A Novel Dynamic Stretching Solution to Eliminate Saturation Effect in NDVI and Its Application in Drought Monitoring

The normalized difference vegetation index(NDVI) is one of the key input variables for developing drought indices.However,the NDVI quickly saturates in high vegetation surfaces,and thus,the generalization of a drought index over different ecosystems becomes a challenge.This paper presents a novel,dynamic stretching algorithm to overcome the saturation effect in NDVI.A scaling transformation function to eliminate saturation effects when the vegetation fraction(VF) is large is proposed.Dynamic range adjustment is conducted using three coefficients,namely,the normalization factor(a),the stretching range controlling factor(m),and the stretching size controlling factor(e).The results show that the stretched NDVI(S-NDVI) is more sensitive to vegetation fraction than NDVI when the VF is large,ranging from 0.75 to 1.00.Moreover,the saturation effect in NDVI is effectively removed by using the S-NDVI.Further analysis suggests that there is a good linear correlation between the S-NDVI and the leaf area index(LAI).At the same time,the proposed S-NDVI significantly reduces or even eliminates the saturation effect over high biomass.A comparative analysis is performed between drought indices derived from NDVI and S-NDVI,respectively.In the experiment,reflectance data from the moderate resolution imaging spectroradiometer(MODIS) products and in-situ observation data from the meteorological sites at a regional scale are used.In this study,the coefficient of determination(R2) of the stretched drought index(S-DI) is above 0.5,indicating the reliability of the proposed algorithm with surface soil moisture content.Thus,the S-DI is suggested to be used as a drought index in extended regions,thus regional heterogeneity should be taken into account when applying stretching method.

A device model for thin silicon-on-insulator SiGe heterojunction bipolar transistors with saturation effects

In this paper, we describe the saturation effect of a silicon germanium （SiGe） heterojunction bipolar transistor （HBT） fabricated on a thin silicon-on-insulator （SOI） with a step-by-step derivation of the model formulation. The collector injection width, the internal base-collector bias, and the hole density at the base-collector junction interface are analysed by considering the unique features of the internal and the external parts of the collector, as they are different from those of a bulk counterpart.

Energetic Ion Effects on the Ion Saturation Current

The ion saturation current is very important in probe theory, which can be used to measure the electron temperature and the floating potential. In this work, the effects of energetic ions on the ion saturation current are studied via particle-in-cell simulations. It is found that the energetic ions and background ions can be treated separately as different species, and they satisfy their individual Bohm criterion at the sheath edge. It is shown that the energetic ions can significantly affect the ion saturation current if their concentration is greater than root T-e/(gamma T-i2(i2)), where T-e is the electron temperature, and gamma(i2) and T-i2 represent the polytropic coefficient and temperature of energetic ions, respectively. As a result, the floating potential and the I-V characteristic profile are strongly influenced by the energetic ions. When the energetic ion current dominates the ion saturation current, an analysis of the ion saturation current will yield the energetic ion temperature rather than the electron temperature.

Direct detection of the transient superresolution effect of nonlinear saturation absorption thin films

Using a strong nonlinear saturation absorption effect is one technique for breaking through the diffraction limit. In this technique, formation of a dynamic and reversible optical pinhole channel and transient superresolution is critical. In this work, a pump–probe transient detection and observation–experimental setup is constructed to explore the formation process directly. A Ge2Sb2Te5 thin film with strong nonlinear saturation absorption is investigated. The dynamic evolution of the optical pinhole channel is detected and imaged, and the transient superresolution spot is directly captured experimentally. Results verify that the superresolution effect originates from the generation of an optical pinhole channel and that the formation of the optical pinhole channel is dynamic and reversible. A good method is provided for direct detection and observation of the transient process of the superresolution effect of nonlinear thin films.

Rapid transition of the hole Rashba effect from strong field dependence to saturation in semiconductor nanowires

With the support by the National Natural Science Foundation of China,a study by the research group led by Prof.Luo Junwei(骆军委)from the Institute of Semiconductors,Chinese Academy of Sciences discovered a rapid transition of the hole Rashba effect from strong field dependence to saturation

Self-synchronized multi-color Q-switched fiber laser using a parallel-integrated fiber Bragg grating

We demonstrate an intracavity self-synchronized multi-color Q-switched fiber laser using a parallel-integrated fiber Bragg grating(PI-FBG), fabricated by a femtosecond laser with a point-by-point parallel inscription method. The multi-color Q-switched pulses can be always self-synchronized when the group delay differences between neighboring spectra range from-3.4 to 3.4 ps.The starting and evolution dynamics indicate that the saturable absorption effect of the carbon nanotube plays a dual role: synchronously triggering the startup of the pulse at successive colors by active Q-switching and spontaneously compensating to some extent the temporal walk-off of the multi-color pulses through the cross saturable absorption modulation. This work unveils the intracavity self-synchronization mechanism of the multi-color Q-switched pulses and also demonstrates the potential of PI-FBGs for the customizable generation of the synchronized multi-color pulse in a single cavity.

Oxidation characteristics of boron particles

The oxidation characteristics of boron particles, boron-A with the diameter of 2.545 μm and boron-B with the diameter of 10.638 μm, at low temperature（1500 K） have been investigated by thermogravimetry（TG） coupled with simultaneous differential scanning calorimetry（DSC）, infrared and mass spectra. A rapid oxidation stage of boron particles, followed by a slow oxidation stage of sintered particles, is found from the TG and DSC curves. The onset temperatures of the oxidation process of boron-A particles are in the range of 806–889 K, which are at least 105 K lower than those of boron-B at the same condition. As the partial pressure of oxygen increases from 5% to 35%, the onset temperature of boron-A or boron-B particles decreases. However, when the partial pressure of oxygen is above 35%, the onset temperature becomes constant, implying a saturation effect of oxygen on the reaction rate. It indicates that the chemical adsorption of oxygen, i.e. chemical reaction, on the particle surface is the rate-limited step at the beginning of the rapid oxidation stage. Therefore, the first-order chemical reaction model is used to simulate the oxidation of boron particles, even that of the sinter. The average activation energies of the particles are 291.3 kJ/mol for boron-A and 338.4 k J/mol for boron-B. While the average activation energies of the sintered particles are 36.35 k J/mol for boron-A and 31.87 kJ/mol for boron-B. The pre-exponential factor of the particles is -10^4, while that of the sinter is 10^-1. The oxidation rate constant of boron is qualitatively mainly affected by the specific surface of the sample and the thickness of the oxide layer.

Supporting Earth Pressures for Foundation Excavation Considering Suction Stress of Soil

The infiltration,evaporation and variation of the groundwater table have significant effects on the suction stress of the soils and the supporting earth pressures of the foundation excavation.The distribution of the suction stresses above the ground water table is derived under different fluxes at the ground surface,according to the soil-water characteristic parameters and the effective degree of saturation.In consideration of the cohesive stress formed from the soil suction stress and the relevant anti sliding effect,the calculation model of supporting earth pressures for foundation excavation is established by the variational limit equilibrium method under the steady flow condition.The evolution of the supporting earth pressures is studied in detail for foundation excavation under different fluxes at the ground surface.The effects of the soil-water characteristic parameters,the ground water table and the internal friction angle on the supporting earth pressures are discussed.The results show that the suction stress is reduced because of the infiltration,and thus the supporting earth pressure increases.The larger the air-entry pressures and the pore size are,the smaller the supporting earth pressures are.The higher the ground water table is,the larger the supporting earth pressures are.In order to reduce the construction risk,the effects of the suction stress and the evolution of the potential critical sliding surface should be considered during the calculation of the supporting earth pressures.