Absorption and dispersion control in a five-level M-type atomic system

We investigate the steady optical response of a coherently driven five-level M-type atomic system in three different situations.When all three coupling fields have the same zero detuning,we just find one deep transparency window accompanied by a steep normal dispersion in the probe absorption and dispersion spectra.When two coupling fields are detuned from the relevant transitions to the same extent,however,a second deep transparency window may be observed in the presence of a narrow absorption line of linewidth ～50 kHz.In this case,two single-photon far-detuned transitions can be replaced by a two-photon resonant transition,so the five-level M system in fact reduces into a four-level quasi-Λ system.Finally,we note that no deep transparency windows and no narrow absorption lines can be found when all three coupling fields have unequal detunings.     

Study of a rectangular coupled cavity extendedinteraction oscillator in sub-terahertz waves

An extended interaction oscillator （EIO） generating 120 GHz wave in sub-terahertz waves is studied by using the three-dimensional electromagnetic simulation software CST and PIC codes. A rectangular reentrant coupled-cavity is proposed as the slow-wave structure of EIO. By CST, the circuit parameters including frequency-phase dispersion, interaction impedance and characteristic impedance are simulated and calculated. The operation mode of EIO is chosen very close to the point where βL = 2π with corresponding frequency 120 GHz, the beam voltage 12 kV and the dimensions of the cavity with the period 0.5mm, the height 3mm and the width 1.4mm. Simulation results of beam-wave interaction by PIC show that the exciting frequency is 120.85 GHz and output peak power 465 W with 12-period coupled-cavity with the perveance 0.17 μP. Simulation results indicate that the EIO has very wide range of the operation voltage.     

Facile approach to obtain individual‐nanotube dispersion at high loading in carbon nanotubes/polyimide composites

A big challenge in making a composite lies in achieving individual‐nanotube dispersion of carbon nanotubes (CNTs) in a polymer matrix, without aggregation and entanglement and excellent interfacial adhesion between the CNTs and the polymers matrix. In this communication, using polyethylene glycol‐200, we successfully prepared CNT‐reinforced polyimide composites that exhibit individual‐nanotube dispertion in the matrix at high‐loading CNT's. The content of CNTs in a composite can reach 43 wt%. Copyright © 2007 John Wiley & Sons, Ltd.     

基于等效层法分析对称性倍频分束镜的半波孔

半波孔问题严重影响了倍频分束镜在高功率激光系统中的应用。为研究半波孔的产生机理,分别模拟了由高、低折射率膜层厚度失配和膜料色散失配引起的1/4波长对称规整膜系的半波孔。基于等效层理论,在Matlab平台上计算膜系的等效折射率E,绘出其对应的反射率极值包络曲线。通过研究对称膜系的等效折射率、反射率光谱和反射率包络之间的关系,从原理上分析了半波孔的大小、位置和变化趋势等特点。结果表明,膜层的厚度失配使对称膜系的等效折射率在光谱半波处产生截止带。膜层数越多,膜层厚度和膜料色散的失配越严重,则倍频分束镜的半波孔越深。     

A highly stable and active mesoporous ruthenium catalyst for ammonia synthesis prepared by a RuCl3/SiO2-templated approach

Molecular nitrogen is relatively inert and the activation of its triple bond is full of challenges and of significance. Hence, searching for an efficiently heterogeneous catalyst with high stability and dispersion is one of the important targets of chemical technology. Here, we report a Ba-K/Ru-MC catalyst with Ru particle size of 1.5–2.5 nm semi-embedded in a mesoporous C matrix and with dual promoters of Ba and K that exhibits a higher activity than the supported Ba-Ru-K/MC catalyst, although both have similar metal particle sizes for ammonia synthesis. Further, the Ba-K/Ru-MC catalyst is more active than commercial fused Fe catalysts and supported Ru catalysts. Characterization techniques such as high-resolution transmission electron microscopy, N₂ physisorption, CO chemisorption, and temperature-programmed reduction suggest that the Ru nanoparticles have strong interactions with the C matrix in Ba-K/Ru-MC, which may facilitate electron transport better than supported nanoparticles.     

Vibrational Dynamics and Heat Capacity of Poly(3-methylthiophene): A Conducting Polymer

Using the Urey–Bradley force field and Wilson's GF matrix method as modified by Higgs, normal modes of vibration and their dispersions in poly(3-methylthiophene) have been obtained. They provide a detailed interpretation of its I.R. and Raman spectra. Characteristic features of the dispersion curves, such as regions of high density-of-states, repulsion, and character mixing of dispersive modes, are discussed. Predictive values of the heat capacity as a function of temperature have been calculated.     

Electromagnetically Induced Transparency of Two Intense Circularly-Polarized Lasers in Cold Plasma: Beat-Wave Second Harmonic Effect

In the present paper the problem of nonlinear interaction of two mildly-relativistic circularly polarized lasers in a cold plasma is studied in order to investigate electromagneticaily induced transparency （EIT）. Based on a relativistic kinetic model, by expansion of relativistic Lorentz factor in terms of lasers amplitude, we obtain the coupled nonlinear dispersion relations. It is observed that due to resonance in the second harmonic of plasma beat-wave, the new EIT pass-band is created in the high intensities of lasers. The effect of amplitude and frequency variation on the dispersion is numerically investigated.     

Change of gain characters for short pulses with third order dispersion in a fiber amplifier

The effect of the third order dispersion (TOD) induced into pulses before entering an Yb-doped fiber amplifier on their power gain characters is investigated using a model of which the pulse amplification process is described by nonlinear time-dependent radiation transfer equations. Numerical results show that peak power gain for the pulse with positive TOD is larger than that with zero and without TOD while the improvement of the peak power gain varies as the obtained output pulse energy changes. The improvement also depends on the ratio of dispersion lengths for group velocity dispersion and TOD.     

Research on target uniform irradiation method using linearly modulated light and special grating dispersion

In indirect-drive laser fusion research, filamentation instability (FI) severely influences laser injection efficiency and beam pointing accuracy in hohlraum. Experiments indicate that high frequency modulations on the far field of the laser driver are responsible for the filamentation. To solve this problem, a new smoothing technique is proposed using linear modulation and special grating dispersion. The linear modulation was realized by stacking a series of 100 ps chirped pulses in fiber delay lines. The temporal smoothing characteristics of the focal spot adopting star and circular grating dispersion were studied. Simulation results indicate that both kinds of grating investigated restrained the high frequency modulations. The star grating has a better smoothing effect when compared to circular grating. The insertion of the star grating could realize “color” variation across the beam in the azimuthal direction and make the far-field pattern rotate with time. Therefore, the time-integrated focal spot would be smoothed in a more comprehensive way. This smoothing method bears the advantage of simple configuration, two-dimensional angular spectral dispersion and not enlarging the far field too much.     

Improved power simulation of AlGaN/GaN HEMT at class-AB operation via an RF drain–source current correction method

A new modified Angelov current–voltage characteristic model equation is proposed to improve the drain–source current（Ids） simulation of an Al Ga N/Ga N-based（gallium nitride） high electron mobility transistor（Al Ga N/Ga N-based HEMT） at high power operation. Since an accurate radio frequency（RF） current simulation is critical for a correct power simulation of the device, in this paper we propose a method of Al Ga N/Ga N high electron mobility transistor（HEMT）nonlinear large-signal model extraction with a supplemental modeling of RF drain–source current as a function of RF input power. The improved results of simulated output power, gain, and power added efficiency（PAE） at class-AB quiescent bias of Vgs =-3.5 V, Vds= 30 V with a frequency of 9.6 GHz are presented.