负电子亲和势GaN光电阴极铯吸附机理研究

为了得到铯吸附与阴极电子亲和势变化之间的定量关系,利用NEA光电阴极激活评估实验系统对GaN光电阴极进行了铯激活.根据半导体光电发射理论和双偶极层模型,通过对电子亲和势随铯覆盖度变化的实验结果进行拟合运算,得到电子亲和势与铯覆盖度之间的函数关系式.分析了铯的吸附机理,得到激活过程中铯的吸附过程与GaN材料有效电子亲和势下降之间的关系.实验表明:负电子亲和势GaN光电阴极材料在铯激活时光电流随着铯覆盖度的增加而从本底值增为极大值,激活过程中GaN电子能量分布曲线低动能截止点的位置决定于铯的覆盖度.当铯的覆盖度从0、1/2、2/3到1个单层变化时,低动能截止点依次向左移动,当覆盖度从0增加到1个单层时,低动能截止点向左移动了约3eV的距离.研究表明,低动能截止点左移本质上是由于对电子逸出起促进作用的有效偶极子[GaN(Mg):Cs]数量的增多造成的,有效偶极子数量的增多带来了材料表面真空能级的下降.     

新一代超高精度锶原子钟

 美国国家标准研究所研究人员认为,在新一代原子钟中将利用锶原子。众所周知,现代铯原子钟走时误差为每5000万年不超过1秒,铯原子钟工作建立在测量铯原子辐射频率基础上,测量精度受原子热运动和辐射波长的限制。锶原子辐射波长比铯原子辐射波长短2个数量级,而最近发现的特性能快速使锶气体冷却到接近绝对零度。     

基于振幅调制的超冷铯原子高分辨光缔合光谱的实验研究

基于振幅调制的超冷铯原子高分辨光谱的实验研究，用相对于铯分子6S_1/2+6P_3/2离解限红失谐的光缔合激光作用于磁光阱中超冷铯原子，观察到通过光缔合产生的激发态超冷分子.在实验中，为了得到高信号-噪声比的光缔合光谱，利用声光调制器对俘获光进行振幅调制，将探测到的超冷铯原子的荧光信号利用lock-in技术解调.同时利用密度矩阵方程系统地分析了实验结果.     

试论王立军实验的意义

 2000年王立军等人在《自然》上发表了一篇重要论文“具有增益的超光速传播”,文中认为他们已成功地实现了光脉冲的超光速传播和从铯室(Ｃｓｃｅｌｌ)中的出射光脉冲在时间上超前于入射光脉冲。图1是实验的示意图:图1王立军实验(ＷＫＤ实验)示意图图1中,入射光脉冲于实验室参照系的时刻ｔ0进入铯室,于实验室参照系的时刻ｔＬ从铯室出去.在铯原子气的反常色散区群速指数ｎｇ=ｎ(ｖ)+ｖｄｎ(ｖ)ｄｖ=-310±5<0(1)光脉冲群速为负速度,即ｖｇ=Ｃｎｇ=-Ｃ310<0.(2)光脉冲在铯室内的通行时间为Δｔ(Ｃｓ)=ｔＣｓＬ-ｔ0=     

氧化铯薄膜的光学特性和介电常数

正常氧化铯薄膜的光学常数和介电常数随铯杂质含量和入射光波长有关。本文给出实测的三种铯杂质含量较多的氧化铯的折射率n。消光系数k和复介电常数在可见光和近红外范围的曲线,并对这些曲线和散射光及反射光颜色随铯杂质含量的关系作了讨论。 关键词：     

时间频率基准装置的研制现状

时间频率基准装置——铯原子喷泉钟, 在标准时间产生和保持、基础物理研究中发挥了重要的作用. 介绍了铯原子喷泉钟的工作原理, 对影响其性能的各项噪声源和频移项给出了分析, 影响频率稳定度性能的主要因素为Dick 效应相关的原子团装载时间、微波激励源相位噪声和探测激光的频率噪声, 影响频率不确定性能主要频移项为: 黑体辐射频移、冷原子碰撞频移、腔相位分布频移和微波泄露频移; 总结和比较了当前具有先进性能的铯原子喷泉钟采用的技术; 介绍了铯原子喷泉钟的主要应用方向、空间冷原子铯钟的研制情况和光学频率原子钟进展.     

CPT原子钟参数设计及其光谱实验研究

通过建立被动型CPT铯原子钟的理论模型,开展CPT信号与铯原子气室特性参量间关系的仿真分析和研究,建立铯原子气室设计及分析方法,得到了被动型CPT铯原子钟最佳设计参数,直径10 mm、长度10 mm、缓冲气体为N₂的圆柱形铯原子气室,最佳工作参数为：工作温度为320 K、气体压强值50 Torr。并经多波长多普勒吸收光谱、CPT信号锁定及频率稳定度测试等实验,验证了理论模型的正确性,为开展高性能芯片级CPT铯原子钟的设计、参数优化提供了一种研究方法。     

两类介质对单色激光的吸收系数的测量与比较

实验测量并比较了铯原子气室和吸收型中性衰减片对852.356 nm单色激光的吸收系数,以及两种介质吸收系数随入射光强变化的行为.对选择吸收和普遍吸收作了分析.采用单色光场与二能级原子系统相互作用的模型,就铯原子气室对852.356 nm共振光的吸收系数随入射光强的变化规律做了分析和拟合.     

铯氧比对砷化镓光电阴极激活结果的影响

实验和理论分析结果表明,激活成功的砷化镓光电阴极的铯氧比存在一个最佳值.砷化镓光电阴极铯氧比的控制可通过调节激活过程中铯源和氧源的加热电流大小来实现.激活实验结果表明,铯氧电流比适中的样品,首次进氧时,光电流上升速度最快,激活后的阴极量子效率最高,稳定性好.当偏离这个比例,过大或过小时,光电流上升速度都会减慢,激活结果也比前者差.随着铯氧电流比的增大,铯氧交替的总次数随之减少.最佳铯氧电流比的调节应以首次进氧时光电流的上升速度最快为准,一旦确定后在整个铯氧交替过程中保持不变.     

一套实用的铯原子喷泉光学系统的设计及建立

介绍了实现铯原子喷泉的过程及条件,并详细讨论了光路中各种光学元件对铯原子喷泉光学系统性能的影响。在对各种光学元件性能综合考虑的基础上,设计并建立了一套用于进行激光冷却和上抛的铯原子喷泉的光学系统,使光强控制、失谐调整、光束开关、光束质量等多种参量的控制达到了技术要求。在此基础上,实现了铯原子磁光阱,为实现铯原子喷泉奠定了基础。     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

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.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

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.     

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.