the latter more dispersed. The HER performance of G/MoS2 was evaluated using eosin Y (EY) as a photosensitizer, and triethanolamine (TEOA) as a sacrificial electron donor under visible-light irradiation (λ > 420 nm, 250 W high pressure Hg lamp as light source). The EY sensitized G/MoS2 composite displays enhanced hydrogen evolution in terms of not only activity but also stability. The average HER activity (9.1 μmol h−1) is three times that of EY sensitized pure MoS2 over 10 h. It is believed that the incorporation of graphene enhances the charge transfer ability and retards the self-degradation path of EY˙−, ultimately improving the HER.

John Disks and K-Quasiconformal Harmonic Mappings

Abstract: The main aim of this article is to establish certain relationships between K-quasiconformal harmonic mappings and John disks. The results of this article are the generalizations of the corresponding results of Pommerenke (J Lond Math Soc 26:77–88, 1982).