稀土La掺杂CrSi2电子结构与光学性质的第一性原理研究

基于密度泛函理论的第一性原理赝势平面波方法, 对稀土La掺杂CrSi2的几何结构, 电子结构和光学性质进行了计算与分析. 结果表明, La掺杂后, CrSi2的晶格常数a, b 和c均增大, 晶格体积增大. La掺杂导致费米面进入价带, 带隙明显变窄仅为0.07eV; 在费米面附近, La原子的5d层电子态密度只占总态密度很小的一部分, 而总态密度仍然由Si的3p层和Cr的3d层电子的分波态密度决定; La掺杂后CrSi2的静态介电常数ε1(0)由28.98增大为91.69, ε2(ω)的两个介电峰均向低能方向偏移且增强, 光学吸收边向低能方向移动, 吸收峰减小. 计算结果为CrSi2材料掺杂改性的实验研究提供了理论依据.     

Al掺杂浓度对CrSi2电子结构及光学性质的影响

采用基于第一性原理的赝势平面波方法,对不同Al掺杂浓度CrSi2的几何结构、能带结构、态密度和光学性质进行了计算和比较。几何结构和电子结构的计算表明:Al掺杂使得CrSi2的晶格常数a和b增大,c变化不大,晶格体积增大;Cr(Si1-xAlx)2仍然是间接带隙半导体,掺杂使得费米面向价带移动,且随着掺杂量的增大而更深地嵌入价带中,费米能级附近的电子态密度主要由Cr的3d态电子贡献。光学性质计算表明,随着掺杂量的增大,Cr(Si1-xAlx)2的静态介电常数、第一介电峰、折射率n0逐渐增大,平均反射效应减弱,表明Al掺杂有效增强了CrSi2对光的吸收,能够提高其光电转换效率。计算结果为CrSi2光电材料的应用和设计提供了理论指导。     

Ac掺杂β-FeSi_2光电性质的第一性原理研究

基于密度泛函理论(DFT)框架下的第一性原理赝势平面波方法,对锕(Ac)掺杂β-FeSi2的几何结构,电子结构和光学性质进行了计算与分析。几何结构的计算表明:Ac掺杂后β-FeSi2晶格常数a、b及c都有所变化,晶胞体积增大。电子结构的计算表明:Ac掺入后导致费米面进入导带,能带结构仍为准直接带隙,但是带隙明显变窄;费米能级附近,总电子态密度主要由Fe的3d层和Si的3p层电子态密度决定,Ac的6d层电子态密度贡献很小。光学性质的计算表明:静态介电常数ε1(0)明显提高,介电函数的虚部ε2的峰值向低能方向移动并且减弱,折射率n0明显提高,消光系数k向低能方向有一微小的偏移,吸收峰增强,平均反射效应变化不大,计算结果为β-FeSi2材料掺杂改性的实验研究提供了理论依据。     

第一性原理计算V-Al共掺杂CrSi_2的光电特性

采用第一性原理赝势平面波方法,对V-Al共掺杂CrSi2的几何结构、电子结构和光学性质进行了理论计算,并与未掺杂、V、Al单掺杂CrSi2的光电性能进行了比较。结果表明:V-Al共掺杂会增大CrSi2的晶格常数a和b,体积相应增大。V-Al:CrSi2是p型间接带隙半导体,带隙宽度为0.256eV,介于V、Al单掺杂CrSi2之间;费米能级附近的电子态密度主要由Cr-3d、V-3d、Si-3p、Al-3p轨道杂化构成。与未掺杂的CrSi2相比,V-Al:CrSi2的静态介电常数和折射率增大,εi(ω)在低能区有一个新的跃迁峰。在光子能量为5eV附近,εi(ω)的跃迁峰强度大幅减弱,吸收系数和光电导率明显降低,吸收边略有红移,平均反射效应减弱。V的掺入会削弱Al单掺杂的电子跃迁,V-Al共掺杂可以对CrSi2的能带结构和光学性质进行更精细的调节。     

稀土（Y、Ce）掺杂β-FeSi2光电性质的第一性原理研究

基于第一性原理赝势平面波方法,对稀土(Y、Ce)掺杂β-FeSi2的几何结构、电子结构和光学性质进行了计算与分析。几何结构的计算表明,Y 或Ce掺杂后β-FeSi2的晶格常数改变,晶胞体积减小。电子结构的计算表明,掺入稀土后β-FeSi2 费米面附近的能带结构变得复杂,带隙变窄;总电子态密度发生了变化,Y 的4d 轨道电子态密度和Ce的4f轨道电子态密度主要贡献给费米面附近。光学性质的计算结果表明,Y 或Ce 掺杂后β-FeSi2 的静态介电常数明显提高,介电函数虚部ε2 的峰值均向低能方向移动并且减弱,折射率n0 明显提高,消光系数k 的峰值减弱,计算结果为β-FeSi2材料掺杂稀土改性的实验研究提供了理论依据。     

稀土(Y、Ce)掺杂b-FeSi_2光电性质的第一性原理研究

基于第一性原理赝势平面波方法,对稀土(Y、Ce)掺杂β-Fe Si2的几何结构、电子结构和光学性质进行了计算与分析。几何结构的计算表明,Y或Ce掺杂后β-Fe Si2的晶格常数改变,晶胞体积减小。电子结构的计算表明,掺入稀土后β-Fe Si2费米面附近的能带结构变得复杂,带隙变窄;总电子态密度发生了变化,Y的4d轨道电子态密度和Ce的4f轨道电子态密度主要贡献给费米面附近。光学性质的计算结果表明,Y或Ce掺杂后β-Fe Si2的静态介电常数明显提高,介电函数虚部ε2的峰值均向低能方向移动并且减弱,折射率n0明显提高,消光系数k的峰值减弱,计算结果为β-Fe Si2材料掺杂稀土改性的实验研究提供了理论依据。     

稀土元素La掺杂β-FeSi2的几何结构和电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理赝势平面波方法,对稀土元素La掺杂β-FeSi2的几何结构、能带结构、态密度进行了理论计算.几何结构的计算表明:La掺杂后使β-FeSi2的晶格常数a、b、c都变大了,使得晶胞体积也相应增大;La掺杂β-FeSi2的置换位置为FeⅡ位.电子结构的计算表明:能带结构为直接带隙,禁带宽度变窄仅为0.013eV;费米面进入价带,能带数目增多,态密度峰值减小,费米面附近载流子浓度显著增大.这些结果为β-FeSi2光电材料掺杂改性的实验和理论研究提供理论依据.     

掺杂MgCNi3超导电性和磁性的第一性原理研究

从第一性原理出发，计算了MgCNi₃的电子能带结构.MgCNi₃中C 2p与Ni 3d轨道杂化使穿梭费米面上的Ni 3d能带表现出平面性，费米面落在态密度范霍夫奇异(vHs)峰的右坡上.vHs峰上大的电子态密度和铁磁相变点附近的自旋涨落是决定MgCNi₃超导电性的重要因素.研究了三种替代式掺杂对其超导电性和磁性的影响，发现电子掺杂使费米能级下滑到态密度较低的位置，导致体系转变为无超导电性的顺磁相；同构等价电子数的金属间化合物的轨道杂化，引起费米面上态密度的减少，降低了超导电性；而空穴掺杂使费米面向vHs峰值方向移动，虽然费米面上电子态密度增大可能提高超导电性，但增强了的Ni原子磁交换作用产生铁磁序，破坏了超导电性. 关键词： 电子结构 超导电性 磁性 掺杂     

锰掺杂二硅化铬电子结构和光学性质的第一性原理计算

采用基于第一性原理的密度泛函理论(DFT)赝势平面波方法计算了锰掺杂二硅化铬(CrSi2)体系的能带结构、态密度和光学性件质.计算结果表明末掺杂CrSi2属于间接带隙半导体间接带隙宽度△ER=0.35 eV;Mn掺杂后费米能级进入导带,带隙变窄,且间接带隙宽度△Eg=0.24 eV,CrSi2转变为n型半导体.光学参数发生改变,静态介电常数由掺杂前的ε1(O)=32变为掺杂后的ε1(O)=58;进一步分析了掺杂对CrSi2的能带结构、态密度和光学性质的影响,为CrSi2材料掺杂改件的研究提供r理论依据.     

C掺杂β-FeSi2的电子结构和光学特性研究

采用基于密度泛函理论的第一性原理赝势平面波方法,对C掺杂p-FeSi2的几何结构、能带结构、态密度和光学性质进行了理论计算.几何结构和电子结构的计算表明:C掺杂后使得β-FeSi2的晶格常数a和b减少,c变化不大,晶格体积减小;C掺杂后的 β-FeSi2能带结构仍为准直接带隙,禁带宽度变窄,直接带隙与间接带隙的能量差值不变,C的掺杂消弱了Fe的3d态电子,费米能级附近的电子态密度主要由Fe的3d态电子贡献.光学性质的计算表明:与未掺杂时相比,介电函数的实部ε1减少,虚部ε2的峰值减少并向高能方向有一微小的偏移,吸收系数有所降低.计算结果为β-FeSi2光电材料的应用和设计提供了理论指导.     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.