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911.
Solar System tests give nowadays constraints on the estimated value of the cosmological constant, which can be accurately
derived from different experiments regarding gravitational redshift, light deflection, gravitational time-delay and geodesic
precession. Assuming that each reasonable theory of gravitation should satisfy Solar System tests, we use these limits on
the estimated value of the cosmological constant to constrain extended theories of Gravity, which are nowadays studied as
possible theories for cosmological models and provide viable solutions to the cosmological constant problem and the explanation
of the present acceleration of the Universe. We obtain that the estimated values, from Solar System tests, for the parameters
appearing in the extended theories of Gravity are orders of magnitude bigger than the values obtained in the framework of
cosmologically relevant theories. 相似文献
912.
The formation of Ag nanoparticles synthesized by homogeneous nucleation, stabilized by polymers (PVA and PVP) was monitored
by UV–Vis spectrophotometry and transmission electron microscopy. Our aim was to differentiate between the two main phases
of particle formation, i.e. nucleation and growth and to characterize their rates with the help of appropriate kinetic equations.
Time resolved spectrophotometric measurements revealed that particle formation is an autocatalytic process: a slow, continuous
nucleation phase (3–5 min) is followed by a rapid, autocatalytic growth phase where the maximal particle size is 5–7 nm. By
freezing the reaction mixture, the process of particle growth can be followed from 5 to 40 min on TEM pictures. The first
order rate constants were calculated and they are strongly depend on the polymer concentration. If the growing particles are
attached by PEI to the surface of a solid support, the formation of silver nanoparticles can also be followed by atomic force
microscopy (AFM) and we can control the particle growth on mica surface. The cross section analysis of the pictures show,
that the particle growing process can be also monitored at solid–liquid interface. 相似文献
913.
Boltzmann-transport equation is analytically solved for two-component magnetoplasma using Chapman-Enskog analysis to include
collisional diffusion transport having anisotropies in both streaming velocity and temperature components. The modified collisional
integrals are analytically solved with flux integrals and perturbed kinetic equation to arrive at drift diffusion velocity
and resulting transport coefficients which are markedly affected by both streaming and temperature anisotropy. The early isotropic
results are recovered in the limit V
0 = 0 and T
‖ = T
⊥ which reduce to eqs (11.30) and (11.31) of [1] and eqs (2.7) and (2.13) of [2]. The electrical resistivity (η⊥) diminishes sharply in fusion temperature limit kT
⊥ = 1 keV. The shape of the curves for both electrical resistivity and thermal conductivity is rectangular hyperbolic. However,
for low thermal ratio (T
‖/T
⊥ < 1), the curves are raised up and for high thermal ratio (T
‖/T
⊥ > 1), they are lowered down the isotropic case (T
‖/T
⊥ > 1), showing comparatively diminished magnitudes of the quantities.
相似文献
914.
Omar Qasaimeh 《Optical and Quantum Electronics》2017,49(3):109
The characteristics of optical bistability in quantum dot vertical cavity semiconductor optical amplifier integrated with MEMS membrane have been investigated, and a closed-form model is derived taking into account the effect of the quantum dot discrete states and the membrane deflection and reflectivity. We show that small membrane deflection significantly adjusts the resonant wavelength, the contrast ratios and the hysteresis width. The shape of the input–output characteristics of the device can be adjusted to display clockwise, butterfly and counterclockwise hysteresis loops depending on the membrane deflection and the initial wavelength detuning. Our analysis reveals that the contrast ratios of the upper and lower bistable levels are totally different. Also, it has been shown that the characteristics of the bistability are strong function of the active layer linewidth enhancement factor and the membrane reflectivity. 相似文献
915.
This paper introduces design and simulation of a three-dimensional complementary metal–oxide–semiconductor CMOS compatible photo-sensor based on a silicon substrate. In the structure of photo-sensor, a vertical n+/p junction as a photosensitive area is formed on one side of a U-groove, and perpendicular to a lateral n-i-p structure on top-side of the silicon surface. This configuration enables a direct butt-coupling of a fiber-optic to the photosensitive area, which is a privilege for many remote monitoring applications. The device analysis is carried out by a two-dimensional simulation using SILVACO TCAD simulator. The thickness of the photo-sensitive area is investigated by considering the figures of merit for the two different thicknesses of 30 and 50 µm. The simulated results (according to the parameters defined for the Si substrate) show a very low dark current of 70 and 100 (fA/μm) for the 30 and 50 µm thicknesses, respectively. In addition, a high photo-current to dark current ratio of ~3000 is achieved under an intensity of 2 mW/cm2 at 633 nm wavelength, according to the wavelength of red He–Ne laser. The sensor demonstrates a responsivity of 0.33 A/W corresponding to 65% external quantum efficiency and a ?3 dB frequency response of 0.2 GHz under a small signal of 2 mW/cm2 at 633 nm wavelength for 10 V reverse bias. 相似文献
916.
Xuefang Zhou Chaoqun Ge Yizhen Wei Guowei Yang Meihua Bi Miao Hu 《Optical and Quantum Electronics》2017,49(4):166
We have experimentally demonstrated a tunable multi-wavelength Brillouin–erbium fiber laser with over 40 GHz spacing utilizing two cascaded double Brillouin-frequency-spacing cavities. In this laser configuration, two segments of 25 km-long single-mode fibers are used as Brillouin gain medium in each ring cavity, and a segment of 8 m-long erbium-doped fiber with 980 nm pump is employed to amplify Brillouin pump (BP). At BP wavelength of 1550 nm, BP power of 8.3 dBm (6.8 mW) and the maximum 980 nm pump power of 27.78 dBm (600 mW), seven output channels with fourfold Brillouin-frequency spacing, and the tuning range of 15 nm from 1545 to 1560 nm are achieved. The proposed multi-wavelength Brillouin–erbium fiber laser has wide applications, such as in microwave signal generation and optical communications. 相似文献
917.
Tellurite glasses (TeO2–ZnO–Nb2O5) mono-doped Er3+ and co-doped Er3+/Ce3+ have been prepared using the melt-quenching technique. To evaluate the effect of Ce3+ on the structural, thermal stability of glass hosts and fluorescence properties of Er3+, X-ray diffraction patterns, Ftir spectra, differential scanning calorimeter curves, absorption spectra, fluorescence emission spectra, fluorescence lifetimes, up-conversion emission spectra of glass samples were measured and investigated. Using Judd–Ofelt theory, we calculated intensity parameters (Ω2, Ω4 and Ω6), spontaneous emission probabilities, the radiative lifetime, luminescence branching factors and the quantum yield of luminescence for 4I13/2 → 4I15/2 transition. The co-doping with Ce3+ was effective on the suppression of up-conversion emission of Er3+ owing to the phonon-assisted energy transfer: Er3+:4I11/2 + Ce3+:2F5/2 → Er3+:4I13/2 + Ce3+:2F7/2 which contributed the effective enhancement of 1.53 µm fluorescence emission. The change in optical properties with the addition of Ce3+ ions have been discussed and compared with other glasses. Using the Mc Cumber method for the 4I13/2 → 4I15/2 transition, absorption cross-section, calculated emission cross-section, and gain cross-section values support that TZNEr1Ce1 glass is a potential material for developing broad-band and high-gain erbium-doped fiber amplifiers applied for 1.53 µm. 相似文献
918.
919.
Ming-Fei Chen Wen-Tse Hsiao Ming-Cheng Wang Kai-Yu Yang Ying-Fang Chen 《Optical and Quantum Electronics》2017,49(11):383
This paper presents the optimal conditions for the ultraviolet laser percussion drilling of alumina materials intended for use in heat sinks. The Taguchi method and grey relational analysis, along with the consideration of multiple quality characteristics, were applied for determining the optimal parameters. The entrance diameter and taper angle of the drilled hole were affected by the material processing parameters, including laser power, pulse duration, focal plane position, and number of pulses. The Taguchi method and grey relational analysis were used for assessing the effects of the operational parameters on multiple performance characteristics. Nine experiments based on an orthogonal array were performed. According to the results, the optimal process parameters were as follows: laser energy density, 3.82 J/cm2; focal plane position, 0.1 mm; number of pulses, 20 shots; and single pulse duration, 3 ms. Analysis of the grey relational grade revealed that the focal plane position was the most dominant parameter. 相似文献
920.
We perform the updated constraints on the Hubble constant H 0 by using the model-independent method, Gaussian processes. Utilizing the latest 30 cosmic chronometer measurements, we obtain H 0 = 67.38 ± 4.72 km s?1 Mpc?1, which is consistent with the Planck 2015 and Riess et al. analysis at 1σ confidence level. Different from the results of Busti et al. by only using 19 H(z) measurements, our reconstruction results of H(z) and the derived values of H 0 are insensitive to the concrete choice of covariance functions of Matérn family. 相似文献